All amino acids are divided into 2 groups. Twenty amino acids are required for protein synthesis. Hair proteins keratins belong to the group

Amino acids are carboxylic acids containing an amino group and a carboxyl group. Natural amino acids are 2-aminocarboxylic acids, or α-amino acids, although there are amino acids such as β-alanine, taurine, γ-aminobutyric acid. The generalized formula of an α-amino acid looks like this:

α-amino acids have four different substituents at carbon 2, that is, all α-amino acids, except glycine, have an asymmetric (chiral) carbon atom and exist in the form of two enantiomers - L- and D-amino acids. Natural amino acids belong to the L-series. D-amino acids are found in bacteria and peptide antibiotics.

All amino acids in aqueous solutions can exist in the form of bipolar ions, and their total charge depends on the pH of the medium. The pH value at which the total charge is zero is called the isoelectric point. At the isoelectric point, an amino acid is a zwitterion, that is, its amine group is protonated, and its carboxyl group is dissociated. In the neutral pH region, most amino acids are zwitterions:

Amino acids do not absorb light in the visible region of the spectrum, aromatic amino acids absorb light in the UV region of the spectrum: tryptophan and tyrosine at 280 nm, phenylalanine at 260 nm.

Amino acids are characterized by some chemical reactions having great importance for laboratory practice: color ninhydrin test for α-amino group, reactions characteristic of sulfhydryl, phenolic and other groups of amino acid radicals, acelation and formation of Schiff bases at amino groups, esterification at carboxyl groups.

Biological role of amino acids:

are structural elements of peptides and proteins, so-called proteinogenic amino acids. Proteins contain 20 amino acids, which are encoded by the genetic code and are included in proteins during translation; some of them can be phosphorylated, acylated or hydroxylated;

can be structural elements of other natural compounds - coenzymes, bile acids, antibiotics;

are signaling molecules. Some of the amino acids are neurotransmitters or precursors of neurotransmitters, hormones and histohormones;

are the most important metabolites, for example, some amino acids are precursors of plant alkaloids, or serve as nitrogen donors, or are vital components of nutrition.

The classification of proteinogenic amino acids is based on the structure and polarity of the side chains:

1. Aliphatic amino acids:

glycine, gly,G,Gly

alanine, ala, A, Ala

valine, shaft,V,Val*

leucine, lei,L,Leu*

isoleucine, silt, I,Ile*

These amino acids do not contain heteroatoms or cyclic groups in the side chain and are characterized by a distinctly low polarity.

cysteine, cis,Cys

methionine, meth,M,Met*

3. Aromatic amino acids:

phenylalanine, hair dryer,F,Phe*

tyrosine, shooting gallery,Y,Tyr

tryptophan, three,W,Trp*

histidine, gis,H,His

Aromatic amino acids contain mesomeric resonance-stabilized cycles. In this group, only the amino acid phenylalanine exhibits low polarity, tyrosine and tryptophan are characterized by noticeable polarity, and histidine even has high polarity. Histidine can also be classified as a basic amino acid.

4. Neutral amino acids:

serine, gray,S,Ser

threonine, tre,T,Thr*

asparagine, asn, N,Asn

glutamine, gln, Q,Gln

Neutral amino acids contain hydroxyl or carboxamide groups. Although the amide groups are nonionic, the asparagine and glutamine molecules are highly polar.

5. Acidic amino acids:

aspartic acid (aspartate), asp,D,Asp

glutamic acid (glutamate), glu, E,Glu

The carboxyl groups of the side chains of acidic amino acids are fully ionized over the entire range of physiological pH values.

6. Essential amino acids:

lysine, l from, K,Lys*

arginine, arg,R,Arg

The side chains of the main amino acids are completely protonated in the neutral pH region. A highly basic and very polar amino acid is arginine, which contains a guanidine group.

7. Imino acid:

proline, about,P,Pro

The side chain of proline consists of a five-membered ring containing an α-carbon atom and an α-amino group. Therefore, proline, strictly speaking, is not an amino acid, but an imino acid. The nitrogen atom in the ring is a weak base and is not protonated at physiological pH values. Due to its cyclic structure, proline causes bends in the polypeptide chain, which is very important for the structure of collagen.

Some of the listed amino acids cannot be synthesized in the human body and must be supplied with food. These essential amino acids are marked with asterisks.

As stated above, proteinogenic amino acids are the precursors of several valuable biologically active molecules.

Two biogenic amines, β-alanine and cysteamine, are part of coenzyme A (coenzymes are derivatives of water-soluble vitamins that form the active center of complex enzymes). β-Alanine is formed by decarboxylation of aspartic acid, and cysteamine by decarboxylation of cysteine:

β-alanine
cysteamine

The glutamic acid residue is part of another coenzyme - tetrahydrofolic acid, a derivative of vitamin B c.

Other biologically valuable molecules are conjugates of bile acids with the amino acid glycine. These conjugates are stronger acids than the base ones, are formed in the liver and are present in bile in the form of salts.

glycocholic acid

Proteinogenic amino acids are precursors of some antibiotics - biologically active substances synthesized by microorganisms and suppress the proliferation of bacteria, viruses and cells. The best known of them are penicillins and cephalosporins, which make up the group of β-lactam antibiotics and are produced by molds of the genus Penicillium. They are characterized by the presence in their structure of a reactive β-lactam ring, with the help of which they inhibit the synthesis of cell walls of gram-negative microorganisms.

general formula of penicillins

From amino acids, biogenic amines are obtained by decarboxylation - neurotransmitters, hormones and histohormones.

The amino acids glycine and glutamate are themselves neurotransmitters in the central nervous system.

dopamine (neurotransmitter) norepinephrine (neurotransmitter)

adrenaline (hormone) histamine (transmitter and histohormone)

serotonin (neurotransmitter and histohormone) GABA (neurotransmitter)

thyroxine (hormone)

A derivative of the amino acid tryptophan is the most famous naturally occurring auxin, indolylacetic acid. Auxins are plant growth regulators, they stimulate the differentiation of growing tissues, the growth of the cambium, roots, accelerate the growth of fruits and the abscission of old leaves; their antagonists are abscisic acid.

indolylacetic acid

Derivatives of amino acids are also alkaloids - natural nitrogen-containing compounds of a basic nature, formed in plants. These compounds are extremely active physiological compounds widely used in medicine. Examples of alkaloids include the phenylalanine derivative papaverine, an isoquinoline alkaloid of poppy somniferous (an antispasmodic), and the tryptophan derivative physostigmine, an indole alkaloid from Calabar beans (an anticholinesterase drug):

papaverine physostigmine

Amino acids are extremely popular objects of biotechnology. There are many options for the chemical synthesis of amino acids, but the result is racemates of amino acids. Since for Food Industry and only L-isomers of amino acids are suitable for medicine; racemic mixtures must be separated into enantiomers, which poses a serious problem. Therefore, the biotechnological approach is more popular: enzymatic synthesis using immobilized enzymes and microbiological synthesis using whole microbial cells. In both last cases, pure L-isomers are obtained.

Amino acids are used as food additives and feed components. Glutamic acid enhances the taste of meat, valine and leucine improve the taste of baked goods, glycine and cysteine are used as antioxidants in canning. D-tryptophan can be a sugar substitute, as it is many times sweeter. Lysine is added to the feed of farm animals, since most plant proteins contain small amounts of the essential amino acid lysine.

Amino acids are widely used in medical practice. These are amino acids such as methionine, histidine, glutamic and aspartic acids, glycine, cysteine, valine.

