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, when heated, give a white precipitate and have the same physical properties like the protein of a chicken egg. 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 together, 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 predominantly 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. V 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. New groups of water molecules, in turn, attach to this surface, 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 sufficiently large distance in terms 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 the water in the tissues of the raw material (saturating it with salt, turning it 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-reaching irreversible deployment.

The most characteristic changes in the 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.

The changes in protein associated with heat 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 when in a dried state.

Protein denaturation plays an important role in a number of technological processes: when baking bread, confectionery, drying meat, fish, vegetables, milk and egg powder, in the manufacture of 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.).



Amino acids - structural components proteins. Proteins, or proteins(Greek protos - primary) are biological heteropolymers, the monomers of which are amino acids.

Amino acids are low molecular weight organic compounds containing carboxyl (-COOH) and amine (-NH 2) groups that are bonded to the same carbon atom. A side chain is attached to the carbon atom - a radical that gives each amino acid certain properties. The general formula of amino acids is:

Most amino acids have one carboxyl group and one amino group; these amino acids are called neutral. There are, however, and essential amino acids- with more than one amino group, and acidic amino acids- with more than one carboxyl group.

About 200 amino acids are known to be found in living organisms, but only 20 of them are part of proteins. These are the so-called main, or protein-forming(proteinogenic), amino acids.

Depending on the type of radical, the main amino acids are divided into three groups: 1) non-polar (alanine, methionine, valine, proline, leucine, isoleucine, tryptophan, phenylalanine); 2) polar uncharged (asparagine, glutamine, serine, glycine, tyrosine, threonine, cysteine); 3) polar charged (arginine, histidine, lysine - positively; aspartic and glutamic acids - negatively).

The side chains of amino acids (radical) can be hydrophobic or hydrophilic, which gives proteins the corresponding properties, which are manifested in the formation of secondary, tertiary and quaternary structures of the protein.

In plants all essential amino acids are synthesized from the primary products of photosynthesis. Man and animals are not able to synthesize a number of proteinogenic amino acids and must receive them ready-made along with food. These amino acids are called irreplaceable. TO these include lysine, valine, leucine, isoleucine, threonine, phenylalanine, tryptophan, methionine; also arginine and histidine - indispensable for children,

In solution, amino acids can act as both acids and bases, that is, they are amphoteric compounds. The carboxyl group —COOH is capable of donating a proton, functioning as an acid, and the amine group — NH2 — accepting a proton, thus exhibiting the properties of a base.

Peptides. The amino group of one amino acid is capable of reacting with the carboxyl group of another amino acid.

The resulting molecule is a dipeptide, and the -CO-NH- bond is called a peptide bond:

There is a free amino group at one end of the dipeptide molecule, and a free carboxyl group at the other. Due to this, the dipeptide can attach other amino acids to itself, forming oligopeptides. If many amino acids (more than ten) are combined in this way, then it turns out polypeptide.

Peptides play an important role in the body. Many oligo- and polypeptides are hormones, antibiotics, toxins.

Oligopeptides include oxytocin, vasopressin, thyrotropin, as well as bradykinin (a pain peptide) and some opiates (“natural drugs” in humans) that provide pain relief. Taking drugs destroys the opiate system of the body, so the addict without a dose of drugs experiences severe pain - "withdrawal", which is normally removed by opiates. Oligopeptides also include some antibiotics (for example, gramicidin S).

Many hormones (insulin, adrenocorticotropic hormone, etc.), antibiotics (for example, gramicidin A), toxins (for example, diphtheria toxin) are polypeptides.

Proteins are polypeptides, the molecule of which contains from fifty to several thousand amino acids with a relative molecular weight of over 10,000.

