What substances are included in proteins. Entertaining chemistry

The chemical composition of proteins.

3.1. Peptide bond

Proteins are irregular polymers built from -amino acid residues, the general formula of which in an aqueous solution at pH values ​​close to neutral can be written as NH 3 + CHRCOO -. The amino acid residues in proteins are linked by an amide bond between the -amino and -carboxyl groups. Peptide bond between two-amino acid residues are usually called peptide bond , and polymers built from -amino acid residues connected by peptide bonds are called polypeptides. A protein as a biologically significant structure can be either one polypeptide or several polypeptides that form a single complex as a result of non-covalent interactions.

3.2. Elemental composition of proteins

Studying the chemical composition of proteins, it is necessary to find out, first, what chemical elements they consist of, and secondly, the structure of their monomers. To answer the first question, the quantitative and qualitative composition of the chemical elements of the protein is determined. Chemical analysis showed presence in all proteins carbon (50-55%), oxygen (21-23%), nitrogen (15-17%), hydrogen (6-7%), sulfur (0.3-2.5%). Phosphorus, iodine, iron, copper and some other macro- and microelements, in various, often very small amounts, are also found in the composition of individual proteins.

The content of the main chemical elements in proteins can vary, with the exception of nitrogen, the concentration of which is characterized by the greatest constancy and averages 16%. In addition, the nitrogen content in other organic substances is low. In accordance with this, it was proposed to determine the amount of protein by the nitrogen included in its composition. Knowing that 1 g of nitrogen is contained in 6.25 g of protein, the found amount of nitrogen is multiplied by a factor of 6.25 and the amount of protein is obtained.

To determine the chemical nature of protein monomers, it is necessary to solve two problems: to divide the protein into monomers and to find out their chemical composition. The breakdown of protein into its constituent parts is achieved by hydrolysis - prolonged boiling of the protein with strong mineral acids (acid hydrolysis) or grounds (alkaline hydrolysis)... The most commonly used boiling at 110  C with HCl for 24 hours. At the next stage, the substances that make up the hydrolyzate are separated. For this purpose, various methods are used, most often - chromatography (for more details, see the chapter "Research Methods ..."). The main part of the separated hydrolysates are amino acids.

3.3. Amino acids

At present, up to 200 different amino acids have been found in various objects of living nature. In the human body, for example, there are about 60 of them. However, proteins contain only 20 amino acids, sometimes called natural.

Amino acids are organic acids in which the hydrogen atom of the -carbon atom is replaced by an amino group - NH 2. Therefore, by chemical nature, these are -amino acids with the general formula:

H - C  - NH 2

It can be seen from this formula that the composition of all amino acids includes the following general groups: - CH 2, - NH 2, - COOH. Side chains (radicals - R) amino acids differ. As can be seen from Appendix I, the chemical nature of radicals is diverse: from the hydrogen atom to cyclic compounds. It is the radicals that determine the structural and functional features of amino acids.

All amino acids, except for the simplest aminoacetic to-you glycine (NH 3 + CH 2 COO ) have a chiral atom C  and can exist in the form of two enantiomers (optical isomers):

COO - COO -

NH 3 + RR NH 3 +

L-isomerD-isomer

All currently studied proteins include only L-series amino acids, in which, if we consider the chiral atom from the side of the H atom, the NH 3 +, COO  groups and the R radical are located clockwise. The need to build a biologically significant polymer molecule from a strictly defined enantiomer is obvious - an unimaginably complex mixture of diastereoisomers would be obtained from a racemic mixture of two enantiomers. The question of why life on Earth is based on proteins built from L- and not D--amino acids is still an intriguing mystery. It should be noted that D-amino acids are widespread in nature and, moreover, are part of biologically significant oligopeptides.

Of the twenty basic -amino acids, proteins are built, however, the rest, rather diverse amino acids are formed from these 20 amino acid residues already in the protein molecule. Among such transformations, the formation of disulfide bridges during the oxidation of two cysteine ​​residues in the already formed peptide chains. As a result, a diaminodicarboxylic acid residue is formed from two cysteine ​​residues cystine (see Appendix I). In this case, cross-linking occurs either within one polypeptide chain, or between two different chains. As a small protein with two polypeptide chains, connected by disulfide bridges, as well as cross-links within one of the polypeptide chains:

GIVEQCCASVCSLYQLENYCN

FVNQHLCGSHLVEALYLVCGERGFFYTPKA

An important example of the modification of amino acid residues is the conversion of proline residues into residues hydroxyproline :

N - CH - CO - N - CH - CO -

CH 2 CH 2 CH 2 CH 2

CH 2 CHOH

This transformation occurs, and on a significant scale, with the formation of an important protein component of connective tissue - collagen .

Another very important type of protein modification is the phosphorylation of the hydroxo groups of serine, threonine and tyrosine residues, for example:

- NH - CH - CO - - NH - CH - CO -

CH 2 OH CH 2 OPO 3 2 -

Amino acids in an aqueous solution are in an ionized state due to the dissociation of amino and carboxyl groups that make up the radicals. In other words, they are amphoteric compounds and can exist either as acids (proton donors) or as bases (donor acceptors).

All amino acids, depending on their structure, are divided into several groups:

Acyclic. Monoaminomonocarboxylic amino acids have in their composition one amine and one carboxyl group, in an aqueous solution they are neutral. Some of them have common structural features, which allows us to consider them together:

Glycine and Alanine. Glycine (glycocol or aminoacetic acid) is optically inactive - it is the only amino acid that does not have enantiomers. Glycine is involved in the formation of nucleic acid and bile to - t, heme, it is necessary for the neutralization of toxic products in the liver. Alanine is used by the body in various carbohydrate and energy metabolism processes. Its isomer -alanine is an integral part of the pantothenic acid vitamin, coenzyme A (CoA), and muscle extractives.

Serine and threonine. They belong to the group of hydroxy acids, because have a hydroxyl group. Serine is part of various enzymes, the main protein of milk - casein, as well as in the composition of many lipoproteins. Threonine is involved in protein biosynthesis, being an essential amino acid.

Cysteine ​​and methionine. Amino acids containing a sulfur atom. The value of cysteine ​​is determined by the presence of a sulfhydryl (- SH) group in its composition, which gives it the ability to easily oxidize and protect the body from substances with a high oxidative capacity (in case of radiation injury, phosphorus poisoning). Methionine is characterized by the presence of an easily mobile methyl group used for the synthesis of important compounds in the body (choline, creatine, thymine, adrenaline, etc.)

Valine, Leucine, and Isoleucine. They are branched-chain amino acids that are actively involved in metabolism and are not synthesized in the body.

Monoaminodicarboxylic amino acids have one amine and two carboxyl groups and give an acidic reaction in aqueous solution. These include aspartic and glutamic acids, asparagine and glutamine. They are part of the inhibitory mediators of the nervous system.

Diaminomonocarboxylic amino acids in aqueous solution have an alkaline reaction due to the presence of two amine groups. The related lysine is necessary for the synthesis of histones as well as in a number of enzymes. Arginine is involved in the synthesis of urea, creatine.

Cyclic... These amino acids contain an aromatic or heterocyclic nucleus and, as a rule, are not synthesized in the human body and must be supplied with food. They are actively involved in a variety of metabolic processes. So

phenyl-alanine is the main source of tyrosine synthesis - a precursor of a number of biologically important substances: hormones (thyroxine, adrenaline), some pigments. Tryptophan, in addition to participating in protein synthesis, is a component of vitamin PP, serotonin, tryptamine, and a number of pigments. Histidine is essential for protein synthesis, is a precursor of histamine, which affects blood pressure and gastric acid secretion.

Properties

Proteins are high molecular weight compounds. These are polymers consisting of hundreds and thousands of amino acid residues - monomers.

Proteins have a high molecular weight, some are soluble in water, capable of swelling, are characterized by optical activity, mobility in electric field and some other properties.

Proteins actively enter into chemical reactions. This property is due to the fact that the amino acids that make up proteins contain different functional groups that can react with other substances. It is important that such interactions also occur inside the protein molecule, as a result of which peptide, hydrogen disulfide and other types of bonds are formed. To amino acid radicals, and, respectively, and molecular mass proteins are in the range of 10,000 - 1,000,000. Thus, ribonuclease (an enzyme that cleaves RNA) contains 124 amino acid residues and its molecular weight is approximately 14,000. Myoglobin (muscle protein), consisting of 153 amino acid residues, has a molecular weight 17,000, and hemoglobin - 64,500 (574 amino acid residues). The molecular weights of other proteins are higher: -globulin (forms antibodies) consists of 1250 amino acids and has a molecular weight of about 150,000, and the molecular weight of the enzyme glutamate dehydrogenase exceeds 1,000,000.

Minerals

The group of macronutrients includes those whose content in the dry substance varies from n ּ 10 -2 to n ּ 10%. These are C, O, H, N, S and P, which are part of the molecular composition of basic substances and Ca, Na, Cl, K, Mg, which are part of supporting tissues, blood, lymph and other tissues.

The group of ultramicroelements includes elements whose content is lower than n ּ 10 -5% (Sb, Hg, Bi, Pb, etc.).

It has been established that most of the elements are biogenic, of great importance for the normal development of biochemical life processes, and the most important biogenic elements are included in IV (C); V (W, P) and VI (O, S) groups of the periodic table. Elements VII (Cl, J, Mn) and VIII (Fe, Co) groups are involved in the formation of substances with high biological value.

The trace element composition of raw materials depends on the habitat or growth. Depending on the concentration of individual elements in the environment and in food, their availability, as well as on the selective ability of certain types of organisms, the degree of use of individual elements in assimilation processes also changes.

Protein

Of the organic substances that make up living organisms, proteins are the most biologically important and the most complex in structure. Almost all manifestations of life (digestion, irritability, contractility, growth and reproduction, movement, metabolism, etc.) are associated with protein substances. Proteins play an important role both in the construction of living matter and in the implementation of the processes of its vital activity.

Specific catalysts of a protein nature - enzymes - accelerate chemical reactions in the body. Various compounds of a protein nature carry out a transport function, supplying the body with oxygen and nutrients. The breakdown of 1 g of protein into final products provides the body with energy of 4.27 kcal.

Proteins isolated from organs and tissues give a precipitate when heated white and have the same physical properties as chicken egg white. Therefore, they came to be called proteins. A synonym for the word "protein" is the word "protein" (from the Greek "proteus" - the first, main).

Proteins are high molecular weight polymers of various amino acids. In fig. 1 shows the formulas of various amino acids.

Fig. 1. Formulas of some amino acids.

Amino acids are divided into 2 large groups: nonessential and irreplaceable. Most amino acids are formed in the body of animals and humans as a result of hydrolysis of food proteins and biosynthesis. But at least eight amino acids are not synthesized in the body. These are valine, leucine, isoleucine, threonine, lysine, phenylalanine, tryptophan and methionine, which are called essential. Proteins that lack one or more of these amino acids are called biologically defective. Animal proteins, including those of aquatic organisms, contain all the essential amino acids.

The amino acids that make up the protein are interconnected by peptide bonds formed between the amine group of one amino acid and the carboxyl group of another. The mechanism of this process is shown in Fig. 2.

Rice. 2. Formation of the primary structure of the protein.

The resulting polypeptides are the basis of all proteins, and a certain sequence of amino acids embedded in them characterizes the primary structure of the protein.

Thus, since protein macromolecules are built of many hundreds of amino acids, there are an infinite number of their isomers in nature, and each type of living creature can have its own protein inherent only to it.