In the last decade, amino acids began to be added to cosmetic products for skin and hair care.

Chemically modified amino acids are also widely used in industry as surfactants in the synthesis of polymers, in the production of detergents, emulsifiers, and fuel additives.

PROTEINS

Proteins are high-molecular substances consisting of amino acids connected by peptide bonds.

It is proteins that are the product of genetic information transmitted from generation to generation and carry out all life processes in the cell.

Functions of proteins:

Catalytic function. The most numerous group of proteins consists of enzymes - proteins with catalytic activity that accelerate chemical reactions. Examples of enzymes are pepsin, alcohol dehydrogenase, glutamine synthetase.

Structure-forming function. Structural proteins are responsible for maintaining the shape and stability of cells and tissues, these include keratins, collagen, fibroin.

Transport function. Transport proteins carry molecules or ions from one organ to another or across membranes within a cell, for example, hemoglobin, serum albumin, ion channels.

Protective function. Proteins of the homeostasis system protect the body from pathogens, foreign information, blood loss - immunoglobulins, fibrinogen, thrombin.

Regulatory function. Proteins perform the functions of signaling substances - some hormones, histohormones and neurotransmitters, are receptors for signaling substances of any structure, and ensure further signal transmission in the biochemical signal chains of the cell. Examples include the growth hormone somatotropin, the hormone insulin, H- and M-cholinergic receptors.

Motor function. With the help of proteins, the processes of contraction and other biological movement are carried out. Examples include tubulin, actin, and myosin.

Spare function. Plants contain storage proteins that are valuable nutrients, in animal organisms muscle proteins serve as reserve nutrients that are mobilized when absolutely necessary.

Proteins are characterized by the presence of several levels of structural organization.

Primary structure A protein is a sequence of amino acid residues in a polypeptide chain. A peptide bond is a carboxamide bond between the α-carboxyl group of one amino acid and the α-amino group of another amino acid.

alanylphenylalanylcysteylproline

The peptide bond has several features:

a) it is resonantly stabilized and therefore is located practically in the same plane - planar; rotation around the C-N bond requires a lot of energy and is difficult;

b) the -CO-NH- bond has a special character, it is smaller than a regular one, but larger than a double one, that is, there is keto-enol tautomerism:

c) substituents in relation to the peptide bond are in trance-position;

d) the peptide backbone is surrounded by side chains of various nature, interacting with the surrounding solvent molecules, free carboxyl and amino groups are ionized, forming cationic and anionic centers of the protein molecule. Depending on their ratio, the protein molecule receives a total positive or negative charge, and is also characterized by one or another pH value of the environment when it reaches the isoelectric point of the protein. Radicals form salt, ether, and disulfide bridges inside the protein molecule, and also determine the range of reactions characteristic of proteins.

Currently, it is agreed that polymers consisting of 100 or more amino acid residues are considered proteins, polypeptides are polymers consisting of 50-100 amino acid residues, and low-molecular peptides are polymers consisting of less than 50 amino acid residues.

Some low molecular weight peptides play an independent biological role. Examples of some of these peptides:

Glutathione - γ-gluc-cis-gly - is one of the most widespread intracellular peptides, takes part in redox processes in cells and the transfer of amino acids across biological membranes.

Carnosine - β-ala-gis - a peptide contained in animal muscles, eliminates lipid peroxide breakdown products, accelerates the breakdown of carbohydrates in muscles and is involved in energy metabolism in muscles in the form of phosphate.

Vasopressin is a hormone of the posterior lobe of the pituitary gland, involved in the regulation of water metabolism in the body:

Phalloidin is a poisonous polypeptide of the fly agaric, in negligible concentrations it causes the death of the body due to the release of enzymes and potassium ions from the cells:

Gramicidin is an antibiotic that acts on many gram-positive bacteria, changes the permeability of biological membranes to low molecular weight compounds and causes cell death:

Meth-enkephalin - tyr-gly-gly-phen-met - a peptide synthesized in neurons and reducing pain.

Protein secondary structure is a spatial structure formed as a result of interactions between functional groups of the peptide backbone.

The peptide chain contains many CO and NH groups of peptide bonds, each of which is potentially capable of participating in the formation of hydrogen bonds. There are two main types of structures that allow this to happen: an α-helix, in which the chain is coiled up like a telephone cord, and a folded β-structure, in which elongated sections of one or more chains are laid side by side. Both of these structures are very stable.

The α-helix is characterized by extremely dense packing of the twisted polypeptide chain; for each turn of the right-handed helix there are 3.6 amino acid residues, the radicals of which are always directed outward and slightly backward, that is, to the beginning of the polypeptide chain.

Main characteristics of the α-helix:

The α-helix is stabilized by hydrogen bonds between the hydrogen atom at the nitrogen of the peptide group and the carbonyl oxygen of the residue located four positions along the chain;

All peptide groups participate in the formation of a hydrogen bond, this ensures maximum stability of the α-helix;

all nitrogen and oxygen atoms of peptide groups are involved in the formation of hydrogen bonds, which significantly reduces the hydrophilicity of α -spiral regions and increases their hydrophobicity;

The α-helix is formed spontaneously and is the most stable conformation of the polypeptide chain, corresponding to the minimum free energy;

In a polypeptide chain of L-amino acids, the right-handed helix, usually found in proteins, is much more stable than the left-handed one.

The possibility of α-helix formation is determined by the primary structure of the protein. Some amino acids prevent the peptide backbone from twisting. For example, the adjacent carboxyl groups of glutamate and aspartate mutually repel each other, which prevents the formation of hydrogen bonds in the α-helix. For the same reason, chain helicalization is difficult in places where positively charged lysine and arginine residues are located close to each other. However, proline plays the largest role in α-helix disruption. Firstly, in proline the nitrogen atom is part of a rigid ring, which prevents rotation around the N-C bond, and secondly, proline does not form a hydrogen bond due to the absence of hydrogen at the nitrogen atom.

β-sheet is a layered structure formed by hydrogen bonds between linearly arranged peptide fragments. Both chains can be independent or belong to the same polypeptide molecule. If the chains are oriented in the same direction, then such a β-structure is called parallel. In the case of opposite chain directions, that is, when the N-terminus of one chain coincides with the C-terminus of another chain, the β-structure is called antiparallel. An antiparallel β-sheet with nearly linear hydrogen bridges is energetically more preferable.

parallel β-sheet antiparallel β-sheet

Unlike the α-helix, which is saturated with hydrogen bonds, each section of the β-sheet chain is open to the formation of additional hydrogen bonds. The side radicals of amino acids are oriented almost perpendicular to the plane of the sheet, alternately up and down.

In those areas where the peptide chain bends quite sharply, there is often a β-loop. This is a short fragment in which 4 amino acid residues are bent by 180° and are stabilized by one hydrogen bridge between the first and fourth residues. Large amino acid radicals interfere with the formation of the β-loop, so it most often includes the smallest amino acid, glycine.

Protein suprasecondary structure– this is some specific order of alternation of secondary structures. A domain is understood as a separate part of a protein molecule that has a certain degree of structural and functional autonomy. Domains are now considered fundamental elements of the structure of protein molecules, and the relationship and nature of the arrangement of α-helices and β-sheets provides more for understanding the evolution of protein molecules and phylogenetic relationships than a comparison of primary structures. The main task of evolution is to design more and more new proteins. There is an infinitesimal chance of accidentally synthesizing an amino acid sequence that would satisfy the packaging conditions and ensure the fulfillment of functional tasks. Therefore, it is common to find proteins with different functions but so similar in structure that they appear to have had a common ancestor or to have evolved from each other. It seems that evolution, when faced with the need to solve a certain problem, prefers not to design proteins for this purpose from the beginning, but to adapt already well-established structures for this purpose, adapting them for new purposes.