Protein structure. Each protein in a particular environment has a specific spatial structure. When characterizing the spatial (three-dimensional) structure, four levels of organization of protein molecules are distinguished (Fig. 1,1).

lie — glu — tre — ala — ala — ala — liz — phen — glu — arg — gln — gis — meth — asp — ser— ser — tre — ser — ala — ala — ser — ser — ser — asn — tyr — cis — asn — glu — met — met— lys — ser — arg — asn — lei — tre — lys — asp — arg — cis — lys — pro — val — asn — tre— fen-— val — gis — deep — ser — lei — ala — asp — val — gln — ala — val — cis — ser — gln— lys — asn — val — ala — cis — lys — asn — gly — gln — tre — asn — cis — three — gln — ser— three — ser — tre — met — ser — il — tre — asp — cis — arg — glu — tre — gly — ser — ser- lie — tyr — pro — asn — cis — ala — tyr — lie — tre — tre — gln — ala — asn — liz — his— ile — ile — val — ala — cis — deep — gly — asn — pro — tyr — val — pro — val — gis — phen— asp-ala-ser-val

Rice. 1.1. Levels of protein structural organization: a — primary structure - amino acid sequence of protein ribonuclease (124 amino acid links); b — secondary structure — the poypeptide chain is twisted in the form of a spiral; v— tertiary structure of myoglobin protein; G — quaternary structure of hemoglobin.

Primary structure- the sequence of amino acids in the polypeptide chain. This structure is specific for each protein and is determined genetic information, that is, it depends on the sequence of nucleotides in the region of the DNA molecule that encodes a given protein. All properties and functions of proteins depend on the primary structure. The replacement of a single amino acid in the composition of protein molecules or disruption of the order in their arrangement usually entails a change in the function of the protein.

Considering that proteins contain 20 types of amino acids, the number of variants of their combinations in the polypeptide chain is truly limitless, which provides a huge number of types of proteins in living cells. For example, more than 10 thousand different proteins have been found in the human body, and they are all built from the same 20 basic amino acids.

In living cells, protein molecules or their individual sections are not an elongated chain, but are twisted into a spiral resembling an extended spring (this is the so-called a-helix), or folded into a folded layer (p-layer). Such a-helices and p-layers are secondary structure. It occurs as a result of the formation of hydrogen bonds within one polypeptide chain (helical configuration) or between two polypeptide chains (folded layers).

The keratin protein has a completely a-helical configuration. It is a structural protein of hair, nails, claws, beak, feathers and horns; it is part of the outer layer of the skin of vertebrates.

In most proteins, helical and non-helical regions of the polypeptide chain fold into a three-dimensional formation of a spherical shape - a globule (characteristic of globular proteins). A globule of a certain configuration is tertiary structure squirrel. This structure is stabilized by ionic, hydrogen, covalent disulfide bonds (formed between the sulfur atoms that make up cysteine, cystine, and megionine), as well as hydrophobic interactions. The most important in the emergence of tertiary structure are hydrophobic interactions; In this case, the protein coagulates in such a way that its hydrophobic side chains are hidden inside the molecule, that is, they are protected from contact with water, and the hydrophilic side chains, on the contrary, are exposed to the outside.

Many proteins with a particularly complex structure consist of several polypeptide chains (subunits), forming quaternary structure protein molecule. Such a structure is found, for example, in the globular protein hemoglobin. Its molecule consists of four separate polypeptide subunits (protomers) located in the tertiary structure, and a non-protein part - heme.

Only in such a structure is hemoglobin capable of performing its transport function.

Under the influence of various chemical and physical factors (treatment with alcohol, acetone, acids, alkalis, high temperature, radiation, high pressure, etc.), the secondary, tertiary and quaternary structures of the protein change due to the rupture of hydrogen and ionic bonds. The process of disrupting the native (natural) structure of a protein is called denaturation. In this case, a decrease in protein solubility, a change in the shape and size of molecules, a loss of enzymatic activity, etc. are observed. The denaturation process can be complete or partial. In some cases, the transition to normal environmental conditions is accompanied by spontaneous restoration of the natural structure of the protein. This process is called renaturation.

Simple and complex proteins. By chemical composition, proteins are distinguished, simple and complex. Forgive me include proteins consisting only of amino acids, and to complicated- proteins containing protein and non-protein (prosthetic); a prosthetic group can be formed by metal ions, phosphoric acid residue, carbohydrates, lipids, etc. Simple proteins are serum albumin of blood, fibrin, some enzymes (trypsin), etc. All proteolipids and glycoproteins are complex proteins; complex proteins are, for example, immunoglobulins (antibodies), hemoglobin, most enzymes, etc.