Polypeptide chains, in turn, can join, forming secondary structures of the protein, mainly due to the bonds that arise between different groups of polypeptides. This is shown schematically in Fig. 3.

a) the formation of hydrogen bonds

b) the formation of a-helix from the polypeptide chain

Rice. 3. Scheme of the formation of the secondary structure of the protein.

The spatial arrangement of the polypeptide chains of a protein molecule determines the tertiary structure of the protein molecule.

Proteins themselves are high molecular weight compounds of complex structure, differing both in physiological functions and in chemical properties. Proteins of food raw materials are mainly in a colloidal state - in the form of gels and sols, which predetermines the instability and variability of properties (denaturation) of protein substances when environmental conditions change.

When acidifying protein solutions to pH 4.5-5.0 (for example, during pickling), proteins lose their solubility and precipitate (coagulate). Many proteins lose their solubility when solutions are saturated with sodium chloride (during salting). In particular, the main muscle proteins, readily soluble in sodium chloride solutions with a concentration of 7.5-10%, are precipitated (salted out) with an increase in its concentration to 15%. When heated (during cooking, frying, baking), the proteins coagulate (coagulate). Thermal denaturation of proteins begins from 28-35 o C. Protein denaturation also occurs during dehydration (dehydration) of their systems (during drying and freezing).

When precipitating (salting out, coagulating) proteins, their connection with water is disrupted.

As a result of the spatial three-dimensional structure "on the surface" of the protein molecule, chemically active groups - NH 2 appear; -COOH; - HE. In an aqueous solution, these groups are in an ionized state with charges of different signs. A protein molecule acquires the corresponding sign and magnitude of charge, depending on the ratio of positively and negatively charged groups. The charge of a protein molecule depends on its state. Any change in the structure of a protein molecule leads to a change in its charge, in particular, a loss of charge leads to denaturation of the protein. The presence of these charges also determines the hydration properties of proteins. For example, water molecules attach to a positively charged protein molecule with their negatively charged ends, and a structure is formed, the center of which is the protein molecule, and around it there is a monomolecular shell of water. Since all the negatively charged ends of water molecules are facing the protein molecule, the same charge remains on the surface of the protein - water structure. This surface, in turn, is joined by new groups of water molecules, etc. In this case, an electrostatically bound hydration layer is formed around each protein molecule. The strength of the bond with the protein decreases in proportion to the square of the distance from the center, i.e. from a protein molecule, and at a distance large enough on the scale of the size of the molecule, this bond is so small that the intrinsic thermal motion of the molecules prevents the action of electrostatic forces. This limits the amount of water retained by the protein surface.

According to existing views, protein tissue can be considered as a colloidal and capillary-porous colloidal body of a very complex structure, the basis of which is a structural network of proteins in a swollen state, containing viscous solutions containing soluble proteins and other nitrogenous and mineral substances that have hydrophilic properties. At the same time, part of the water that is part of the muscle tissue is firmly held by the proteins of the structural network, as well as by the molecules of dissolved proteins and other hydrophilic substances.

Along with the water retained by the force field on the outer and inner surfaces of the protein particles, there is water in the muscle tissue, which is held by osmotically and by the forces of mechanical connection (capillary-retained water). This water is in liquids (solutions) containing various nitrogenous and organic substances and mineral salts, enclosed in closed cells (micropores) inside protein structures and penetrating the latter micro- and macrocapillaries. According to the data available in the literature, 1 g of protein during hydration binds on average 0.3 g of water.

All processing methods, technological modes are aimed at changing water in the tissues of the raw material (saturation with salt, transformation into ice, heating to a temperature close to the boiling point, evaporation). A change in the internal energy of water leads to a violation of the equilibrium state between the protein and water, which forms a hydration shell. The protein molecule reacts to this by rearranging its own structure and, accordingly, changing the magnitude of the charge. When these changes are completed with a sharp decrease or complete disappearance of the charge, protein denaturation occurs.

Depending on the intensity and duration of external exposure, protein denaturation can be either reversible, or partially reversible, or irreversible.

The depth of denaturation can be determined by the ability of muscle tissue to restore completely or partially its connection with water.

Currently used methods of processing food raw materials with a high protein content lead mainly to changes that can be characterized as partial denaturation. The scheme of denaturation of a protein molecule is shown in Fig. 4.

Rice. 4. Scheme of denaturation of a protein molecule:

A - initial state, B - incipient reversible deployment, C - far gone irreversible deployment.

The most characteristic changes in protein during thermal denaturation (temperature 70-100 ° C) are the loss of its native properties (the ability to dissolve in water, solutions of salts and alcohols), as well as a decrease in the ability to swell.

Protein changes associated with thermal denaturation are the more significant, the higher the temperature and duration of heating, the effect of pressure, and in an aqueous solution the protein denatures faster than in a dried state.

Protein denaturation plays an important role in a number of technological processes: baking bread, confectionery, drying meat, fish, vegetables, milk and egg powder, making canned food, etc.

Under the conditions of bringing the product to full readiness, usually with prolonged exposure to temperatures close to 100 ° C, proteins undergo further changes associated with the destruction of their macromolecules - hydrolysis.

At the beginning of the process, volatile products can be split off from protein molecules: carbon dioxide, hydrogen sulfide, ammonia, phosphorous hydrogen and other substances involved in the formation of taste and aroma of finished products. With prolonged exposure to water and heat, water-soluble nitrogenous substances are formed due to depolymerization of the protein molecule, which occurs, for example, during the transition of collagen to glutin.

Protein hydrolysis can be caused by proteolytic enzymes used to intensify some technological processes (softening tough meat, making yeast dough, etc.).



Protein- high molecular weight organic compounds consisting of α-amino acid residues.

V protein composition includes carbon, hydrogen, nitrogen, oxygen, sulfur. Some proteins form complexes with other molecules containing phosphorus, iron, zinc and copper.

Proteins have a high molecular weight: egg albumin - 36,000, hemoglobin - 152,000, myosin - 500,000. For comparison: the molecular weight of alcohol is 46, acetic acid is 60, benzene is 78.

Amino acid composition of proteins

Protein- non-batch polymers, the monomers of which are α-amino acids... Usually, 20 types of α-amino acids are called protein monomers, although more than 170 of them have been found in cells and tissues.

Depending on whether amino acids can be synthesized in the human body and other animals, a distinction is made between: nonessential amino acids- can be synthesized; essential amino acids- cannot be synthesized. Essential amino acids must be ingested with food. Plants synthesize all kinds of amino acids.

Depending on the amino acid composition, proteins are: complete- contain the entire set of amino acids; inferior- some amino acids are missing in their composition. If proteins are made up of only amino acids, they are called simple... If proteins contain, in addition to amino acids, a non-amino acid component (prosthetic group), they are called complex... The prosthetic group can be represented by metals (metalloproteins), carbohydrates (glycoproteins), lipids (lipoproteins), nucleic acids (nucleoproteins).

Everything amino acids contain: 1) a carboxyl group (-COOH), 2) an amino group (-NH 2), 3) a radical or an R-group (the rest of the molecule). The structure of the radical is different for different types of amino acids. Depending on the number of amino groups and carboxyl groups that make up amino acids, there are: neutral amino acids having one carboxyl group and one amino group; essential amino acids having more than one amino group; acidic amino acids having more than one carboxyl group.

Amino acids are amphoteric compounds, since in solution they can act both as acids and bases. In aqueous solutions, amino acids exist in different ionic forms.

Peptide bond

Peptides- organic substances consisting of amino acid residues linked by a peptide bond.

The formation of peptides occurs as a result of the condensation reaction of amino acids. When the amino group of one amino acid interacts with the carboxyl group of the other, a covalent nitrogen-carbon bond arises between them, which is called peptide... Depending on the number of amino acid residues that make up the peptide, a distinction is made between dipeptides, tripeptides, tetrapeptides etc. The formation of a peptide bond can be repeated many times. This leads to education polypeptides... At one end of the peptide there is a free amino group (called the N-end), and at the other end there is a free carboxyl group (called the C-end).

Spatial organization of protein molecules

The performance of certain specific functions by proteins depends on the spatial configuration of their molecules; in addition, it is energetically unfavorable for the cell to keep proteins in an unfolded form, in the form of a chain, therefore, polypeptide chains are folded, acquiring a certain three-dimensional structure, or conformation. Allocate 4 levels spatial organization of proteins.

Primary protein structure- the sequence of the arrangement of amino acid residues in the polypeptide chain that makes up the protein molecule. The connection between amino acids is peptide.

If a protein molecule consists of only 10 amino acid residues, then the number of theoretically possible variants of protein molecules that differ in the order of alternation of amino acids is 10 20. With 20 amino acids, you can make up more large quantity various combinations. In the human body, about ten thousand different proteins have been found, which differ both from each other and from the proteins of other organisms.

It is the primary structure of a protein molecule that determines the properties of protein molecules and its spatial configuration. Replacement of just one amino acid for another in the polypeptide chain leads to a change in the properties and functions of the protein. For example, the replacement of the sixth glutamic amino acid with valine in the β-subunit of hemoglobin leads to the fact that the hemoglobin molecule as a whole cannot perform its main function - oxygen transport; in such cases, a person develops a disease - sickle cell anemia.

Secondary structure- ordered folding of the polypeptide chain into a spiral (looks like an extended spring). The turns of the helix are strengthened by hydrogen bonds between carboxyl groups and amino groups. Almost all CO and NH groups are involved in the formation of hydrogen bonds. They are weaker than peptide ones, but, being repeated many times, give stability and rigidity to this configuration. At the level of the secondary structure, there are proteins: fibroin (silk, cobweb), keratin (hair, nails), collagen (tendons).

Tertiary structure- folding of polypeptide chains into globules, resulting from the emergence of chemical bonds (hydrogen, ionic, disulfide) and the establishment of hydrophobic interactions between the radicals of amino acid residues. The main role in the formation of the tertiary structure is played by hydrophilic-hydrophobic interactions. In aqueous solutions, hydrophobic radicals tend to hide from water, grouping inside a globule, while hydrophilic radicals, as a result of hydration (interaction with water dipoles), tend to be on the surface of the molecule. In some proteins, the tertiary structure is stabilized by disulfide covalent bonds between the sulfur atoms of two cysteine ​​residues. At the level of the tertiary structure, there are enzymes, antibodies, and some hormones.

Quaternary structure characteristic of complex proteins, the molecules of which are formed by two or more globules. Subunits are held in the molecule by ionic, hydrophobic and electrostatic interactions. Sometimes, during the formation of a quaternary structure, disulfide bonds arise between the subunits. The most studied protein with a quaternary structure is hemoglobin... It is formed by two α-subunits (141 amino acid residues) and two β-subunits (146 amino acid residues). Associated with each subunit is a heme molecule containing iron.

If for some reason the spatial conformation of proteins deviates from the normal, the protein cannot perform its functions. For example, mad cow disease (spongiform encephalopathy) is caused by abnormal conformation of prions, the surface proteins of nerve cells.

Protein properties

The amino acid composition, the structure of the protein molecule determine it properties... Proteins combine basic and acidic properties, determined by amino acid radicals: the more acidic amino acids in a protein, the more pronounced its acidic properties. The ability to give and attach H + is determined by buffering properties of proteins; one of the most powerful buffers is hemoglobin in erythrocytes, which maintains blood pH at a constant level. There are soluble proteins (fibrinogen), there are insoluble proteins that perform mechanical functions (fibroin, keratin, collagen). There are chemically active proteins (enzymes), there are chemically inactive, resistant to various environmental conditions and extremely unstable.