Some examples of frequently repeated suprasecondary structures:

αα’ – proteins containing only α-helices (myoglobin, hemoglobin);

ββ’ – proteins containing only β-structures (immunoglobulins, superoxide dismutase);

βαβ’ – β-barrel structure, each β-layer is located inside the barrel and is connected to an α-helix located on the surface of the molecule (triose phosphoisomerase, lactate dehydrogenase);

“zinc finger” - a protein fragment consisting of 20 amino acid residues, the zinc atom is linked to two cysteine residues and two histidine residues, resulting in the formation of a “finger” of approximately 12 amino acid residues, which can bind to the regulatory regions of the DNA molecule;

“leucine zipper” - interacting proteins have an α-helical region containing at least 4 leucine residues, they are located 6 amino acids apart, that is, they are on the surface of every second turn and can form hydrophobic bonds with leucine residues of another protein . With the help of leucine zippers, for example, molecules of strongly basic histone proteins can be complexed, overcoming a positive charge.

Protein tertiary structure- this is the spatial arrangement of the protein molecule, stabilized by the bonds between the side radicals of amino acids.

Types of bonds that stabilize the tertiary structure of a protein:

electrostatic hydrogen hydrophobic disulfide

interaction communication interaction communication

Depending on the folding of the tertiary structure, proteins can be classified into two main types - fibrillar and globular.

Fibrillar proteins are long, thread-like molecules insoluble in water, the polypeptide chains of which are elongated along one axis. These are mainly structural and contractile proteins. Some examples of the most common fibrillar proteins:

α-Keratins. Synthesized by epidermal cells. They account for almost all the dry weight of hair, fur, feathers, horns, nails, claws, quills, scales, hooves and turtle shell, as well as a significant portion of the weight of the outer layer of skin. This is a whole family of proteins; they are similar in amino acid composition, contain many cysteine residues and have the same spatial arrangement of polypeptide chains. In hair cells, the polypeptide chains of keratin are first organized into fibers, from which structures are then formed like a rope or twisted cable, eventually filling the entire space of the cell. The hair cells become flattened and finally die, and the cell walls form a tubular sheath called the cuticle around each hair. In α-keratin, the polypeptide chains have the shape of an α-helix, twisted around one another into a three-core cable with the formation of cross-disulfide bonds. The N-terminal residues are located on one side (parallel). Keratins are insoluble in water due to the predominance of amino acids in their composition with non-polar side radicals that face the aqueous phase. During perm, the following processes occur: first, disulfide bridges are destroyed by reduction with thiols, and then, when the hair is given the required shape, it is dried by heating, while due to oxidation with atmospheric oxygen, new disulfide bridges are formed, which retain the shape of the hairstyle.

β-Keratins. These include silk and spider web fibroin. They are antiparallel β-pleated layers with a predominance of glycine, alanine and serine in the composition.

Collagen. The most common protein in higher animals and the main fibrillar protein of connective tissues. Collagen is synthesized in fibroblasts and chondrocytes - specialized connective tissue cells, from which it is then expelled. Collagen fibers are found in skin, tendons, cartilage and bones. They do not stretch, are stronger than steel wire, and collagen fibrils are characterized by transverse striations. When boiled in water, fibrous, insoluble and indigestible collagen is converted into gelatin by hydrolysis of some of the covalent bonds. Collagen contains 35% glycine, 11% alanine, 21% proline and 4-hydroxyproline (an amino acid unique to collagen and elastin). This composition determines the relatively low nutritional value of gelatin as a food protein. Collagen fibrils are composed of repeating polypeptide subunits called tropocollagen. These subunits are arranged along the fibril in the form of parallel bundles in a head-to-tail fashion. The displacement of the heads gives the characteristic transverse striations. The voids in this structure, if necessary, can serve as a site for the deposition of crystals of hydroxyapatite Ca 5 (OH)(PO 4) 3, which plays an important role in bone mineralization.

Tropocollagen subunits consist of three polypeptide chains tightly coiled into a three-strand rope, distinct from α- and β-keratins. In some collagens, all three chains have the same amino acid sequence, while in others only two chains are identical, and the third is different. The polypeptide chain of tropocollagen forms a left-handed helix, with only three amino acid residues per turn due to the chain bends caused by proline and hydroxyproline. The three chains are connected to each other, in addition to hydrogen bonds, by a covalent-type bond formed between two lysine residues located in adjacent chains:

As we get older, everything is formed in and between the tropocollagen subunits larger number cross-links, which makes collagen fibrils more rigid and brittle, and this changes the mechanical properties of cartilage and tendons, makes bones more brittle and reduces the transparency of the cornea.

Elastin. Contained in the yellow elastic tissue of ligaments and the elastic layer of connective tissue in the walls of large arteries. The main subunit of elastin fibrils is tropoelastin. Elastin is rich in glycine and alanine, contains a lot of lysine and little proline. The spiral sections of elastin stretch when tension is applied, but return to their original length when the load is removed. The lysine residues of four different chains form covalent bonds with each other and allow elastin to stretch reversibly in all directions.

Globular proteins are proteins whose polypeptide chain is folded into a compact globule and are capable of performing a wide variety of functions.

It is most convenient to consider the tertiary structure of globular proteins using the example of myoglobin. Myoglobin is a relatively small oxygen-binding protein found in muscle cells. It stores bound oxygen and promotes its transfer to mitochondria. The myoglobin molecule contains one polypeptide chain and one hemogroup (heme) - a complex of protoporphyrin with iron. Main properties of myoglobin:

a) the myoglobin molecule is so compact that only 4 water molecules can fit inside it;

b) all polar amino acid residues, with the exception of two, are located on the outer surface of the molecule, and all of them are in a hydrated state;

c) most of the hydrophobic amino acid residues are located inside the myoglobin molecule and, thus, are protected from contact with water;

d) each of the four proline residues in the myoglobin molecule is located at the bend site of the polypeptide chain; serine, threonine and asparagine residues are located at other bend sites, since such amino acids prevent the formation of an α-helix if they are located next to each other;

e) a flat heme group lies in a cavity (pocket) near the surface of the molecule, the iron atom has two coordination bonds directed perpendicular to the heme plane, one of them is connected to histidine residue 93, and the other serves to bind an oxygen molecule.

Starting from the tertiary structure, the protein becomes capable of performing its inherent biological functions. The basis of the functioning of proteins is that when a tertiary structure is laid on the surface of the protein, areas are formed that can attach other molecules, called ligands. The high specificity of the interaction of the protein with the ligand is ensured by the complementarity of the structure of the active center to the structure of the ligand. Complementarity is the spatial and chemical correspondence of interacting surfaces. For most proteins, the tertiary structure is the maximum level of folding.

Quaternary protein structure– characteristic of proteins consisting of two or more polypeptide chains connected to each other exclusively by non-covalent bonds, mainly electrostatic and hydrogen. Most often, proteins contain two or four subunits; more than four subunits usually contain regulatory proteins.