Functions of proteins.

1. Structural. Proteins are part of cell membranes and the matrix of cell organelles. The walls of blood vessels, cartilage, tendons, hair, nails, and claws in higher animals consist mainly of proteins.
2. Catalytic (enzymatic). Protein enzymes catalyze all chemical reactions in the body. They ensure the breakdown of nutrients in the digestive tract, carbon fixation during photosynthesis, etc.
3. Transport. Some proteins are capable of attaching and carrying various substances. Blood albumin transport fatty acids, globulins - metal ions and hormones, hemoglobin - oxygen and carbon dioxide. Protein molecules that make up the plasma membrane take part in the transport of substances into the cell.
4. Protective. It is performed by immunoglobulins (antibodies) in the blood, which provide the body's immune defense. Fibrinogen and thrombin are involved in blood clotting and prevent bleeding.
5. Contractile. Due to the sliding of actin and myosin protofibrils relative to each other, muscle contraction occurs, as well as non-muscle intracellular contractions. The movement of cilia and flagella is associated with sliding relative to each other of microtubules, which are of a protein nature.
6. Regulatory. Many hormones are oligopeptides or poor (eg insulin, glucagon [insulin antagonist], adrenocorticotropic hormone, etc.).
7. Receptor. Some proteins built into the cell membrane are able to change their structure under the influence of the external environment. This is how signals are received from the outside and information is transmitted to the cell. An example is phyto-chromium- is a light-sensitive protein that regulates the photoperiodic response of plants, and opsin - component rhodopsin, pigment found in the cells of the retina.
8. Energy. Proteins can serve as a source of energy in the cell (after hydrolysis). Usually, proteins are spent for energy needs in extreme cases, when the reserves of carbohydrates and fats are depleted.

Enzymes (enzymes). These are specific proteins that are present in all living organisms and play the role of biological catalysts.

Chemical reactions in a living cell occur at a certain temperature, normal pressure and corresponding acidity of the environment. Under such conditions, the reactions of synthesis or decomposition of substances would proceed very slowly in the cell if they were not exposed to the effects of enzymes. Enzymes speed up the reaction without changing its overall result by reducing activation energy, that is, when they are present, much less energy is required to impart reactivity to the molecules that react, or the reaction proceeds along a different path with a lower energy barrier.

All processes in a living organism are directly or indirectly carried out with the participation of enzymes. For example, under their action, the constituent components of food (proteins, carbohydrates, lipids, etc.) are split into simpler compounds, and then new macromolecules characteristic of this type are synthesized from them. Therefore, disturbances in the formation and activity of enzymes often lead to serious illnesses.

In terms of spatial organization, enzymes consist of several sex and peptide chains and usually have a quaternary structure. In addition, enzymes can include non-proteinaceous structures. The protein part wears name apoenzyme, and non-protein - cofactor(if these are cations or anions of inorganic substances, for example, Zn 2-Mn 2+, etc.) or coenzyme (coenzyme)(if it is a low molecular weight organic substance).

Vitamins are precursors or constituents of many coenzymes. So, pantothenic acid is a component of coenzyme A, nicotinic acid (vitamin PP) is a precursor of NAD and NADP, etc.

Enzymatic catalysis obeys the same laws as non-enzymatic catalysis in the chemical industry, however, unlike it, it is characterized by an unusually a high degree of specificity(the enzyme catalyzes only one reaction or acts on only one type of bond). This ensures the fine regulation of all vital processes (respiration, digestion, photosynthesis, etc.) that take place in the cell and the body. For example, the urease enzyme catalyzes the cleavage of only one substance - urea (H 2 N-CO-NH 2 + H 2 O -> - »2NH 3 + CO 2), without exerting a catalytic effect on structurally related compounds.