External factors (heat, ultraviolet radiation, heavy metals and their salts, pH changes, radiation, dehydration)

can cause disruption of the structural organization of the protein molecule. The process of loss of the three-dimensional conformation inherent in a given protein molecule is called denaturation... Denaturation is caused by the breaking of bonds that stabilize a certain protein structure. Initially, the weakest ties are broken, and with tougher conditions, the stronger ones. Therefore, first the quaternary is lost, then the tertiary and secondary structures. A change in the spatial configuration leads to a change in the properties of the protein and, as a consequence, makes it impossible for the protein to perform its biological functions. If denaturation is not accompanied by the destruction of the primary structure, then it can be reversible, in this case, self-restoration of the conformation inherent in the protein occurs. For example, membrane receptor proteins undergo such denaturation. The process of restoring the protein structure after denaturation is called renaturation... If the restoration of the spatial configuration of the protein is impossible, then denaturation is called irreversible.

Protein functions

Function	Examples and explanations
Construction	Proteins are involved in the formation of cellular and extracellular structures: they are part of cell membranes (lipoproteins, glycoproteins), hair (keratin), tendons (collagen), etc.
Transport	Blood protein hemoglobin attaches oxygen and transports it from the lungs to all tissues and organs, and from them transfers carbon dioxide to the lungs; the composition of cell membranes includes special proteins that provide active and strictly selective transfer of certain substances and ions from the cell to the external environment and vice versa.
Regulatory	Protein hormones are involved in the regulation of metabolic processes. For example, the hormone insulin regulates blood glucose levels, promotes the synthesis of glycogen, and increases the formation of fats from carbohydrates.
Protective	In response to the penetration of foreign proteins or microorganisms (antigens) into the body, special proteins are formed - antibodies that can bind and neutralize them. Fibrin, formed from fibrinogen, helps to stop bleeding.
Motor	The contractile proteins actin and myosin provide muscle contraction in multicellular animals.
Signal	Protein molecules are built into the surface membrane of the cell, which are capable of changing their tertiary structure in response to the action of environmental factors, thus carrying out the reception of signals from the external environment and the transmission of commands to the cell.
Storing	In the body of animals, proteins, as a rule, are not stored, with the exception of egg albumin, milk casein. But thanks to proteins in the body, some substances can be stored in reserve, for example, during the breakdown of hemoglobin, iron is not excreted from the body, but is stored, forming a complex with the protein ferritin.
Energy	When 1 g of protein breaks down to final products, 17.6 kJ is released. Proteins first break down to amino acids, and then to final products - water, carbon dioxide and ammonia. However, as a source of energy, proteins are used only when other sources (carbohydrates and fats) are used up.
Catalytic	One of the most important functions of proteins. Provided with proteins - enzymes that accelerate biochemical reactions in cells. For example, ribulose biphosphate carboxylase catalyzes CO 2 fixation during photosynthesis.

Enzymes

Enzymes, or enzymes, Is a special class of proteins that are biological catalysts. Thanks to enzymes, biochemical reactions proceed at a tremendous speed. The rate of enzymatic reactions is tens of thousands of times (and sometimes millions) higher than the rate of reactions involving inorganic catalysts. The substance on which the enzyme acts is called substrate.

Enzymes - globular proteins structural features enzymes can be divided into two groups: simple and complex. Simple enzymes are simple proteins, i.e. consist only of amino acids. Complex enzymes are complex proteins, i.e. in addition to the protein part, they include a group of non-protein nature - cofactor... For some enzymes, vitamins act as cofactors. In the enzyme molecule, a special part is secreted, called the active center. Active center- a small section of the enzyme (from three to twelve amino acid residues), where the substrate or substrates bind to form an enzyme-substrate complex. Upon completion of the reaction, the enzyme-substrate complex decomposes into an enzyme and a reaction product (s). Some enzymes have (except active) allosteric centers- the sites to which the enzyme rate regulators are attached ( allosteric enzymes).

Enzymatic catalysis reactions are characterized by: 1) high efficiency, 2) strict selectivity and direction of action, 3) substrate specificity, 4) fine and precise regulation. The substrate and reaction specificity of enzymatic catalysis reactions is explained by the hypotheses of E. Fischer (1890) and D. Koshland (1959).

E. Fisher ("key-lock" hypothesis) suggested that the spatial configurations of the active center of the enzyme and the substrate should exactly correspond to each other. The substrate is compared to a "key", the enzyme is compared to a "lock".

D. Koshland (hypothesis "hand-glove") suggested that the spatial correspondence of the structure of the substrate and the active center of the enzyme is created only at the moment of their interaction with each other. This hypothesis is also called the induced correspondence hypothesis.

The rate of enzymatic reactions depends on: 1) temperature, 2) enzyme concentration, 3) substrate concentration, 4) pH. It should be emphasized that since enzymes are proteins, their activity is highest under physiologically normal conditions.

Most enzymes can only work at temperatures between 0 and 40 ° C. Within these limits, the reaction rate increases by about 2 times with an increase in temperature for every 10 ° C. At temperatures above 40 ° C, the protein undergoes denaturation and the enzyme activity decreases. At temperatures close to the freezing point, enzymes are inactivated.

With an increase in the amount of substrate, the rate of the enzymatic reaction increases until the number of substrate molecules becomes equal to the number of enzyme molecules. With a further increase in the amount of substrate, the rate will not increase, since the saturation of the active centers of the enzyme occurs. An increase in the concentration of the enzyme leads to an increase in catalytic activity, since a larger number of substrate molecules undergo transformations per unit time.

For each enzyme, there is an optimal pH value at which it exhibits maximum activity (pepsin - 2.0, salivary amylase - 6.8, pancreatic lipase - 9.0). At higher or lower pH values, the activity of the enzyme decreases. With sharp shifts in pH, the enzyme denatures.

The rate of work of allosteric enzymes is regulated by substances that attach to allosteric centers. If these substances accelerate the reaction, they are called activators if they slow down - inhibitors.

Enzyme classification

By the type of catalyzed chemical transformations, enzymes are divided into 6 classes:

1. oxyreductase(transfer of hydrogen atoms, oxygen or electrons from one substance to another - dehydrogenase),
2. transferases(transfer of a methyl, acyl, phosphate or amino group from one substance to another - transaminase),
3. hydrolases(hydrolysis reactions, in which two products are formed from the substrate - amylase, lipase),
4. lyases(non-hydrolytic attachment to the substrate or elimination of a group of atoms from it, while the C-C, C-N, C-O, C-S bonds - decarboxylase can be broken),
5. isomerase(intramolecular rearrangement - isomerase),
6. ligases(the connection of two molecules as a result of the formation of bonds C-C, C-N, C-O, C-S - synthetase).

Classes are in turn subdivided into subclasses and sub-subclasses. In the current international classification, each enzyme has a specific cipher consisting of four numbers separated by dots. The first number is the class, the second is the subclass, the third is the sub-subclass, the fourth is the ordinal number of the enzyme in this sub-subclass, for example, the arginase cipher is 3.5.3.1.

Go to lectures number 2"Structure and function of carbohydrates and lipids"

Go to lectures No. 4"Structure and function of ATP nucleic acids"

The main properties of proteins depend on their chemical structure. Proteins are high molecular weight compounds, the molecules of which are built from their alpha-amino acid residues, i.e. amino acids in which the primary amino group and the carboxyl group are bonded to the same carbon atom (the first carbon from the carbonyl group).

19-32 types of alpha-amino acids are isolated from proteins by hydrolysis, but usually 20 alpha-amino acids are obtained (these are the so-called proteinogenic amino acids). Their general formula:

common part for all amino acids

R is a radical, i.e. a grouping of atoms in an amino acid molecule associated with an alpha-carbon atom and not taking part in the formation of the spine of the polypeptide chain.

Among the products of hydrolysis of many proteins, proline and hydroxyproline were found, which contain an imino group = NH, and not an amino group H 2 N- and are actually imino acids, not amino acids.

Amino acids are colorless crystalline substances that melt with decomposition at high temperatures (above 250 ° C). They are readily soluble, for the most part, in water and insoluble in ether and other organic solvents.

Amino acids simultaneously contain two groups capable of ionization: a carboxyl group, which has acidic properties, and an amino group, which has basic properties, i.e. amino acids are amphoteric electrolytes.

In strongly acidic solutions, amino acids are present in the form of positively charged ions, and in alkaline solutions, in the form of negative ions.

Depending on the pH of the medium, any amino acid can have either a positive or a negative charge.

The pH value of the medium at which the amino acid particles are electrically neutral is designated as their isoelectric point.

All amino acids obtained from proteins, with the exception of glycine, are optically active, since they contain an asymmetric carbon atom in the alpha position.

Of the 17 optically active protein amino acids, 7 are characterized by right / + / and 10 - left / - / rotation of the plane of the polarized beam, but they all belong to the L-series.

D-series amino acids have been found in some natural compounds and biological objects (for example, in bacteria and in the antibiotics gramicidin and actinomycin). The physiological significance of D- and L-amino acids is different. D-series amino acids, as a rule, are either completely not assimilated by animals and plants, or poorly assimilated, since the enzyme systems of animals and plants are specifically adapted to L-amino acids. It is noteworthy that optical isomers can be distinguished by taste: the L-series amino acids are bitter or tasteless, and the D-series amino acids are sweet.

All groups of amino acids are characterized by reactions involving amino groups or carboxyl groups, or both at the same time. In addition, amino acid radicals are capable of various interactions. Amino acid radicals react:

Salt formation;

Redox reactions;

Acylation reactions;

Esterification;

Amidation;

Phosphorylation.

These reactions, leading to the formation of colored products, are widely used for the identification and semi-quantitative determination of individual amino acids and proteins, for example, the xanthoprotein reaction (amidation), Millon (salt formation), biuret (salt formation), ninhydrin reaction (oxidation), etc.

The physical properties of amino acid radicals are also very diverse. This concerns, first of all, their volume, charge. The variety of amino acid radicals in terms of chemical nature and physical properties determines the polyfunctional and specific features of the proteins they form.

The classification of amino acids found in proteins can be carried out according to various criteria: by the structure of the carbon skeleton, by the content of -COOH and H 2 N-groups, etc. The most rational classification is based on differences in the polarity of amino acid radicals at pH 7, i.e. at a pH value corresponding to intracellular conditions. Accordingly, the amino acids that make up proteins can be divided into four classes:

Amino acids with non-polar radicals;

Amino acids with uncharged polar radicals;

Amino acids with negatively charged polar radicals;

Amino acids with positively charged polar radicals

Let's consider the structure of these amino acids.

Amino acids with non-polar R-groups (radicals)

This class includes four aliphatic amino acids (alanine, valine, isoleucine, leucine), two aromatic amino acids (phenylalanine, tryptophan), one sulfur-containing amino acid (methionine), and one imino acid (proline). A common property of these amino acids is their lower water solubility compared to polar amino acids. Their structure is as follows:

Alanine (α-aminopropionic acid)

Valine (α-aminoisovaleric acid)

Leucine (α-aminoisocaproic acid)

Isoleucine (α-amino-β-methylvaleric acid)

Phenylalanine (α-amino-β-phenylpropionic acid)

Tryptophan (α-amino-β-indolepropionic acid)

Methionine (α-amino-γ-methylthiobutyric acid)

Proline (pyrrolidine-α-carboxylic acid)

2. Amino acids with uncharged polar R-groups (radicals)

This class includes one aliphatic amino acid, glycine (glycol), two hydroxy amino acids, serine and threonine, one sulfur-containing amino acid, cysteine, one aromatic amino acid, tyrosine, and two amides, asparagine and glutamine.