Proteins that have a quaternary structure are often called oligomeric. There are homomeric and heteromeric proteins. Homomeric proteins include proteins in which all subunits have the same structure, for example, the enzyme catalase consists of four absolutely identical subunits. Heteromeric proteins have different subunits; for example, the enzyme RNA polymerase consists of five structurally different subunits that perform different functions.

The interaction of one subunit with a specific ligand causes conformational changes in the entire oligomeric protein and changes the affinity of other subunits for ligands; this property underlies the ability of oligomeric proteins to undergo allosteric regulation.

The quaternary structure of a protein can be considered using the example of hemoglobin. Contains four polypeptide chains and four heme prosthetic groups, in which the iron atoms are in the ferrous form Fe 2+. The protein part of the molecule, globin, consists of two α-chains and two β-chains, containing up to 70% α-helices. Each of the four chains has a characteristic tertiary structure, and one hemogroup is associated with each chain. The hemes of different chains are located relatively far from each other and have different inclination angles. Few direct contacts are formed between two α-chains and two β-chains, while numerous contacts of the α 1 β 1 and α 2 β 2 type formed by hydrophobic radicals arise between the α and β chains. Between α 1 β 1 and α 2 β 2 a channel remains.

Unlike myoglobin, hemoglobin is characterized by a significantly lower affinity for oxygen, which allows it, at the low partial pressures of oxygen existing in the tissues, to give them a significant part of the bound oxygen. Oxygen is more easily bound by hemoglobin iron at higher pH values and low CO 2 concentrations characteristic of the alveoli of the lungs; the release of oxygen from hemoglobin is favored by lower pH values and high concentrations of CO 2 characteristic of tissues.

In addition to oxygen, hemoglobin carries hydrogen ions, which bind to histidine residues in the chains. Hemoglobin also carries carbon dioxide, which attaches to the terminal amino group of each of the four polypeptide chains, resulting in the formation of carbaminohemoglobin:

The substance 2,3-diphosphoglycerate (DPG) is present in erythrocytes in fairly high concentrations; its content increases when rising to high altitudes and during hypoxia, facilitating the release of oxygen from hemoglobin in the tissues. DPG is located in the channel between α 1 β 1 and α 2 β 2, interacting with positively contaminated groups of β-chains. When hemoglobin binds oxygen, DPG is forced out of the cavity. The red blood cells of some birds contain not DPG, but inositol hexa-phosphate, which further reduces the affinity of hemoglobin for oxygen.

2,3-Diphosphoglycerate (DPG)

HbA is normal adult hemoglobin, HbF is fetal hemoglobin, has a greater affinity for O 2, HbS is hemoglobin in sickle cell anemia. Sickle cell anemia is a serious inherited disease caused by a genetic abnormality of hemoglobin. In the blood of sick people there is an unusually large number of thin sickle-shaped red blood cells, which, firstly, easily rupture, and secondly, clog the blood capillaries. At the molecular level, hemoglobin S differs from hemoglobin A by one amino acid residue at position 6 of the β-chains, where valine is found instead of a glutamic acid residue. Thus, hemoglobin S contains two less negative charges; the appearance of valine leads to the appearance of a “sticky” hydrophobic contact on the surface of the molecule; as a result, during deoxygenation, deoxyhemoglobin S molecules stick together and form insoluble, abnormally long thread-like aggregates, leading to deformation of red blood cells.

There is no reason to think that there is independent genetic control over the formation of levels of protein structural organization above the primary, since the primary structure determines the secondary, tertiary, and quaternary (if any). The native conformation of a protein is the thermodynamically most stable structure under given conditions.

Proteins form the material basis of the chemical activity of the cell. The functions of proteins in nature are universal. Name proteins, the most accepted term in Russian literature corresponds to the term proteins(from Greek proteios- first). To date, great strides have been made in establishing the relationship between the structure and functions of proteins, the mechanism of their participation in the most important processes of the body's life, and in understanding the molecular basis of the pathogenesis of many diseases.

Depending on their molecular weight, peptides and proteins are distinguished. Peptides have a lower molecular weight than proteins. Peptides are more likely to have a regulatory function (hormones, enzyme inhibitors and activators, ion transporters across membranes, antibiotics, toxins, etc.).

12.1. α -Amino acids

12.1.1. Classification

Peptides and proteins are built from α-amino acid residues. Total number There are more than 100 naturally occurring amino acids, but some of them are found only in a certain community of organisms; the 20 most important α-amino acids are constantly found in all proteins (Scheme 12.1).

α-Amino acids are heterofunctional compounds whose molecules contain both an amino group and a carboxyl group at the same carbon atom.

Scheme 12.1.The most important α-amino acids*

* Abbreviations are used only to write amino acid residues in peptide and protein molecules. ** Essential amino acids.

The names of α-amino acids can be constructed using substitutive nomenclature, but their trivial names are more often used.

Trivial names for α-amino acids are usually associated with sources of isolation. Serine is part of silk fibroin (from lat. serieus- silky); Tyrosine was first isolated from cheese (from the Greek. tyros- cheese); glutamine - from cereal gluten (from German. Gluten- glue); aspartic acid - from asparagus sprouts (from lat. asparagus- asparagus).

Many α-amino acids are synthesized in the body. Some amino acids necessary for protein synthesis are not produced in the body and must come from outside. These amino acids are called irreplaceable(see diagram 12.1).

Essential α-amino acids include:

valine isoleucine methionine tryptophan

leucine lysine threonine phenylalanine

α-Amino acids are classified in several ways depending on the characteristic that serves as the basis for their division into groups.

One of the classification features is the chemical nature of the radical R. Based on this feature, amino acids are divided into aliphatic, aromatic and heterocyclic (see diagram 12.1).

Aliphaticα -amino acids. This is the largest group. Within it, amino acids are divided using additional classification features.

Depending on the number of carboxyl groups and amino groups in the molecule, the following are distinguished:

Neutral amino acids - one NH group each 2 and COOH;

Basic amino acids - two NH groups 2 and one group

COOH;

Acidic amino acids - one NH 2 group and two COOH groups.

It can be noted that in the group of aliphatic neutral amino acids the number of carbon atoms in the chain does not exceed six. At the same time, there are no amino acids with four carbon atoms in the chain, and amino acids with five and six carbon atoms have only a branched structure (valine, leucine, isoleucine).

An aliphatic radical may contain “additional” functional groups:

Hydroxyl - serine, threonine;

Carboxylic - aspartic and glutamic acids;

Thiol - cysteine;

Amide - asparagine, glutamine.

Aromaticα -amino acids. This group includes phenylalanine and tyrosine, constructed in such a way that the benzene rings in them are separated from the common α-amino acid fragment by the methylene group -CH 2-.

Heterocyclic α -amino acids. Histidine and tryptophan belonging to this group contain heterocycles - imidazole and indole, respectively. The structure and properties of these heterocycles are discussed below (see 13.3.1; 13.3.2). General principle The construction of heterocyclic amino acids is the same as that of aromatic ones.

Heterocyclic and aromatic α-amino acids can be considered as β-substituted derivatives of alanine.

The amino acid also belongs to gerocyclic proline, in which the secondary amino group is included in the pyrrolidine

In the chemistry of α-amino acids, much attention is paid to the structure and properties of the “side” radicals R, which play an important role in the formation of the structure of proteins and the performance of their biological functions. Of great importance are such characteristics as the polarity of the “side” radicals, the presence of functional groups in the radicals and the ability of these functional groups to ionize.

Depending on the side radical, amino acids with non-polar(hydrophobic) radicals and amino acids c polar(hydrophilic) radicals.