To understand the mechanism of action of enzymes with high specificity, very the theory of the active center is important. According to her, v molecule of each enzyme there is one an area or more in which catalysis occurs due to close (at many points) contact between the molecules of the enzyme and a specific substance (substrate). The active center is either a functional group (for example, the OH-group of serine), or a separate amino acid. Usually, for catalytic action, a combination of several (on average from 3 to 12) amino acid residues arranged in a certain order is required. The active center is also formed by enzyme-bound metal ions, vitamins and other compounds of a non-protein nature - coenzymes, or cofactors. Moreover, the shape and chemical structure of the active center are such that with it can bind only certain substrates due to their ideal correspondence (complementarity or complementarity) to each other. The role of the remaining amino acid residues in the large molecule of the enzyme is to provide its molecule with the corresponding globular form, which is necessary for the effective operation of the active center. In addition, a strong electric field arises around the large enzyme molecule. In such a field, it becomes possible to orient the substrate molecules and acquire an asymmetric shape. This leads to a weakening of chemical bonds, and the catalyzed reaction occurs with a lower initial energy consumption, and, therefore, at a much higher rate. For example, one molecule of the enzyme catalase can break down more than 5 million molecules of hydrogen peroxide (H2O2) in 1 minute, which occurs during the oxidation of various compounds in the body.

In some enzymes, in the presence of a substrate, the configuration of the active site undergoes changes, i.e., the enzyme orientates its functional groups in such a way as to provide the greatest catalytic activity.

At the final stage of the chemical reaction, the enzyme-substrate complex is separated to form the final products and free enzyme. The active center released in this process can accept new substrate molecules.

Enzymatic reaction rate depends on many factors: the nature and concentration of the enzyme and substrate, temperature, pressure, acidity of the medium, the presence of inhibitors, etc. For example, at temperatures close to zero, the rate of biochemical reactions slows down to a minimum. This property is widely used in various sectors of the national economy, especially in agriculture and medicine. In particular, conservation various organs (kidneys, heart, spleen, liver) before their transplantation to the patient occurs with cooling in order to reduce the intensity of biochemical reactions and prolong the life of organs. Rapid freezing of food products prevents the growth and reproduction of microorganisms (bacteria, fungi, etc.), and also inactivates their digestive enzymes, so that they are no longer able to cause food decomposition.

A source : ON. Lemeza L. V. Kamlyuk N. D. Lisov "A guide to biology for university applicants"

Squirrel rich in vitamins and minerals such as: vitamin B2 - 11.7%, vitamin PP - 20%, potassium - 12.2%, phosphorus - 21.5%, iron - 26.1%, selenium - 16.9%

Why Protein is useful

• Vitamin B2 participates in redox reactions, enhances the color sensitivity of the visual analyzer and dark adaptation. Insufficient intake of vitamin B2 is accompanied by a violation of the condition of the skin, mucous membranes, impaired light and twilight vision.
• Vitamin PP participates in redox reactions of energy metabolism. Insufficient vitamin intake is accompanied by disruption of the normal state of the skin, gastrointestinal tract and nervous system.
• Potassium is the main intracellular ion that takes part in the regulation of water, acid and electrolyte balance, participates in the processes of nerve impulses, pressure regulation.
• Phosphorus takes part in many physiological processes, including energy metabolism, regulates acid-base balance, is a part of phospholipids, nucleotides and nucleic acids, is necessary for the mineralization of bones and teeth. Deficiency leads to anorexia, anemia, rickets.
• Iron is a part of proteins of various functions, including enzymes. Participates in the transport of electrons, oxygen, ensures the course of redox reactions and activation of peroxidation. Insufficient consumption leads to hypochromic anemia, myoglobin-deficient atony of skeletal muscles, increased fatigue, myocardiopathy, atrophic gastritis.
• Selenium- an essential element of the antioxidant defense system of the human body, has an immunomodulatory effect, participates in the regulation of the action of thyroid hormones. Deficiency leads to Kashin-Beck disease (osteoarthritis with multiple deformities of the joints, spine and extremities), Keshan disease (endemic myocardiopathy), hereditary thrombastenia.