These amino acids are more soluble in water than amino acids with non-polar R ‑ groups, since their polar groups can form hydrogen bonds with water molecules. Their structure is as follows:

Glycine or Glycocol (α-Aminoacetic Acid)

Serine (α-amino-β-hydroxypropionic acid)

Threonine (α-amino-β-hydroxybutyric acid)

Cysteine ​​(α-amino-β-thiopropionic acid)

Tyrosine (α-amino-β-parahydroxyphenylpropionic acid)

Asparagine

The content of the article

PROTEINS (article 1)- a class of biological polymers present in every living organism. With the participation of proteins, the main processes that ensure the vital activity of the body take place: respiration, digestion, muscle contraction, transmission of nerve impulses. Bone tissue, skin, hair, horny formations of living beings are composed of proteins. For most mammals, the growth and development of the body occurs at the expense of products containing proteins as a food component. The role of proteins in the body and, accordingly, their structure is very diverse.

Protein composition.

All proteins are polymers, the chains of which are assembled from amino acid fragments. Amino acids are organic compounds containing (in accordance with the name) an amino group NH 2 and an organic acidic group, i.e. carboxyl, COOH group. Of the whole variety of existing amino acids (theoretically, the number of possible amino acids is unlimited), only those in which there is only one carbon atom between the amino group and the carboxyl group participate in the formation of proteins. In general, the amino acids involved in the formation of proteins can be represented by the formula: H 2 N – CH (R) –COOH. The R group attached to the carbon atom (the one between the amino and carboxyl group) determines the difference between the amino acids that make up proteins. This group can only consist of carbon and hydrogen atoms, but more often contains, in addition to C and H, various functional (capable of further transformations) groups, for example, HO-, H 2 N-, etc. There is also a variant when R = H.

The organisms of living beings contain more than 100 different amino acids, however, not all are used in the construction of proteins, but only 20, the so-called "fundamental" ones. Table 1 shows their names (most of the names developed historically), structural formula, as well as a widely used abbreviation. All structural formulas are arranged in the table so that the main amino acid fragment is on the right.

Table 1. AMINO ACIDS PARTICIPATING IN THE CREATION OF PROTEINS

Name	Structure	Designation
GLYCINE		GLI
ALANIN		ALA
VALIN		SHAFT
Leucine		LEY
Isoleucine		ILE
SERIN		CEP
THREONINE		TRE
CYSTEINE		CIS
METIONIN		MET
LYSINE		LIZ
ARGININE		ARG
ASPARAGIC ACID		ASN
ASPARAGIN		ASN
GLUTAMIC ACID		GLU
GLUTAMINE		GLN
Phenylalanine		HAIR DRYER
Tyrosine		TIR
TRIPTOFAN		THREE
HISTIDINE		GIS
Proline		Missile defense
In international practice, the abbreviated designation of the listed amino acids is accepted using the Latin three-letter or one-letter abbreviations, for example, glycine - Gly or G, alanine - Ala or A.

Among these twenty amino acids (Table 1), only proline contains an NH group next to the carboxyl group COOH (instead of NH 2), since it is part of the cyclic fragment.

Eight amino acids (valine, leucine, isoleucine, threonine, methionine, lysine, phenylalanine and tryptophan), placed in the table on a gray background, are called essential, since the body must constantly receive them from protein foods for normal growth and development.

A protein molecule is formed as a result of the sequential connection of amino acids, while the carboxyl group of one acid interacts with the amino group of the neighboring molecule, as a result a peptide bond –CO – NH– is formed and a water molecule is released. In fig. 1 shows the serial connection of alanine, valine and glycine.

Rice. 1 SERIAL COMPOUND OF AMINO ACIDS during the formation of a protein molecule. The path from the terminal amino group H 2 N to the terminal carboxyl group COOH was chosen as the main direction of the polymer chain.

To describe the structure of a protein molecule in a compact manner, abbreviations of amino acids (Table 1, third column) involved in the formation of the polymer chain are used. A fragment of the molecule shown in Fig. 1 is written as follows: H 2 N-ALA-VAL-GLI-COOH.

Protein molecules contain from 50 to 1500 amino acid residues (shorter chains are called polypeptides). The individuality of a protein is determined by the set of amino acids that make up the polymer chain and, no less important, by the order in which they alternate along the chain. For example, an insulin molecule consists of 51 amino acid residues (this is one of the shortest-chain proteins) and consists of two parallel chains of unequal length connected to each other. The sequence of amino acid fragments is shown in Fig. 2.

Rice. 2 INSULIN MOLECULE built of 51 amino acid residues, fragments of the same amino acids are marked with the corresponding background color. Cysteine ​​amino acid residues contained in the chain (abbreviated designation CIS) form disulfide bridges -S-S-, which bind two polymer molecules, or form bridges within one chain.

Cysteine ​​amino acid molecules (Table 1) contain reactive sulfhydride groups –SH, which interact with each other to form –S – S– disulfide bridges. The role of cysteine ​​in the world of proteins is special, with its participation cross-links are formed between polymer protein molecules.

The combination of amino acids into a polymer chain occurs in a living organism under the control of nucleic acids, it is they who provide a strict assembly order and regulate the fixed length of the polymer molecule ( cm... NUCLEIC ACIDS).

Protein structure.

The composition of a protein molecule, presented in the form of alternating amino acid residues (Fig. 2), is called the primary structure of the protein. Hydrogen bonds ( cm... HYDROGEN BOND), as a result, the protein molecule acquires a certain spatial shape, called the secondary structure. The most common are two types of secondary structure of proteins.

The first option, called the α-helix, is realized using hydrogen bonds within one polymer molecule. The geometric parameters of the molecule, determined by the bond lengths and bond angles, are such that the formation of hydrogen bonds is possible for groups H-N and C = O, between which there are two peptide fragments H-N-C = O (Fig. 3).

The composition of the polypeptide chain shown in Fig. 3 are written in abbreviated form as follows:

H 2 N-ALA VAL-ALA-LEI-ALA-ALA-ALA-ALA-VAL-ALA-ALA-ALA-COOH.

As a result of the contracting action of hydrogen bonds, the molecule acquires the shape of a spiral - the so-called α-helix, it is depicted as a curved spiral-shaped ribbon passing through the atoms forming a polymer chain (Fig. 4)

Rice. 4 VOLUME MODEL OF A PROTEIN MOLECULE in the form of an α-helix. Hydrogen bonds are shown with green dashed lines. The cylindrical shape of the spiral is visible at a certain angle of rotation (hydrogen atoms are not shown in the figure). The color of individual atoms is given in accordance with international rules that recommend black for carbon atoms, blue for nitrogen, red for oxygen, yellow for sulfur (white is recommended for hydrogen atoms not shown in the figure, in this case the entire structure depicted on a dark background).

Another variant of the secondary structure, called the β-structure, is also formed with the participation of hydrogen bonds, the difference is that the H-N and C = O groups of two or more polymer chains located in parallel interact. Since the polypeptide chain has a direction (Fig. 1), variants are possible when the direction of the chains coincides (parallel β-structure, Fig. 5), or they are opposite (antiparallel β-structure, Fig. 6).

Polymer chains of various compositions can participate in the formation of the β-structure, while the organic groups framing the polymer chain (Ph, CH 2 OH, etc.), in most cases, play a secondary role, the interposition of the H-N and C = O groups is of decisive importance. Since, relative to the polymer chain, the H-N and C = O groups are directed in different directions (in the figure - up and down), it becomes possible to simultaneously interact with three or more chains.

The composition of the first polypeptide chain in Fig. 5:

H 2 N-LEY-ALA-FEN-GLI-ALA-ALA-COOH

The composition of the second and third chain:

H 2 N-GLI-ALA-SER-GLI-TRE-ALA-COOH

The composition of the polypeptide chains shown in Fig. 6, the same as in Fig. 5, the difference is that the second chain has the opposite (in comparison with Fig. 5) direction.

The formation of a β-structure inside one molecule is possible, when a chain fragment in a certain region turns out to be rotated by 180 °, in this case two branches of one molecule have the opposite direction, as a result of which an antiparallel β-structure is formed (Fig. 7).

The structure shown in Fig. 7 in a flat image is shown in Fig. 8 in the form of a volumetric model. The sections of the β-structure are conventionally denoted in a simplified manner by a flat wavy ribbon that passes through the atoms forming the polymer chain.

In the structure of many proteins, sections of the α-helix and ribbon-like β-structures, as well as single polypeptide chains, alternate. Their interposition and alternation in the polymer chain is called the tertiary structure of the protein.

Methods for depicting the structure of proteins are shown below using the plant protein cambin as an example. The structural formulas of proteins, which often contain up to hundreds of amino acid fragments, are complex, cumbersome and difficult to understand, therefore, sometimes simplified structural formulas are used - without symbols of chemical elements (Fig. 9, option A), but at the same time they retain the color of the valence lines in accordance with international rules (fig. 4). In this case, the formula is presented not in a flat, but in a spatial image, which corresponds to the real structure of the molecule. This method makes it possible, for example, to distinguish between disulfide bridges (similar to those in insulin, Fig. 2), phenyl groups in the lateral framing of the chain, etc. The image of molecules in the form of volumetric models (balls connected by rods) is somewhat more clear (Fig. 9, option B). However, both methods do not allow one to show the tertiary structure, so the American biophysicist Jane Richardson suggested depicting α-structures in the form of spirally twisted ribbons (see Fig. 4), β-structures in the form of flat wavy ribbons (Fig. 8), and the connecting them single chains - in the form of thin bundles, each type of structure has its own color. Nowadays, this method of imaging the tertiary structure of a protein is widely used (Fig. 9, variant B). Sometimes, for more informational content, they show together a tertiary structure and a simplified structural formula (Fig. 9, option D). There are also modifications of the method proposed by Richardson: α-helices are depicted in the form of cylinders, and β-structures - in the form of flat arrows indicating the direction of the chain (Fig. 9, variant E). Less common is the method in which the entire molecule is depicted as a bundle, where unequal structures are distinguished by different colors, and disulfide bridges are shown in the form of yellow bridges (Fig. 9, option E).

Variant B is most convenient for perception, when, when depicting the tertiary structure, the structural features of the protein (amino acid fragments, the order of their alternation, hydrogen bonds) do not indicate, while proceeding from the fact that all proteins contain "details" taken from a standard set of twenty amino acids ( Table 1). The main task when imaging a tertiary structure is to show the spatial arrangement and alternation of secondary structures.

Rice. nine DIFFERENT IMAGE OPTIONS OF CRAMBIN PROTEIN STRUCTURE.
A - structural formula in the spatial image.
B - structure in the form of a volumetric model.
B - tertiary structure of the molecule.
D - a combination of options A and B.
D is a simplified representation of the tertiary structure.
E - tertiary structure with disulfide bridges.

The most convenient for perception is the volumetric tertiary structure (variant B), freed from the details of the structural formula.

A protein molecule with a tertiary structure, as a rule, takes on a certain configuration, which is formed by polar (electrostatic) interactions and hydrogen bonds. As a result, the molecule takes the form of a compact coil - globular proteins (globules, lat... ball), or threadlike - fibrillar proteins (fibra, lat... fiber).