The first group includes amino acids with aliphatic side radicals - alanine, valine, leucine, isoleucine, methionine - and aromatic side radicals - phenylalanine, tryptophan.

The second group includes amino acids that have polar functional groups in their radicals that are capable of ionization (ionogenic) or are unable to transform into an ionic state (nonionic) under body conditions. For example, in tyrosine the hydroxyl group is ionic (phenolic in nature), in serine it is nonionic (alcoholic in nature).

Polar amino acids with ionic groups in radicals under certain conditions can be in an ionic (anionic or cationic) state.

12.1.2. Stereoisomerism

The main type of construction of α-amino acids, i.e., the bond of the same carbon atom with two different functional groups, a radical and a hydrogen atom, in itself predetermines the chirality of the α-carbon atom. The exception is the simplest amino acid glycine H 2 NCH 2 COOH, which has no center of chirality.

The configuration of α-amino acids is determined by the configuration standard - glyceraldehyde. The location of the amino group in the standard Fischer projection formula on the left (similar to the OH group in l-glyceraldehyde) corresponds to the l-configuration, and on the right - to the d-configuration of the chiral carbon atom. By R, In the S-system, the α-carbon atom in all α-amino acids of the l-series has an S-configuration, and in the d-series, an R-configuration (the exception is cysteine, see 7.1.2).

Most α-amino acids contain one asymmetric carbon atom per molecule and exist as two optically active enantiomers and one optically inactive racemate. Almost all natural α-amino acids belong to the l-series.

The amino acids isoleucine, threonine and 4-hydroxyproline contain two chirality centers in the molecule.

Such amino acids can exist as four stereoisomers, representing two pairs of enantiomers, each of which forms a racemate. To build animal proteins, only one of the enantiomers is used.

The stereoisomerism of isoleucine is similar to the previously discussed stereoisomerism of threonine (see 7.1.3). Of the four stereoisomers, proteins contain l-isoleucine with the S configuration of both asymmetric carbon atoms C-α and C-β. The names of another pair of enantiomers that are diastereomers with respect to leucine use the prefix Hello-.

Cleavage of racemates. The source of α-amino acids of the l-series are proteins, which are subjected to hydrolytic cleavage for this purpose. Due to the great need for individual enantiomers (for the synthesis of proteins, medicinal substances, etc.) chemical methods for breaking down synthetic racemic amino acids. Preferred enzymatic method of digestion using enzymes. Currently, chromatography on chiral sorbents is used to separate racemic mixtures.

12.1.3. Acid-base properties

The amphotericity of amino acids is determined by acidic (COOH) and basic (NH 2) functional groups in their molecules. Amino acids form salts with both alkalis and acids.

In the crystalline state, α-amino acids exist as dipolar ions H3N+ - CHR-COO- (commonly used notation

The structure of the amino acid in non-ionized form is for convenience only).

IN aqueous solution amino acids exist as an equilibrium mixture of dipolar ion, cationic and anionic forms.

The equilibrium position depends on the pH of the medium. For all amino acids, cationic forms predominate in strongly acidic (pH 1-2) and anionic forms in strongly alkaline (pH > 11) environments.

The ionic structure determines the series specific properties amino acids: high melting point (above 200? C), solubility in water and insolubility in non-polar organic solvents. The ability of most amino acids to dissolve well in water is an important factor in ensuring their biological functioning; the absorption of amino acids, their transport in the body, etc. are associated with it.

A fully protonated amino acid (cationic form), from the standpoint of Brønsted’s theory, is a dibasic acid,

By donating one proton, such a dibasic acid turns into a weak monobasic acid - a dipolar ion with one acid group NH 3 + . Deprotonation of the dipolar ion leads to the production of the anionic form of the amino acid - the carboxylate ion, which is a Brønsted base. The values characterize

The basic acidic properties of the carboxyl group of amino acids usually range from 1 to 3; values pK a2 characterizing the acidity of the ammonium group - from 9 to 10 (Table 12.1).

Table 12.1.Acid-base properties of the most important α-amino acids

The equilibrium position, i.e., the ratio of different forms of an amino acid, in an aqueous solution at certain pH values significantly depends on the structure of the radical, mainly on the presence of ionic groups in it, playing the role of additional acidic and basic centers.

The pH value at which the concentration of dipolar ions is maximum, and the minimum concentrations of cationic and anionic forms of an amino acid are equal, is calledisoelectric point (p/).

Neutralα -amino acids. These amino acids matterpIslightly lower than 7 (5.5-6.3) due to the greater ability to ionize the carboxyl group under the influence of the -/- effect of the NH 2 group. For example, alanine has an isoelectric point at pH 6.0.

Sourα -amino acids. These amino acids have an additional carboxyl group in the radical and are in a fully protonated form in a strongly acidic environment. Acidic amino acids are tribasic (according to Brøndsted) with three meaningspK a,as can be seen in the example of aspartic acid (p/ 3.0).

For acidic amino acids (aspartic and glutamic), the isoelectric point is at a pH much lower than 7 (see Table 12.1). In the body at physiological pH values (for example, blood pH 7.3-7.5), these acids are in anionic form, since both carboxyl groups are ionized.

Basicα -amino acids. In the case of basic amino acids, the isoelectric points are located in the pH region above 7. In a strongly acidic environment, these compounds are also tribasic acids, the ionization stages of which are illustrated by the example of lysine (p/ 9.8).

In the body, basic amino acids are found in the form of cations, that is, both amino groups are protonated.

In general, no α-amino acid in vivois not at its isoelectric point and does not fall into a state corresponding to the lowest solubility in water. All amino acids in the body are in ionic form.

12.1.4. Analytically important reactions α -amino acids

α-Amino acids, as heterofunctional compounds, enter into reactions characteristic of both the carboxyl and amino groups. Some chemical properties of amino acids are due to the functional groups in the radical. This section discusses reactions that are of practical importance for the identification and analysis of amino acids.

Esterification.When amino acids react with alcohols in the presence of an acid catalyst (for example, hydrogen chloride gas), they produce esters in the form of hydrochlorides. To isolate free esters, the reaction mixture is treated with ammonia gas.

Amino acid esters do not have a dipolar structure, therefore, unlike the parent acids, they dissolve in organic solvents and are volatile. Thus, glycine is a crystalline substance with a high melting point (292°C), and its methyl ester is a liquid with a boiling point of 130°C. Analysis of amino acid esters can be carried out using gas-liquid chromatography.

Reaction with formaldehyde. Of practical importance is the reaction with formaldehyde, which underlies the quantitative determination of amino acids by the method formol titration(Sørensen method).

The amphoteric nature of amino acids does not allow direct titration with alkali for analytical purposes. The interaction of amino acids with formaldehyde produces relatively stable amino alcohols (see 5.3) - N-hydroxymethyl derivatives, the free carboxyl group of which is then titrated with alkali.

Qualitative reactions. A feature of the chemistry of amino acids and proteins is the use of numerous qualitative (color) reactions, which previously formed the basis of chemical analysis. Nowadays, when research is carried out using physicochemical methods, many qualitative reactions continue to be used for the detection of α-amino acids, for example, in chromatographic analysis.

Chelation. With cations of heavy metals, α-amino acids as bifunctional compounds form intra-complex salts, for example, with freshly prepared copper(11) hydroxide under mild conditions, well-crystallizing chelates are obtained

copper salts(11) of blue color(one of the nonspecific methods for detecting α-amino acids).