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Protein

Lecture 2

Protein functions

Chemical composition proteins

Characterization of proteinogenic amino acids

Protein structure

Protein classification

Protein properties and research methods

Proteins are structural components of organs and tissues, exhibit enzymatic activity (enzymes), are involved in the regulation of metabolism. Transport proteins that carry protons and electrons across membranes provide bioenergetics: absorption of light, respiration, production of ATP. Spare proteins (typical mainly for plants) are deposited in seeds and are used to feed seedlings during germination. By burning ATP, proteins provide mechanical activity, are involved in the movement of the cytoplasm and other cellular organelles. Important protective function of proteins: hydrolytic enzymes of lysosomes and vacuoles break down harmful substances that have entered the cell; glycoproteins are involved in plant protection against pathogens; proteins perform cryoprotective and antifreeze functions. One protein can perform two or more functions (some membrane proteins can perform structural and enzymatic functions).

The amazing variety of functions of proteins and their high prevalence are reflected in their name - proteins(from Greek " protos» - primary, essential). As a rule, the protein content in plants is lower than in animals: in vegetative organs, the amount of protein is usually 5-15% of dry weight. So, the leaves of timothy contain 7% protein, and the leaves of clover and vetch - 15%. More protein in seeds: in cereals, on average, 10-20%, in legumes and oilseeds - 25-35%. Soybean seeds are the richest in protein - up to 40%, and sometimes even higher.

In plant cells, proteins are usually associated with carbohydrates, lipids and other compounds, as well as with membranes, so they are difficult to extract and obtain pure preparations, especially from vegetative organs... In this regard, seed proteins are better studied in plants, where there are more of them and from where they are more easily extracted.

Proteins - organic compounds having the following elemental composition: carbon 51-55 %; oxygen 21-23 %; hydrogen 6,6-7,3 %; nitrogen 15-18 %; sulfur 0.3-2.4%. Some proteins also include phosphorus (0,2-2 %), iron and other elements. A characteristic indicator of the elemental composition of proteins in all organisms is the presence nitrogen, on average it is taken equal to 16 % ... The relative constancy of this indicator makes it possible to use it for the quantitative determination of protein: the relative value of the protein nitrogen content, in percent, is multiplied by the conversion factor - 6,25 (100: 16 = 6.25). By the chemical nature of proteins - heteropolymers built from leftovers amino acids. Amino Acids (AA) organic compounds are called, in the molecules of which one or more hydrogen atoms are replaced amino groups(- NH 2).

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 with 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 have developed historically), the structural formula, as well as the 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 is 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, and sulfur for sulfur. yellow(for hydrogen atoms not shown in the figure, white is recommended; in this case, the entire structure is depicted against 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 relatively polymer chains 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, therefore the American biophysicist Jane Richardson proposed to depict 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, assumes 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 certain type of tertiary structure 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 the 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 with an increase in temperature, the destruction of secondary and tertiary structures occurs without damage to 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, therefore 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 make up 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 about 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 side 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), and 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 conditions of the course 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 dextrorotatory 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 exclusively 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 inhaling large amounts carbon monoxide) 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 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 mammals, accounting 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 in the process of growth and development of the organism are not renewed or 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 protein resilin has the greatest elasticity, 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). Mammals contain up to 30 different types of 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 "plastically" 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 areas 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 - groups of the amino acid. 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, a bacterium), after which the body begins to produce the desired protein. In this way, significant amounts of hard-to-obtain proteins and peptides, as well as their analogs, 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 the body with building materials for the synthesis of new protein molecules and, in addition, provide 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 animal feed, 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 respond 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 are 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 of 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. like the left hand to the right. One configuration is called left-handed, or levogyrate (L), and the other, right-handed, or dextrorotatory (D), since two such isomers differ in the direction of rotation of the plane of polarized light. Proteins contain only L-amino acids (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 rolled up 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, with the correct stacking of chains, cavities of a certain shape appear, 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; this changes the configuration of the electron cloud of the molecule 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 types plants and animals, and therefore bear the same name, 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 any two biological species, the less differences are 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, such as 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 in the form of 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 an enzyme is often required for this attachment.

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 the 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 learned how 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 to amino acids, the latter are absorbed, and already proteins characteristic of this organism... None of the 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 protein mass 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 required to maintain nitrogen balance, then there is apparently 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 provide 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. The body of the animal itself is able to synthesize some of the amino acids. 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.