An example of a globular structure is albumin protein; the albumin class includes chicken egg protein. The albumin polymer chain is assembled mainly from alanine, aspartic acid, glycine, and cysteine, alternating in a specific order. The tertiary structure contains α-helices connected by single chains (Fig. 10).

Rice. ten GLOBULAR STRUCTURE OF ALBUMIN

An example of a fibrillar structure is fibroin protein. They contain a large number of residues of glycine, alanine and serine (every second amino acid residue is glycine); residues of cysteine ​​containing sulfhydride groups are absent. Fibroin, the main component of natural silk and spider webs, contains β-structures connected by single chains (Fig. 11).

Rice. eleven FIBRILLARY PROTEIN FIBROIN

The possibility of the formation of a tertiary structure of a certain type is inherent in the primary structure of the protein, i.e. predetermined by the order of alternation of amino acid residues. From certain sets of such residues, α-helices predominantly arise (there are quite a few such sets), another set leads to the appearance of β-structures, and single chains are characterized by their composition.

Some protein molecules, while retaining a tertiary structure, are able to combine into large supramolecular aggregates, while they are held together by polar interactions, as well as hydrogen bonds. Such formations are called the quaternary structure of the protein. For example, the protein ferritin, which consists mainly of leucine, glutamic acid, aspartic acid, and histidine (all 20 amino acid residues in ferricin, in varying amounts), forms a tertiary structure of four parallel-folded α-helices. When molecules are combined into a single ensemble (Fig. 12), a quaternary structure is formed, which can include up to 24 ferritin molecules.

Fig. 12 FORMATION OF THE QUATERNARY STRUCTURE OF THE GLOBULAR PROTEIN FERRITIN

Another example of supramolecular formations is the structure of collagen. It is a fibrillar protein whose chains are built mainly from glycine alternating with proline and lysine. The structure contains single chains, triple α-helices, alternating with ribbon-like β-structures, stacked in the form of parallel bundles (Fig. 13).

Fig. 13 SUPERMOLECULAR STRUCTURE OF COLLAGEN FIBRILLARY PROTEIN

Chemical properties of proteins.

Under the action of organic solvents, waste products of some bacteria (lactic acid fermentation) or when the temperature rises, the secondary and tertiary structures are destroyed without damaging its primary structure, as a result, the protein loses its solubility and loses its biological activity, this process is called denaturation, that is, the loss of natural properties. for example, curdling sour milk, curdled protein of a boiled chicken egg. At elevated temperatures, proteins of living organisms (in particular, microorganisms) quickly denature. Such proteins are not able to participate in biological processes, as a result, microorganisms die, so boiled (or pasteurized) milk can last longer.

The peptide bonds H-N-C = O, which form the polymer chain of the protein molecule, are hydrolyzed in the presence of acids or alkalis, and the polymer chain is broken, which, ultimately, can lead to the original amino acids. Peptide bonds that are part of α-helices or β-structures are more resistant to hydrolysis and various chemical influences (in comparison with the same bonds in single chains). A more delicate disassembly of the protein molecule into its constituent amino acids is carried out in an anhydrous medium using hydrazine H 2 N – NH 2, while all amino acid fragments, except for the last one, form the so-called hydrazides of carboxylic acids containing the C (O) –HN – NH 2 ( fig. 14).

Rice. fourteen. DECOMPOSITION OF POLYPEPTIDE

Such an analysis can provide information on the amino acid composition of a particular protein, but it is more important to know their sequence in a protein molecule. One of the methods widely used for this purpose is the action on the polypeptide chain of phenyl isothiocyanate (FITC), which in an alkaline medium is attached to the polypeptide (from the end that contains the amino group), and when the reaction of the medium changes to acidic, it detaches from the chain, taking with it fragment of one amino acid (Fig. 15).

Rice. 15 SEQUENTIAL DEGRADATION OF POLYPEPTIDE

Many special techniques have been developed for such an analysis, including those that begin to "disassemble" a protein molecule into its constituent components, starting from the carboxyl end.

The transverse S-S disulfide bridges (formed during the interaction of cysteine ​​residues, Figs. 2 and 9) cleave, converting them into HS-groups by the action of various reducing agents. The action of oxidizing agents (oxygen or hydrogen peroxide) again leads to the formation of disulfide bridges (Fig. 16).

Rice. 16. SPLITTING OF DISULFIDE BRIDGES

To create additional cross-links in proteins, the reactivity of amino and carboxyl groups is used. More accessible for various interactions are amino groups that are in the lateral framing of the chain - fragments of lysine, asparagine, lysine, proline (Table 1). When such amino groups interact with formaldehyde, the condensation process takes place and cross bridges –NH – CH2 – NH– appear (Fig. 17).

Rice. 17 CREATION OF ADDITIONAL CROSS-Bridges BETWEEN PROTEIN MOLECULES.

The terminal carboxyl groups of a protein are capable of reacting with complex compounds of some polyvalent metals (chromium compounds are more often used), while cross-linking also occurs. Both processes are used in leather tanning.

The role of proteins in the body.

The role of proteins in the body is varied.

Enzymes(fermentatio lat... - fermentation), their other name is enzymes (en zumh Greek... - in yeast) are proteins with catalytic activity, they are able to increase the speed of biochemical processes thousands of times. Under the action of enzymes, the constituent components of food - proteins, fats and carbohydrates - are broken down into simpler compounds, from which new macromolecules are then synthesized, which are necessary for the body of a certain type. Enzymes are also involved in many biochemical synthesis processes, for example, in the synthesis of proteins (some proteins help to synthesize others). Cm... ENZYMES

Enzymes are not only highly efficient catalysts, but also selective (direct the reaction strictly in a given direction). In their presence, the reaction proceeds with almost 100% yield without the formation of by-products, and at the same time the flow conditions are mild: normal atmospheric pressure and temperature of a living organism. For comparison, the synthesis of ammonia from hydrogen and nitrogen in the presence of a catalyst - activated iron - is carried out at 400–500 ° C and a pressure of 30 MPa, the ammonia yield is 15–25% per cycle. Enzymes are considered unsurpassed catalysts.

Intensive research on enzymes began in the middle of the 19th century, now more than 2000 different enzymes have been studied, this is the most diverse class of proteins.

The names of enzymes are as follows: to the name of the reagent with which the enzyme interacts, or to the name of the catalyzed reaction, add the ending -ase, for example, arginase decomposes arginine (Table 1), decarboxylase catalyzes decarboxylation, ie. elimination of CO 2 from the carboxyl group:

- COOH → - CH + CO 2

Often, for a more accurate designation of the role of an enzyme, both the object and the type of reaction are indicated in its name, for example, alcohol dehydrogenase - an enzyme that dehydrates alcohols.

For some enzymes, discovered a long time ago, the historical name has been preserved (without the end -aza), for example, pepsin (pepsis, Greek... digestion) and trypsin (thrypsis Greek... liquefaction), these enzymes break down proteins.

For systematization, enzymes are combined into large classes, the classification is based on the type of reaction, the classes are named according to the general principle - the name of the reaction and the ending - aza. Some of these classes are listed below.

Oxidoreductase- enzymes that catalyze redox reactions. Dehydrogenases belonging to this class carry out proton transfer, for example, alcohol dehydrogenase (ADH) oxidizes alcohols to aldehydes, the subsequent oxidation of aldehydes to carboxylic acids catalyze aldehyde dehydrogenases (ALDH). Both processes occur in the body during the conversion of ethanol into acetic acid (Fig. 18).

Rice. eighteen TWO-STAGE OXIDATION OF ETHANOL to acetic acid

It is not ethanol that has a narcotic effect, but the intermediate product acetaldehyde, the lower the activity of the ALDH enzyme, the slower the second stage passes - the oxidation of acetaldehyde to acetic acid and the longer and stronger the intoxicating effect of ethanol ingestion is manifested. The analysis showed that more than 80% of representatives of the yellow race have a relatively low ALDH activity and therefore a significantly more severe alcohol tolerance. The reason for this innate decreased ALDH activity is that some of the glutamic acid residues in the "weakened" ALDH molecule are replaced by lysine fragments (Table 1).

Transferases- enzymes that catalyze the transfer of functional groups, for example, transiminase catalyzes the movement of the amino group.

Hydrolases- enzymes that catalyze hydrolysis. The previously mentioned trypsin and pepsin hydrolyze peptide bonds, and lipases cleave the ester bond in fats:

–RС (О) ОR 1 + Н 2 О → –RС (О) ОН + HOR 1

Lyases- enzymes that catalyze reactions that are not hydrolytic, as a result of such reactions, the C-C, C-O, C-N bonds are broken and new bonds are formed. The enzyme decarboxylase belongs to this class

Isomerase- enzymes that catalyze isomerization, for example, the conversion of maleic acid into fumaric acid (Fig. 19), this is an example of cis - trans isomerization (see ISOMERIA).

Rice. 19. ISOMERIZATION OF MALEIC ACID into fumaric acid in the presence of an enzyme.

The work of enzymes is observed general principle, in accordance with which there is always a structural correspondence between the enzyme and the reagent of the accelerated reaction. According to the figurative expression of E. Fischer, one of the founders of the enzyme theory, the reagent approaches the enzyme like a key to a lock. In this regard, each enzyme catalyzes a specific chemical reaction or a group of reactions of the same type. Sometimes an enzyme can act on one single compound, for example, urease (uron Greek... - urine) catalyzes only the hydrolysis of urea:

(H 2 N) 2 C = O + H 2 O = CO 2 + 2NH 3

The finest selectivity is shown by enzymes that distinguish between optically active antipodes - left- and right-handed isomers. L-arginase acts only on levogyrate arginine and does not affect the dextrorotatory isomer. L-lactate dehydrogenase acts only on levorotatory lactic acid esters, the so-called lactates (lactis lat... milk), while D-lactate dehydrogenase only breaks down D-lactates.

Most of the enzymes act not on one, but on a group of related compounds, for example, trypsin "prefers" to cleave peptide bonds formed by lysine and arginine (Table 1.)

The catalytic properties of some enzymes, such as hydrolases, are determined solely by the structure of the protein molecule itself, another class of enzymes - oxidoreductases (for example, alcohol dehydrogenase) can be active only in the presence of non-protein molecules associated with them - vitamins that activate Mg, Ca, Zn, Mn ions and fragments of nucleic acids (Fig. 20).

Rice. twenty ALCOHOLDEHYDROGENASE MOLECULE

Transport proteins bind and transfer various molecules or ions across cell membranes (both inside the cell and outside), as well as from one organ to another.

For example, hemoglobin binds oxygen as blood passes through the lungs and delivers it to various tissues of the body, where oxygen is released and then used to oxidize food components, this process serves as a source of energy (sometimes the term "burning" of food in the body is used).

In addition to the protein part, hemoglobin contains a complex compound of iron with a cyclic porphyrin molecule (porphyros Greek... - purple), which causes the red color of the blood. It is this complex (Fig. 21, left) that plays the role of an oxygen carrier. In hemoglobin, the iron porphyrin complex is located inside the protein molecule and is retained by polar interactions, as well as by coordination with nitrogen in histidine (Table 1), which is part of the protein. The O2 molecule, which is carried by hemoglobin, attaches by means of a coordination bond to the iron atom on the side opposite to that to which histidine is attached (Fig. 21, right).