Ninhydrin reaction. The general qualitative reaction of α-amino acids is the reaction with ninhydrin. The reaction product has a blue-violet color, which is used for visual detection of amino acids on chromatograms (on paper, in a thin layer), as well as for spectrophotometric determination on amino acid analyzers (the product absorbs light in the region of 550-570 nm).

Deamination. In laboratory conditions, this reaction is carried out by the action of nitrous acid on α-amino acids (see 4.3). In this case, the corresponding α-hydroxy acid is formed and nitrogen gas is released, the volume of which is used to determine the amount of amino acid that has reacted (Van-Slyke method).

Xanthoprotein reaction. This reaction is used to detect aromatic and heterocyclic amino acids - phenylalanine, tyrosine, histidine, tryptophan. For example, when concentrated nitric acid acts on tyrosine, a nitro derivative is formed, colored yellow. In an alkaline environment, the color becomes orange due to ionization of the phenolic hydroxyl group and an increase in the contribution of the anion to conjugation.

There are also a number of private reactions that allow the detection of individual amino acids.

Tryptophan detected by reaction with p-(dimethylamino)benzaldehyde in sulfuric acid by the appearance of a red-violet color (Ehrlich reaction). This reaction is used for quantitative analysis tryptophan in protein breakdown products.

Cysteine detected through several qualitative reactions based on the reactivity of the mercapto group it contains. For example, when a protein solution with lead acetate (CH3COO)2Pb is heated in an alkaline medium, a black precipitate of lead sulfide PbS is formed, which indicates the presence of cysteine in proteins.

12.1.5. Biologically important chemical reactions

In the body, under the influence of various enzymes, a number of important chemical transformations of amino acids are carried out. Such transformations include transamination, decarboxylation, elimination, aldol cleavage, oxidative deamination, and oxidation of thiol groups.

Transamination is the main pathway for the biosynthesis of α-amino acids from α-oxoacids. The donor of the amino group is an amino acid present in cells in sufficient quantity or excess, and its acceptor is an α-oxoacid. In this case, the amino acid is converted into an oxoacid, and the oxoacid into an amino acid with the corresponding structure of radicals. As a result, transamination represents reversible process interchange of amino and oxo groups. An example of such a reaction is the production of l-glutamic acid from 2-oxoglutaric acid. The donor amino acid can be, for example, l-aspartic acid.

α-Amino acids contain an electron-withdrawing amino group (more precisely, a protonated amino group NH) in the α-position to the carboxyl group 3 +), and therefore capable of decarboxylation.

Eliminationcharacteristic of amino acids in which the side radical in the β-position to the carboxyl group contains an electron-withdrawing functional group, for example, hydroxyl or thiol. Their elimination leads to intermediate reactive α-enamino acids, which easily transform into tautomeric imino acids (analogy with keto-enol tautomerism). As a result of hydration at the C=N bond and subsequent elimination of the ammonia molecule, α-imino acids are converted into α-oxo acids.

This type of transformation is called elimination-hydration. An example is the production of pyruvic acid from serine.

Aldol cleavage occurs in the case of α-amino acids, which contain a hydroxyl group in the β-position. For example, serine is broken down to form glycine and formaldehyde (the latter is not released in free form, but immediately binds to the coenzyme).

Oxidative deamination can be carried out with the participation of enzymes and the coenzyme NAD+ or NADP+ (see 14.3). α-Amino acids can be converted into α-oxoacids not only through transamination, but also through oxidative deamination. For example, α-oxoglutaric acid is formed from l-glutamic acid. At the first stage of the reaction, glutamic acid is dehydrogenated (oxidized) to α-iminoglutaric acid

acids. In the second stage, hydrolysis occurs, resulting in α-oxoglutaric acid and ammonia. The hydrolysis stage occurs without the participation of an enzyme.

The reaction of reductive amination of α-oxo acids occurs in the opposite direction. α-oxoglutaric acid, always contained in cells (as a product of carbohydrate metabolism), is converted in this way into L-glutamic acid.

Oxidation of thiol groups underlies the interconversions of cysteine and cystine residues, providing a number of redox processes in the cell. Cysteine, like all thiols (see 4.1.2), is easily oxidized to form a disulfide, cystine. The disulfide bond in cystine is easily reduced to form cysteine.

Due to the ability of the thiol group to easily oxidize, cysteine performs a protective function when the body is exposed to substances with high oxidative capacity. In addition, it was the first drug to show anti-radiation effects. Cysteine is used in pharmaceutical practice as a stabilizer for drugs.

Conversion of cysteine to cystine results in the formation of disulfide bonds, such as in reduced glutathione

(see 12.2.3).

12.2. Primary structure of peptides and proteins

Conventionally, it is believed that peptides contain up to 100 amino acid residues in a molecule (which corresponds to a molecular weight of up to 10 thousand), and proteins contain more than 100 amino acid residues (molecular weight from 10 thousand to several million).

In turn, in the group of peptides it is customary to distinguish oligopeptides(low molecular weight peptides) containing no more than 10 amino acid residues in the chain, and polypeptides, the chain of which includes up to 100 amino acid residues. Macromolecules with a number of amino acid residues approaching or slightly exceeding 100 do not distinguish between polypeptides and proteins; these terms are often used as synonyms.

A peptide and protein molecule can be formally represented as a product of polycondensation of α-amino acids, which occurs with the formation of a peptide (amide) bond between monomer units (Scheme 12.2).

The design of the polyamide chain is the same for the entire variety of peptides and proteins. This chain has an unbranched structure and consists of alternating peptide (amide) groups -CO-NH- and fragments -CH(R)-.

One end of the chain containing an amino acid with a free NH group 2, is called the N-terminus, the other is called the C-terminus,

Scheme 12.2.The principle of constructing a peptide chain

which contains an amino acid with a free COOH group. Peptide and protein chains are written from the N-terminus.

12.2.1. Structure of the peptide group

In the peptide (amide) group -CO-NH- the carbon atom is in a state of sp2 hybridization. The lone pair of electrons of the nitrogen atom enters into conjugation with the π-electrons of the C=O double bond. From the standpoint of electronic structure, the peptide group is a three-center p,π-conjugated system (see 2.3.1), the electron density in which is shifted towards the more electronegative oxygen atom. The C, O, and N atoms forming a conjugated system are located in the same plane. The electron density distribution in the amide group can be represented using the boundary structures (I) and (II) or the electron density shift as a result of the +M- and -M-effects of the NH and C=O groups, respectively (III).

As a result of conjugation, some alignment of bond lengths occurs. The C=O double bond is extended to 0.124 nm compared to the usual length of 0.121 nm, and the C-N bond becomes shorter - 0.132 nm compared to 0.147 nm in the usual case (Fig. 12.1). The planar conjugated system in the peptide group causes difficulty in rotation around the C-N bond (the rotation barrier is 63-84 kJ/mol). Thus, the electronic structure determines a fairly rigid flat structure of the peptide group.

As can be seen from Fig. 12.1, the α-carbon atoms of amino acid residues are located in the plane of the peptide group on opposite sides of the C-N bond, i.e., in a more favorable trans position: the side radicals R of amino acid residues in this case will be the most distant from each other in space.

The polypeptide chain has a surprisingly uniform structure and can be represented as a series of each other located at an angle.

Rice. 12.1.Planar arrangement of the peptide group -CO-NH- and α-carbon atoms of amino acid residues

to each other planes of peptide groups connected to each other through α-carbon atoms by Cα-N and Cα-Csp bonds 2 (Fig. 12.2). Rotation around these single bonds is very limited due to difficulties in the spatial placement of side radicals of amino acid residues. Thus, the electronic and spatial structure of the peptide group largely determines the structure of the polypeptide chain as a whole.