Rice. 21 STRUCTURE OF THE IRON COMPLEX

The structure of the complex in the form of a volumetric model is shown on the right. The complex is retained in the protein molecule by a coordination bond (blue dotted line) between the Fe atom and the N atom in histidine, which is part of the protein. The O 2 molecule, which is transported by hemoglobin, is coordinatively attached (red dotted line) to the Fe atom from the opposite country of the flat complex.

Hemoglobin is one of the most thoroughly studied proteins; it consists of a-helices connected by single chains and contains four iron complexes. Thus, hemoglobin is like a bulky package for the transfer of four oxygen molecules at once. In shape, hemoglobin corresponds to globular proteins (Fig. 22).

Rice. 22 GLOBULAR FORM OF HEMOGLOBIN

The main "advantage" of hemoglobin is that the addition of oxygen and its subsequent elimination during transmission to various tissues and organs is quick. Carbon monoxide, CO (carbon monoxide), binds to Fe in hemoglobin even faster, but, unlike O 2, forms a complex that is difficult to decompose. As a result, such hemoglobin is unable to bind O 2, which leads (when large amounts of carbon monoxide is inhaled) to the death of the body from suffocation.

The second function of hemoglobin is the transfer of exhaled CO 2, but in the process of temporary binding of carbon dioxide, it is not the iron atom that is involved, but the H 2 N-group of the protein.

The "performance" of proteins depends on their structure, for example, the replacement of a single amino acid residue of glutamic acid in the hemoglobin polypeptide chain with a valine residue (a rarely observed congenital anomaly) leads to a disease called sickle cell anemia.

There are also transport proteins that can bind fats, glucose, amino acids and transport them both inside and outside cells.

Transport proteins of a special type do not carry the substances themselves, but perform the functions of a "transport regulator", passing certain substances through the membrane (outer wall of the cell). Such proteins are often called membrane proteins. They have the shape of a hollow cylinder and, being built into the membrane wall, provide the movement of some polar molecules or ions into the cell. An example of a membrane protein is porin (Fig. 23).

Rice. 23 PORINE PROTEIN

Food and storage proteins, as the name suggests, serve as sources of internal nutrition, more often for the embryos of plants and animals, as well as in the early stages of development of young organisms. Food proteins include albumin (Fig. 10) - the main component of egg white, as well as casein - the main protein in milk. Under the action of the enzyme pepsin, casein is curdled in the stomach, this ensures its retention in the digestive tract and effective assimilation. Casein contains fragments of all the amino acids the body needs.

Iron ions are stored in ferritin (Fig. 12), which is contained in animal tissues.

Storage proteins also include myoglobin, which resembles hemoglobin in composition and structure. Myoglobin is concentrated mainly in muscles, its main role is to store oxygen, which hemoglobin gives it. It is quickly saturated with oxygen (much faster than hemoglobin), and then gradually transfers it to various tissues.

Structural proteins perform a protective function (skin) or support - they hold the body together and give it strength (cartilage and tendons). Their main component is the fibrillar protein collagen (Fig. 11), the most abundant protein in the animal world, in the body of mammals, it accounts for almost 30% of the total mass of proteins. Collagen has a high tensile strength (the strength of the skin is known), but due to the low content of cross-links in the collagen of the skin, animal skins are not very suitable in their raw form for the manufacture of various products. To reduce the swelling of the skin in water, shrinkage during drying, as well as to increase the strength in the watered state and increase the elasticity in collagen, additional crosslinks are created (Fig.15a), this is the so-called leather tanning process.

In living organisms, collagen molecules that have arisen during the growth and development of the body are not renewed and are not replaced by newly synthesized ones. As the body ages, the number of cross-links in collagen increases, which leads to a decrease in its elasticity, and since renewal does not occur, age-related changes appear - an increase in the fragility of cartilage and tendons, the appearance of wrinkles on the skin.

The articular ligaments contain elastin, a structural protein that is easily stretched in two dimensions. The greatest elasticity is possessed by the protein resilin, which is located in the places of the hinged attachment of the wings of some insects.

Horny formations - hair, nails, feathers, consisting mainly of the protein keratin (Fig. 24). Its main difference is a noticeable content of cysteine ​​residues, which forms disulfide bridges, which gives high elasticity (the ability to restore its original shape after deformation) to hair and woolen fabrics.

Rice. 24. FRAGMENT OF FIBRILLARY PROTEIN KERATIN

For an irreversible change in the shape of a keratin object, you must first destroy the disulfide bridges with the help of a reducing agent, give a new shape, and then re-create the disulfide bridges with the help of an oxidizing agent (Fig. 16), this is how, for example, perming hair is done.

With an increase in the content of cysteine ​​residues in keratin and, accordingly, an increase in the number of disulfide bridges, the ability to deform disappears, but at the same time a high strength appears (the horns of ungulates and the shells of turtles contain up to 18% of cysteine ​​fragments). The body of mammals contains up to 30 different types keratin.

The keratin-related fibrillar protein fibroin, secreted by silkworm caterpillars during cocoon curling, as well as by spiders when weaving a web, contains only β-structures connected by single chains (Fig. 11). Unlike keratin, fibroin does not have transverse disulfide bridges, it is very tear-resistant (strength per unit cross-section is higher for some web samples than for steel cables). Due to the absence of cross-linking, fibroin is inelastic (it is known that woolen fabrics are almost indestructible, and silk fabrics easily wrinkle).

Regulatory proteins.

Regulatory proteins, more commonly referred to as hormones, are involved in various physiological processes. For example, the hormone insulin (Fig. 25) consists of two α-chains connected by disulfide bridges. Insulin regulates metabolic processes with the participation of glucose, its absence leads to diabetes.

Rice. 25 PROTEIN INSULIN

In the pituitary gland of the brain, a hormone is synthesized that regulates the growth of the body. There are regulatory proteins that control the biosynthesis of various enzymes in the body.

The contractile and motor proteins give the body the ability to contract, change shape and move, especially in the muscles. 40% of the mass of all proteins contained in muscles is myosin (mys, myos, Greek... - muscle). Its molecule contains both a fibrillar and a globular part (Fig. 26)

Rice. 26 MYOSIN MOLECULE

Such molecules are combined into large aggregates containing 300–400 molecules.

When the concentration of calcium ions in the space surrounding the muscle fibers changes, a reversible change in the conformation of molecules occurs - a change in the shape of the chain due to the rotation of individual fragments around the valence bonds. This leads to muscle contraction and relaxation, the signal to change the concentration of calcium ions comes from the nerve endings in the muscle fibers. Artificial muscle contraction can be caused by the action of electrical impulses, leading to a sharp change in the concentration of calcium ions, this is the basis for the stimulation of the heart muscle to restore the work of the heart.

Protective proteins help to protect the body from the invasion of attacking bacteria, viruses and from the penetration of foreign proteins (the generalized name of foreign bodies - antigens). The role of protective proteins is played by immunoglobulins (their other name is antibodies), they recognize antigens that have entered the body and firmly bind to them. In mammals, including humans, there are five classes of immunoglobulins: M, G, A, D and E, their structure, as the name suggests, is globular, in addition, they are all built in a similar way. The molecular organization of antibodies is shown below using the example of class G immunoglobulin (Fig. 27). The molecule contains four polypeptide chains united by three disulfide bridges S-S(in Fig. 27 they are shown with thickened valence bonds and large S symbols), in addition, each polymer chain contains intrachain disulfide bridges. Two large polymer chains (highlighted in blue) contain 400-600 amino acid residues. The other two chains (highlighted in green) are almost twice as short, they contain approximately 220 amino acid residues. All four chains are arranged in such a way that the end H 2 N-groups are directed in the same direction.

Rice. 27 SCHEMATIC IMAGE OF THE IMMUNOGLOBULIN STRUCTURE

After contact of the body with a foreign protein (antigen), the cells of the immune system begin to produce immunoglobulins (antibodies), which accumulate in the blood serum. At the first stage, the main work is done by the sections of the chains containing the end H 2 N (in Fig. 27, the corresponding sections are marked in light blue and light green). These are antigen capture areas. In the process of immunoglobulin synthesis, these areas are formed in such a way that their structure and configuration correspond as much as possible to the structure of the approaching antigen (like a key to a lock, like enzymes, but the tasks in this case are different). Thus, for each antigen, a strictly individual antibody is created as an immune response. Not a single known protein can change the structure so "plasticly" depending on external factors, in addition to immunoglobulins. Enzymes solve the problem of structural correspondence to the reagent in a different way - with the help of a gigantic set of various enzymes, counting on all possible cases, and immunoglobulins rebuild the "working tool" every time. Moreover, the hinge region of the immunoglobulin (Fig. 27) provides the two capture regions with some independent mobility, as a result, the immunoglobulin molecule can "find" the two most convenient sites for capture in the antigen in order to securely fix it, this resembles the actions of a crustacean creature.

Next, a chain of sequential reactions of the body's immune system turns on, immunoglobulins of other classes are connected, as a result, a foreign protein is deactivated, and then the destruction and removal of the antigen (foreign microorganism or toxin).

After contact with the antigen, the maximum concentration of immunoglobulin is reached (depending on the nature of the antigen and the individual characteristics of the organism itself) within several hours (sometimes several days). The body retains the memory of such a contact, and with a repeated attack with the same antigen, immunoglobulins accumulate in the blood serum much faster and in greater quantities - acquired immunity arises.

The above classification of proteins is to a certain extent arbitrary, for example, the protein thrombin, mentioned among the protective proteins, is essentially an enzyme that catalyzes the hydrolysis of peptide bonds, that is, belongs to the class of proteases.

Protective proteins are often referred to as snake venom proteins and toxic proteins from some plants, since their task is to protect the body from damage.

There are proteins whose functions are so unique that it is difficult to classify them. For example, the monellin protein found in one African plant is very sweet in taste and has become the subject of research as a non-toxic substance that can be used in place of sugar to prevent obesity. The blood plasma of some Antarctic fish contains proteins with antifreeze properties, which prevents the blood of these fish from freezing.

Artificial synthesis of proteins.

The condensation of amino acids leading to the polypeptide chain is a well-studied process. You can carry out, for example, the condensation of any one amino acid or a mixture of acids and get, respectively, a polymer containing the same units, or different units alternating in a random order. Such polymers have little resemblance to natural polypeptides and have no biological activity. The main task is to combine amino acids in a strictly defined, predetermined order in order to reproduce the sequence of amino acid residues in natural proteins. American scientist Robert Merrifield proposed an original method to solve this problem. The essence of the method is that the first amino acid is attached to an insoluble polymer gel, which contains reactive groups that can combine with –COOH - amino acid groups. A crosslinked polystyrene with chloromethyl groups introduced into it was taken as such a polymer substrate. So that the amino acid taken for the reaction does not react with itself and so that it does not attach with the H 2 N-group to the support, the amino group of this acid is pre-blocked with a bulky substituent [(C 4 H 9) 3] 3 OC (O) -group. After the amino acid has attached to the polymer support, the blocking group is removed and another amino acid is introduced into the reaction mixture, in which the H 2 N group is also pre-blocked. In such a system, only the interaction of the H 2 N-group of the first amino acid and the –COOH group of the second acid is possible, which is carried out in the presence of catalysts (phosphonium salts). Then the whole scheme is repeated by introducing the third amino acid (Fig. 28).

Rice. 28. SCHEME OF SYNTHESIS OF POLYPEPTIDE CHAINS

In the last step, the resulting polypeptide chains are separated from the polystyrene support. Now the whole process is automated, there are automatic peptide synthesizers operating according to the described scheme. This method has been used to synthesize many peptides used in medicine and agriculture. It was also possible to obtain improved analogs of natural peptides with selective and enhanced action. Some small proteins are synthesized, such as insulin hormone and some enzymes.

There are also methods of protein synthesis that copy natural processes: they synthesize fragments of nucleic acids that are tuned to obtain certain proteins, then these fragments are inserted into a living organism (for example, into a bacterium), after which the body begins to produce the desired protein. In this way, significant amounts of hard-to-reach proteins and peptides, as well as their analogues, are now obtained.

Proteins as food sources.

Proteins in a living organism are constantly split into the original amino acids (with the indispensable participation of enzymes), some amino acids pass into others, then the proteins are synthesized again (also with the participation of enzymes), i.e. the body is constantly renewing itself. Some proteins (collagen of the skin, hair) are not renewed, the body constantly loses them and synthesizes new ones instead. Proteins as food sources perform two main functions: they supply to the body construction material for the synthesis of new protein molecules and, in addition, supply the body with energy (sources of calories).

Carnivorous mammals (including humans) get the necessary proteins from plant and animal food. None of the proteins obtained from food is incorporated into the body unchanged. In the digestive tract, all absorbed proteins are broken down to amino acids, and already from them proteins necessary for a particular organism are built, while of the 8 essential acids (Table 1), the other 12 can be synthesized in the body if they are not supplied in sufficient quantities with food, but essential acids must be supplied with food without fail. The body receives sulfur atoms in cysteine ​​with an essential amino acid - methionine. Part of the proteins breaks down, releasing the energy necessary to maintain vital activity, and the nitrogen contained in them is excreted from the body in the urine. Usually, the human body loses 25-30 g of protein per day, so protein food must be constantly present in the right amount. The minimum daily protein requirement is 37 g for men and 29 g for women, but the recommended intake is almost twice as high. When evaluating food, it is important to consider the quality of the protein. In the absence or low content of essential amino acids, protein is considered of low value, therefore, such proteins should be consumed in greater quantities. So, proteins of legumes contain little methionine, and proteins of wheat and corn have a low content of lysine (both amino acids are essential). Animal proteins (excluding collagens) are classified as complete foods. A complete set of all essential acids contains milk casein, as well as cottage cheese and cheese made from it, therefore, a vegetarian diet, if it is very strict, i.e. "Dairy-free", requires increased consumption of legumes, nuts and mushrooms to supply the body with essential amino acids in the right amount.

Synthetic amino acids and proteins are also used as food products, adding them to feeds that contain small amounts of essential amino acids. There are bacteria that can process and assimilate oil hydrocarbons, in this case, for the full synthesis of proteins, they need to be fed with nitrogen-containing compounds (ammonia or nitrates). The protein obtained in this way is used as feed for livestock and poultry. A set of enzymes, carbohydrases, are often added to the compound feed for domestic animals, which catalyze the hydrolysis of difficult-to-decompose components of carbohydrate food (cell walls of cereals), as a result of which plant food is absorbed more fully.

Mikhail Levitsky

PROTEINS (article 2)

(proteins), a class of complex nitrogen-containing compounds, the most characteristic and important (along with nucleic acids) components of living matter. Proteins have many and varied functions. Most proteins are enzymes that catalyze chemical reactions. Many hormones that regulate physiological processes are also proteins. Structural proteins such as collagen and keratin are the main components of bone, hair and nails. The contractile proteins of muscles have the ability to change their length, using chemical energy to perform mechanical work. Proteins include antibodies that bind and neutralize toxic substances. Some proteins that can react to external influences (light, smell) serve as receptors in the sense organs that perceive irritation. Many proteins located inside the cell and on the cell membrane perform regulatory functions.

In the first half of the 19th century. many chemists, among them in the first place J. von Liebig, gradually came to the conclusion that proteins represent a special class of nitrogenous compounds. The name "proteins" (from the Greek protos - the first) was proposed in 1840 by the Dutch chemist G. Mulder.

PHYSICAL PROPERTIES

Proteins are white in solid state, and colorless in solution, unless they carry some chromophore (colored) group, such as hemoglobin. Water solubility varies greatly between proteins. It also changes depending on pH and on the concentration of salts in the solution, so that conditions can be selected under which one protein will selectively precipitate in the presence of other proteins. This "salting-out" method is widely used for the isolation and purification of proteins. Purified protein often precipitates out of solution in the form of crystals.

In comparison with other compounds, the molecular weight of proteins is very high - from several thousand to many millions of daltons. Therefore, during ultracentrifugation, proteins are precipitated, and moreover, at different rates. Due to the presence of positively and negatively charged groups in protein molecules, they move at different speeds and in an electric field. This is the basis of electrophoresis, a method used to isolate individual proteins from complex mixtures. Protein purification is also carried out by chromatography.

CHEMICAL PROPERTIES

Structure.

Proteins are polymers, i.e. molecules built, like chains, from repeating monomeric units, or subunits, the role of which is played by alpha-amino acids. General amino acid formula

where R is a hydrogen atom or some organic group.

A protein molecule (polypeptide chain) can consist of only a relatively small number of amino acids or several thousand monomeric units. The combination of amino acids in a chain is possible because each of them has two different chemical groups: an amino group with basic properties, NH2, and an acidic carboxyl group, COOH. Both of these groups are attached to the a-carbon atom. The carboxyl group of one amino acid can form an amide (peptide) bond with the amino group of another amino acid:

After the two amino acids have joined in this way, the chain can be extended by adding a third to the second amino acid, etc. As you can see from the above equation, when the peptide bond is formed, a water molecule is released. In the presence of acids, alkalis, or proteolytic enzymes, the reaction proceeds in the opposite direction: the polypeptide chain is split into amino acids with the addition of water. This reaction is called hydrolysis. Hydrolysis occurs spontaneously, and energy is required to combine amino acids into a polypeptide chain.

A carboxyl group and an amide group (or a similar imide group - in the case of the amino acid proline) are present in all amino acids, the differences between amino acids are determined by the nature of that group, or "side chain", which is indicated above by the letter R. The role of the side chain can be played by one a hydrogen atom, like the amino acid glycine, and some bulky grouping, like histidine and tryptophan. Some side chains are chemically inert, while others are markedly reactive.

Many thousands of different amino acids can be synthesized, and many different amino acids are found in nature, but only 20 types of amino acids are used for protein synthesis: alanine, arginine, asparagine, aspartic acid, valine, histidine, glycine, glutamine, glutamic acid, isoleucine, leucine, lysine , methionine, proline, serine, tyrosine, threonine, tryptophan, phenylalanine and cysteine ​​(in proteins, cysteine ​​can be present as a dimer - cystine). True, some proteins also contain other amino acids besides the regularly occurring twenty, but they are formed as a result of modification of any of the twenty listed after it has been incorporated into the protein.

Optical activity.

All amino acids, with the exception of glycine, have four different groups attached to the alpha carbon. From the point of view of geometry, four different groups can be attached in two ways, and accordingly there are two possible configurations, or two isomers related to each other, like an object to its mirror image, i.e. how left hand to the right. One configuration is called left, or levogyrate (L), and the other - right, or dextrorotatory (D), since two such isomers differ in the direction of rotation of the plane of polarized light. Only L-amino acids are found in proteins (the exception is glycine; it can be represented only in one form, since it has two of the four groups that are the same), and they all have optical activity (since there is only one isomer). D-amino acids are rare in nature; they are found in some antibiotics and in the cell wall of bacteria.

Amino acid sequence.

The amino acids in the polypeptide chain are not arranged randomly, but in a certain fixed order, and it is this order that determines the functions and properties of the protein. By varying the order of the 20 types of amino acids, you can get a huge number of different proteins, just as you can make up many different texts from the letters of the alphabet.

In the past, it often took several years to determine the amino acid sequence of a protein. Direct determination is still a rather laborious task, although devices have been created that allow it to be carried out automatically. It is usually easier to determine the nucleotide sequence of the corresponding gene and deduce the amino acid sequence of the protein from it. To date, the amino acid sequences of many hundreds of proteins have already been determined. The functions of the decoded proteins are usually known, and this helps to imagine the possible functions of similar proteins, formed, for example, in malignant neoplasms.

Complex proteins.

Proteins consisting only of amino acids are called simple proteins. Often, however, a metal atom or some chemical compound other than an amino acid is attached to the polypeptide chain. These proteins are called complex proteins. An example is hemoglobin: it contains iron porphyrin, which determines its red color and allows it to act as an oxygen carrier.

The names of most complex proteins contain an indication of the nature of the attached groups: there are sugars in glycoproteins, and fats in lipoproteins. If the catalytic activity of the enzyme depends on the attached group, then it is called a prosthetic group. Often, some vitamin plays the role of a prosthetic group or is part of it. Vitamin A, for example, attached to one of the retinal proteins, determines its sensitivity to light.

Tertiary structure.

It is not so much the amino acid sequence of the protein itself (primary structure) that is important, but the way of its packing in space. Along the entire length of the polypeptide chain, hydrogen ions form regular hydrogen bonds, which give it the shape of a spiral or a layer (secondary structure). The combination of such helices and layers gives rise to a compact form of the next order - the tertiary structure of the protein. Rotations through small angles are possible around the bonds holding the monomeric links of the chain. Therefore, from a purely geometric point of view, the number of possible configurations for any polypeptide chain is infinitely large. In reality, however, each protein normally exists in only one configuration, determined by its amino acid sequence. This structure is not rigid, it seems to "breathe" - it oscillates around a certain average configuration. The chain folds into such a configuration in which free energy (the ability to perform work) is minimal, just as a released spring is compressed only to a state corresponding to a minimum of free energy. Often, one part of the chain is rigidly linked to the other by disulfide (–S – S–) bonds between two cysteine ​​residues. This is partly why cysteine ​​plays a particularly important role among amino acids.

The complexity of the structure of proteins is so great that it is still impossible to calculate the tertiary structure of a protein, even if its amino acid sequence is known. But if it is possible to obtain crystals of a protein, then its tertiary structure can be determined by X-ray diffraction.

In structural, contractile, and some other proteins, the chains are elongated and several adjacent slightly folded chains form fibrils; fibrils, in turn, fold into larger formations - fibers. However, most proteins in solution have a globular shape: the chains are coiled in a globule, like yarn in a ball. Free energy in this configuration is minimal, since hydrophobic ("water repelling") amino acids are hidden inside the globule, while hydrophilic ("water attracting") amino acids are located on its surface.

Many proteins are complexes of several polypeptide chains. This structure is called the quaternary protein structure. The hemoglobin molecule, for example, has four subunits, each of which is a globular protein.

Structural proteins, due to their linear configuration, form fibers with a very high tensile strength, while the globular configuration allows proteins to enter into specific interactions with other compounds. On the surface of the globule at correct styling chains arise of a certain shape of the cavity, in which reactive chemical groups are located. If the given protein is an enzyme, then another, usually smaller, molecule of some substance enters such a cavity just like a key enters a lock; in this case, the configuration of the electron cloud of the molecule changes under the influence of the chemical groups in the cavity, and this forces it to react in a certain way. In this way, the enzyme catalyzes the reaction. Antibody molecules also have cavities in which various foreign substances are bound and thereby rendered harmless. The “key and lock” model, which explains the interaction of proteins with other compounds, makes it possible to understand the specificity of enzymes and antibodies; their ability to react only with certain compounds.

Proteins in different types of organisms.