Rice. 12.2.The relative position of the planes of peptide groups in the polypeptide chain

12.2.2. Composition and amino acid sequence

With a uniformly constructed polyamide chain, the specificity of peptides and proteins is determined by two most important characteristics - amino acid composition and amino acid sequence.

The amino acid composition of peptides and proteins is the nature and quantitative ratio of their α-amino acids.

The amino acid composition is determined by analyzing peptide and protein hydrolysates, mainly by chromatographic methods. Currently, such analysis is carried out using amino acid analyzers.

Amide bonds are capable of hydrolysis in both acidic and alkaline environments (see 8.3.3). Peptides and proteins are hydrolyzed to form either shorter chains - this is the so-called partial hydrolysis, or mixtures of amino acids (in ionic form) - complete hydrolysis. Hydrolysis is usually carried out in an acidic environment, since many amino acids are unstable under alkaline hydrolysis conditions. It should be noted that the amide groups of asparagine and glutamine are also subject to hydrolysis.

The primary structure of peptides and proteins is the amino acid sequence, i.e. the order of alternation of α-amino acid residues.

The primary structure is determined by sequentially removing amino acids from either end of the chain and identifying them.

12.2.3. Structure and nomenclature of peptides

Peptide names are constructed by sequentially listing amino acid residues, starting from the N-terminus, with the addition of a suffix-il, except for the last C-terminal amino acid, for which its full name is retained. In other words, the names

amino acids that entered into the formation of a peptide bond due to “their” COOH group end in the name of the peptide with -il: alanil, valyl, etc. (for aspartic and glutamic acid residues the names “aspartyl” and “glutamyl” are used, respectively). The names and symbols of amino acids indicate their belonging to l -row, unless otherwise indicated ( d or dl).

Sometimes in the abbreviated notation the symbols H (as part of an amino group) and OH (as part of a carboxyl group) indicate the unsubstitution of the functional groups of terminal amino acids. This method is convenient for depicting functional derivatives of peptides; for example, the amide of the above peptide at the C-terminal amino acid is written H-Asn-Gly-Phe-NH2.

Peptides are found in all organisms. Unlike proteins, they have a more heterogeneous amino acid composition, in particular, they quite often include amino acids d -row. Structurally, they are also more diverse: they contain cyclic fragments, branched chains, etc.

One of the most common representatives of tripeptides is glutathione- found in the body of all animals, plants and bacteria.

Cysteine in the composition of glutathione makes it possible for glutathione to exist in both reduced and oxidized forms.

Glutathione is involved in a number of redox processes. It functions as a protein protector, i.e., a substance that protects proteins with free SH thiol groups from oxidation with the formation of disulfide bonds -S-S-. This applies to those proteins for which such a process is undesirable. In these cases, glutathione takes on the action of an oxidizing agent and thus “protects” the protein. During the oxidation of glutathione, intermolecular cross-linking of two tripeptide fragments occurs due to a disulfide bond. The process is reversible.

12.3. Secondary structure of polypeptides and proteins

High molecular weight polypeptides and proteins, along with the primary structure, are also characterized by higher levels of organization, which are called secondary, tertiary And quaternary structures.

The secondary structure is described by the spatial orientation of the main polypeptide chain, the tertiary structure by the three-dimensional architecture of the entire protein molecule. Both secondary and tertiary structure are associated with the ordered arrangement of the macromolecular chain in space. The tertiary and quaternary structure of proteins is discussed in a biochemistry course.

It was shown by calculation that one of the most favorable conformations for a polypeptide chain is an arrangement in space in the form of a right-handed helix, called α-helix(Fig. 12.3, a).

The spatial arrangement of an α-helical polypeptide chain can be imagined by imagining that it wraps around a certain

Rice. 12.3.α-helical conformation of the polypeptide chain

cylinder (see Fig. 12.3, b). On average, there are 3.6 amino acid residues per turn of the helix, the pitch of the helix is 0.54 nm, and the diameter is 0.5 nm. The planes of two neighboring peptide groups are located at an angle of 108°, and the side radicals of amino acids are located on the outside of the helix, i.e., they are directed as if from the surface of the cylinder.

The main role in securing such a chain conformation is played by hydrogen bonds, which in the α-helix are formed between the carbonyl oxygen atom of each first and the hydrogen atom of the NH group of each fifth amino acid residue.

Hydrogen bonds are directed almost parallel to the axis of the α-helix. They keep the chain twisted.

Typically, protein chains are not completely helical, but only partially. Proteins such as myoglobin and hemoglobin contain fairly long α-helical regions, such as the myoglobin chain

75% spiralized. In many other proteins, the proportion of helical regions in the chain may be small.

Another type of secondary structure of polypeptides and proteins is β-structure, also called folded sheet, or folded layer. Elongated polypeptide chains are arranged in folded sheets, linked by many hydrogen bonds between the peptide groups of these chains (Fig. 12.4). Many proteins contain both α-helical and β-sheet structures.

Rice. 12.4.Secondary structure of the polypeptide chain in the form of a folded sheet (β-structure)

Modern protein nutrition cannot be imagined without considering the role of individual amino acids. Even with an overall positive protein balance, the animal’s body may experience a lack of protein. This is due to the fact that the absorption of individual amino acids is interconnected with each other; a deficiency or excess of one amino acid can lead to a deficiency of another.
Some amino acids are not synthesized in the human and animal bodies. They are called irreplaceable. There are only ten such amino acids. Four of them are critical (limiting) - they most often limit the growth and development of animals.
In diets for poultry, the main limiting amino acids are methionine and cystine, in diets for pigs – lysine. The body must receive a sufficient amount of the main limiting acid with food so that other amino acids can be effectively used for protein synthesis.

This principle is illustrated by the "Liebig barrel", where the fill level of the barrel represents the level of protein synthesis in the animal's body. The shortest board in a barrel “limites” the ability to hold liquid in it. If this board is extended, then the volume of liquid held in the barrel will increase to the level of the second limiting board.
Most important factor, which determines the productivity of animals, is the balance of the amino acids it contains in accordance with physiological needs. Numerous studies have proven that in pigs, depending on the breed and sex, the need for amino acids differs quantitatively. But the ratio of essential amino acids for the synthesis of 1 g of protein is the same. This ratio of essential amino acids to lysine, as the main limiting amino acid, is called the “ideal protein” or “ideal amino acid profile.” (

Lysine

is part of almost all proteins of animal, plant and microbial origin, but cereal proteins are poor in lysine.

Lysine regulates reproductive function; its deficiency disrupts the formation of sperm and eggs.
Necessary for the growth of young animals and the formation of tissue proteins. Lysine takes part in the synthesis of nucleoproteins, chromoproteins (hemoglobin), thereby regulating the pigmentation of animal fur. Regulates the amount of protein breakdown products in tissues and organs.
Promotes calcium absorption
Participates in the functional activity of the nervous and endocrine systems, regulates the metabolism of proteins and carbohydrates, however, when reacting with carbohydrates, lysine becomes inaccessible to absorption.
Lysine is the starting substance in the formation of carnitine, which plays an important role in fat metabolism.