Proteins that perform the same function in different plant and animal species and therefore bear the same name also have a similar configuration. They, however, differ somewhat in their amino acid sequence. As species diverge from a common ancestor, some amino acids at certain positions are replaced by others as a result of mutations. Harmful mutations that cause hereditary diseases are discarded by natural selection, but beneficial or at least neutral ones can remain. The closer to each other are two biological species, the less difference is found in their proteins.

Some proteins change relatively quickly, while others are very conservative. The latter include, for example, cytochrome c, a respiratory enzyme found in most living organisms. In humans and chimpanzees, its amino acid sequences are identical, while in the cytochrome c of wheat, only 38% of the amino acids were different. Even comparing humans and bacteria, the similarity of cytochromes with (the differences affect 65% of the amino acids here) can still be seen, although the common ancestor of bacteria and humans lived on Earth about two billion years ago. Nowadays, comparison of amino acid sequences is often used to build a phylogenetic (genealogical) tree reflecting evolutionary relationships between different organisms.

Denaturation.

The synthesized protein molecule, folding, acquires its characteristic configuration. This configuration, however, can be destroyed by heating, by changing the pH, by the action of organic solvents, and even by simple agitation of the solution until bubbles appear on its surface. The protein changed in this way is called denatured; it loses its biological activity and usually becomes insoluble. Well-known examples of denatured protein are boiled eggs or whipped cream. Small proteins containing only about a hundred amino acids are capable of annealing, i.e. re-acquire the original configuration. But most of the proteins are simply converted into a mass of entangled polypeptide chains and does not restore the previous configuration.

One of the main difficulties in isolating active proteins is associated with their extreme sensitivity to denaturation. This property of proteins finds useful application in food preservation: high temperature irreversibly denatures enzymes of microorganisms, and microorganisms die.

PROTEIN SYNTHESIS

For protein synthesis, a living organism must have a system of enzymes capable of attaching one amino acid to another. A source of information is also needed that would determine which amino acids should be combined. Since there are thousands of types of proteins in the body, and each of them consists of an average of several hundred amino acids, the information required must be truly enormous. It is stored (just as a tape is stored) in the nucleic acid molecules that make up the genes.

Enzyme activation.

A polypeptide chain synthesized from amino acids is not always a protein in its final form. Many enzymes are synthesized first in the form of inactive precursors and become active only after another enzyme has removed several amino acids at one end of the chain. In this inactive form, some of the digestive enzymes are synthesized, for example trypsin; these enzymes are activated in the digestive tract as a result of the removal of the end of the chain. The hormone insulin, the molecule of which in its active form consists of two short chains, is synthesized as one chain, the so-called. proinsulin. Then the middle part of this chain is removed, and the remaining fragments bind to each other, forming an active hormone molecule. Complex proteins are formed only after a certain chemical group is attached to the protein, and this attachment often requires an enzyme.

Metabolic circulation.

After feeding the animal amino acids labeled with radioactive isotopes of carbon, nitrogen or hydrogen, the label is quickly incorporated into its proteins. If the labeled amino acids cease to enter the body, then the amount of the label in proteins begins to decrease. These experiments show that the formed proteins are not stored in the body until the end of life. All of them, with a few exceptions, are in a dynamic state, constantly decaying to amino acids, and then synthesized again.

Some proteins break down when cells die and break down. This constantly happens, for example, with erythrocytes and epithelial cells lining the inner surface of the intestine. In addition, degradation and resynthesis of proteins also take place in living cells. Ironically, less is known about the breakdown of proteins than about their synthesis. It is clear, however, that proteolytic enzymes are involved in the breakdown, similar to those that break down proteins into amino acids in the digestive tract.

The half-life of different proteins is different - from several hours to many months. The only exception is collagen molecules. Once formed, they remain stable, not renewed or replaced. Over time, however, some of their properties change, in particular elasticity, and since they are not renewed, certain age-related changes are the result of this, for example, the appearance of wrinkles on the skin.

Synthetic proteins.

Chemists have long since learned to polymerize amino acids, but amino acids combine in this disordered manner, so that the products of such polymerization are not very similar to natural ones. True, it is possible to combine amino acids in a given order, which makes it possible to obtain some biologically active proteins, in particular insulin. The process is quite complicated, and in this way it is possible to obtain only those proteins, the molecules of which contain about a hundred amino acids. It is preferable to instead synthesize or isolate the nucleotide sequence of the gene corresponding to the desired amino acid sequence, and then introduce this gene into the bacterium, which will produce a large amount of the desired product by replication. This method, however, also has its drawbacks.

PROTEIN AND NUTRITION

When proteins in the body are broken down into amino acids, these amino acids can be used again to synthesize proteins. At the same time, the amino acids themselves are subject to degradation, so that they are not completely reused. It is also clear that during growth, pregnancy and wound healing, protein synthesis must exceed decay. The body is constantly losing some proteins; these are proteins of hair, nails and the surface layer of the skin. Therefore, for the synthesis of proteins, each organism must receive amino acids from food.

Sources of amino acids.

Green plants synthesize all 20 amino acids found in proteins from CO2, water and ammonia or nitrates. Many bacteria are also able to synthesize amino acids in the presence of sugar (or some equivalent) and fixed nitrogen, but sugar is ultimately supplied by green plants. In animals, the ability to synthesize amino acids is limited; they get amino acids by eating green plants or other animals. In the digestive tract, the absorbed proteins are broken down into amino acids, the latter are absorbed, and proteins characteristic of the given organism are already built from them. No absorbed protein is incorporated into the structures of the body as such. The only exception is that in many mammals, part of the maternal antibodies can pass through the placenta into the fetal bloodstream intact, and through breast milk (especially in ruminants) be transferred to the newborn immediately after birth.

Protein requirements.

It is clear that in order to maintain life, the body must receive a certain amount of protein from food. However, the extent of this need depends on a number of factors. The body needs food both as a source of energy (calories) and as a material for building its structures. In the first place is the need for energy. This means that when there are few carbohydrates and fats in the diet, dietary proteins are used not to synthesize their own proteins, but as a source of calories. With prolonged fasting, even one's own proteins are spent on meeting energy needs. If there are enough carbohydrates in the diet, then protein intake can be reduced.

Nitrogen balance.

On average approx. 16% of the total mass of protein is nitrogen. When the amino acids that were part of the proteins are broken down, the nitrogen contained in them is excreted from the body in the urine and (to a lesser extent) in the feces in the form of various nitrogenous compounds. Therefore, it is convenient to use an indicator such as nitrogen balance to assess the quality of protein nutrition, i.e. the difference (in grams) between the amount of nitrogen entering the body and the amount of nitrogen excreted per day. With a normal diet in an adult, these amounts are equal. In a growing organism, the amount of excreted nitrogen is less than the amount received, i.e. the balance is positive. With a lack of protein in the diet, the balance is negative. If there are enough calories in the diet, but proteins are completely absent in it, the body conserves proteins. At the same time, protein metabolism slows down, and the re-utilization of amino acids in protein synthesis proceeds with the highest possible efficiency. However, losses are inevitable, and nitrogenous compounds are still excreted in the urine and partly in the feces. The amount of nitrogen excreted from the body per day during protein starvation can serve as a measure of the daily lack of protein. It is natural to assume that by introducing into the diet an amount of protein equivalent to this deficiency, it is possible to restore the nitrogen balance. However, it is not. Having received this amount of protein, the body begins to use amino acids less efficiently, so that some additional amount of protein is required to restore nitrogen balance.

If the amount of protein in the diet exceeds what is needed to maintain nitrogen balance, then there is probably no harm from this. Excess amino acids are simply used as an energy source. As a particularly striking example, we can cite the Eskimos, who are low in carbohydrates and about ten times more protein than is required to maintain nitrogen balance. In most cases, however, the use of protein as an energy source is disadvantageous, since a certain amount of carbohydrates can produce many more calories than from the same amount of protein. In poor countries, the population gets the necessary calories from carbohydrates and consumes the minimum amount of protein.

If the body receives the required number of calories in the form of non-protein foods, then the minimum amount of protein that maintains the nitrogen balance is approx. 30 g per day. About four slices of bread or 0.5 liters of milk contains about the same amount of protein. A slightly larger amount is usually considered optimal; recommended from 50 to 70 g.

Essential amino acids.

Until now, protein has been viewed as a whole. Meanwhile, in order for protein synthesis to proceed, all the necessary amino acids must be present in the body. Some of the amino acids the body of the animal is able to synthesize itself. They are called non-essential because they do not have to be present in the diet - it is only important that the overall intake of protein as a source of nitrogen is sufficient; then, with a shortage of nonessential amino acids, the body can synthesize them at the expense of those that are present in excess. The rest, "irreplaceable", amino acids cannot be synthesized and must enter the body with food. Valine, leucine, isoleucine, threonine, methionine, phenylalanine, tryptophan, histidine, lysine and arginine are indispensable for humans. (Although arginine can be synthesized in the body, it is classified as an essential amino acid, since it is not produced in sufficient quantities in newborns and growing children. On the other hand, for a mature person, the intake of some of these amino acids from food may become unnecessary.)

This list of essential amino acids is approximately the same in other vertebrates and even in insects. The nutritional value of proteins is usually determined by feeding them to growing rats and monitoring the weight gain of the animals.

The nutritional value of proteins.

The nutritional value of protein is determined by the essential amino acid that is most lacking. Let us illustrate this with an example. The proteins of our body contain on average approx. 2% tryptophan (by weight). Let's say that the diet includes 10 g of protein, containing 1% tryptophan, and that there are enough other essential amino acids in it. In our case, 10 g of this defective protein is essentially equivalent to 5 g of complete protein; the remaining 5 g can only serve as a source of energy. Note that, since amino acids are practically not stored in the body, and in order for protein synthesis to proceed, all amino acids must be present at the same time, the effect of the intake of essential amino acids can be found only if all of them enter the body at the same time.

The average composition of most animal proteins is close to the average composition of proteins human body, so amino acid deficiency is unlikely to threaten us if our diet is rich in foods such as meat, eggs, milk and cheese. However, there are proteins, such as gelatin (a product of collagen denaturation), which contain very few essential amino acids. Vegetable proteins, although they are better than gelatin in this sense, are also poor in essential amino acids; they are especially low in lysine and tryptophan. Nevertheless, a purely vegetarian diet cannot be considered harmful at all, if only a slightly larger amount of plant proteins is consumed, sufficient to provide the body with essential amino acids. Most of the protein is found in the seeds of plants, especially in the seeds of wheat and various legumes. Young shoots such as asparagus are also rich in protein.

Synthetic proteins in the diet.

By adding small amounts of synthetic essential amino acids or proteins rich in them to deficient proteins, such as maize proteins, it is possible to significantly increase the nutritional value of the latter, i.e. thereby, as it were, to increase the amount of protein consumed. Another possibility is to grow bacteria or yeast on petroleum hydrocarbons with the addition of nitrates or ammonia as a nitrogen source. The microbial protein obtained in this way can serve as feed for poultry or livestock, or it can be directly consumed by humans. The third, widely used, method uses the features of the physiology of ruminants. In ruminants in the initial part of the stomach, the so-called. the rumen, there are special forms of bacteria and protozoa, which convert defective plant proteins into more complete microbial proteins, and these, in turn, after being digested and absorbed, turn into animal proteins. Urea, a cheap synthetic nitrogen-containing compound, can be added to livestock feed. The microorganisms inhabiting the rumen use urea nitrogen to convert carbohydrates (which are much more abundant in the feed) into protein. About a third of all nitrogen in livestock feed can come in the form of urea, which in fact means, to a certain extent, chemical protein synthesis.