Methionine and cystine sulfur-containing amino acids. In this case, methionine can be transformed into cystine, so these amino acids are rationed together, and if there is a deficiency, methionine supplements are introduced into the diet. Both of these amino acids are involved in the formation of skin derivatives - hair, feathers; together with vitamin E, they regulate the removal of excess fat from the liver and are necessary for the growth and reproduction of cells and red blood cells. If there is a lack of methionine, cystine is inactive. However, a significant excess of methionine in the diet should not be allowed.

Methionine

promotes the deposition of fat in muscles, is necessary for the formation of new organic compounds choline (vitamin B4), creatine, adrenaline, niacin (vitamin B5), etc.
A deficiency of methionine in diets leads to a decrease in the level of plasma proteins (albumin), causes anemia (the level of hemoglobin in the blood decreases), while a simultaneous lack of vitamin E and selenium contributes to the development of muscular dystrophy. An insufficient amount of methionine in the diet causes stunted growth of young animals, loss of appetite, decreased productivity, increased feed costs, fatty liver, impaired renal function, anemia and emaciation.
Excess methionine impairs the use of nitrogen, causes degenerative changes in the liver, kidneys, pancreas, and increases the need for arginine and glycine. With a large excess of methionine, an imbalance is observed (the balance of amino acids is disturbed, which is based on sharp deviations from the optimal ratio of essential amino acids in the diet), which is accompanied by metabolic disorders and inhibition of growth rate in young animals.
Cystine is a sulfur-containing amino acid, interchangeable with methionine, participates in redox processes, the metabolism of proteins, carbohydrates and bile acids, promotes the formation of substances that neutralize intestinal poisons, activates insulin, together with tryptophan, cystine participates in the synthesis in the liver of bile acids necessary for absorption products of digestion of fats from the intestines, used for the synthesis of glutathione. Cystine has the ability to absorb ultraviolet rays. With a lack of cystine, there is cirrhosis of the liver, delayed feathering and feather growth in young birds, fragility and loss (plucking) of feathers in adult birds, and decreased resistance to infectious diseases.

Tryptophan

determines the physiological activity of enzymes of the digestive tract, oxidative enzymes in cells and a number of hormones, participates in the renewal of blood plasma proteins, determines the normal functioning of the endocrine and hematopoietic apparatus, the reproductive system, the synthesis of gamma globulins, hemoglobin, nicotinic acid, eye purple, etc. In case of deficiency in the diet of tryptophan, the growth of young animals slows down, the egg production of laying hens decreases, the cost of feed for production increases, the endocrine and gonads atrophy, blindness occurs, anemia develops (the number of red blood cells and the level of hemoglobin in the blood decreases), the resistance and immune properties of the body, fertilization and hatchability of eggs decrease . In pigs fed a diet low in tryptophan, feed intake decreases, a perverted appetite appears, bristles become coarser and emaciated, and fatty liver is noted. A deficiency of this amino acid also leads to sterility, increased excitability, convulsions, cataract formation, negative balance nitrogen and loss of live weight. Tryptophan, being a precursor (provitamin) of nicotinic acid, prevents the development of pellagra.

Amino acids, proteins and peptides are examples of the compounds described below. Many biologically active molecules contain several chemically different functional groups that can interact with each other and with each other's functional groups.

Amino acids.

Amino acids- organic bifunctional compounds, which include a carboxyl group - UNS, and the amino group is N.H. 2 .

Separate α And β - amino acids:

Mostly found in nature α -acids. Proteins contain 19 amino acids and one imino acid ( C 5 H 9NO 2 ):

The simplest amino acid- glycine. The remaining amino acids can be divided into the following main groups:

1) homologues of glycine - alanine, valine, leucine, isoleucine.

Obtaining amino acids.

Chemical properties of amino acids.

Amino acids- these are amphoteric compounds, because contain 2 opposite functional groups - an amino group and a hydroxyl group. Therefore, they react with both acids and alkalis:

Acid-base transformation can be represented as:

Lecture No. 3

Topic: “Amino acids - structure, classification, properties, biological role”

Amino acids – nitrogen containing organic compounds, whose molecules contain an amino group –NH2 and a carboxyl group –COOH

The simplest representative is aminoethanoic acid H2N - CH2 - COOH

Classification of amino acids

There are 3 main classifications of amino acids:

Physico-chemical – based on differences in the physicochemical properties of amino acids

Hydrophobic amino acids (non-polar). The components of radicals usually contain hydrocarbon groups, where the electron density is evenly distributed and there are no charges or poles. They may also contain electronegative elements, but they are all in a hydrocarbon environment.
Hydrophilic uncharged (polar) amino acids . The radicals of such amino acids contain polar groups: -OH, -SH, -CONH2
Negatively charged amino acids. These include aspartic and glutamic acids. They have an additional COOH group in the radical - in a neutral environment they acquire a negative charge.
Positively charged amino acids : arginine, lysine and histidine. They have an additional NH 2 group (or an imidazole ring, like histidine) in the radical - in a neutral environment they acquire a positive charge.

Biological classification – if possible, synthesized in the human body

Irreplaceable amino acids, they are also called “essential”. They cannot be synthesized in the human body and must be supplied with food. There are 8 of them and 2 more amino acids that are classified as partially essential.

Indispensable: methionine, threonine, lysine, leucine, isoleucine, valine, tryptophan, phenylalanine.

Partially irreplaceable: arginine, histidine.

Replaceable(can be synthesized in the human body). There are 10 of them: glutamic acid, glutamine, proline, alanine, aspartic acid, asparagine, tyrosine, cysteine, serine and glycine.

Chemical classification - in accordance with the chemical structure of the amino acid radical (aliphatic, aromatic).

Amino acids are classified according to their structural characteristics.

1. Depending on relative position amino and carboxyl groups, amino acids are divided into α-, β-, γ-, δ-, ε- etc.

The need for amino acids decreases: For congenital disorders associated with the absorption of amino acids. In this case, some protein substances can cause allergic reactions in the body, including problems in the gastrointestinal tract, itching and nausea.
Amino Acid Digestibility

The speed and completeness of absorption of amino acids depends on the type of products containing them. The amino acids contained in egg whites, low-fat cottage cheese, lean meat and fish are well absorbed by the body.

Amino acids are also quickly absorbed with the right combination of products: milk is combined with buckwheat porridge and white bread, all kinds of flour products with meat and cottage cheese.
Beneficial features amino acids, their effect on the body

Each amino acid has its own effect on the body. So methionine is especially important for improving fat metabolism in the body; it is used as a prevention of atherosclerosis, cirrhosis and fatty liver degeneration.

For certain neuropsychiatric diseases, glutamine and aminobutyric acids are used. Glutamic acid is also used in cooking as a flavoring additive. Cysteine is indicated for eye diseases.

The three main amino acids - tryptophan, lysine and methionine, are especially necessary for our body. Tryptophan is used to accelerate the growth and development of the body, and it also maintains nitrogen balance in the body.

Lysine ensures normal growth of the body and participates in the processes of blood formation.

The main sources of lysine and methionine are cottage cheese, beef, and some types of fish (cod, pike perch, herring). Tryptophan is found in optimal quantities in offal, veal and game.heart attack.

Amino acids for health, energy and beauty

To successfully build muscle mass in bodybuilding, amino acid complexes consisting of leucine, isoleucine and valine are often used.

To maintain energy during training, athletes use methionine, glycine and arginine, or products containing them, as dietary supplements.

For any person leading an active healthy lifestyle, special foods are needed that contain a number of essential amino acids to maintain excellent physical shape, quickly restore strength, burn excess fat or build muscle mass.