Almost all natural biological compounds containing a chiral center occur in only one stereo isomeric form - D or L. With the exception of glycine, which does not have an asymmetric carbon atom, all amino acids that make up protein molecules are L-stereoisomers. This conclusion was reached on the basis of numerous carefully conducted chemical studies in which optical properties amino acids were compared with their behavior in chemical reactions. Below we will see that some D-amino acids are also found in living nature, but they are never part of proteins.

The presence in proteins of only L-stereoisomers of amino acids is quite remarkable, since the usual chemical reactions used to synthesize compounds with an asymmetric carbon atom always produce optically inactive products. This is because in conventional chemical reactions, both D- and L-stereoisomers are formed at the same rate. The result is a racemic mixture, or racemate, an equimolar mixture of D- and L-isomers that does not rotate the plane of polarization in either direction. A racemic mixture can only be separated into D- and L-isomers by very laborious methods based on differences in the physical properties of the stereoisomers. The separated D- and L-isomers revert back to a racemic mixture over time (see Appendix 5-2).

Appendix 5-2. How to determine a person's age using amino acid chemistry

The optical isomers of amino acids undergo a very slow and spontaneous non-enzymatic racemization, so that over a very long period of time, a pure L- or D-isomer can turn into an equimolar mixture of D- and L-isomers. The racemization of each L-amino acid at a given temperature proceeds at a certain rate. This circumstance can be used to determine the age of people and animals or fossil remains of organisms. For example, in the dentin protein found in hard tooth enamel, L-aspartate spontaneously racemizes at human body temperature at a rate of one year. In children during the period of tooth formation, the dentin contains only L-aspartate. It is possible to extract dentin from just one tooth and determine the content of D-aspartate in it. Such analyzes were done on the dentin of the inhabitants of the mountain villages of Ecuador, many of whom attributed too much age to themselves. Since this was doubtful in some cases, a racemization test was used for verification, which turned out to be quite accurate. So, for a 97-year-old woman, whose age was documented, according to the test, the age was set to 99 years.

Tests performed on the fossil remains of prehistoric animals - elephants, dolphins and bears - showed that the data obtained by this method are in good agreement with the results of dating based on the rate of decay of radioactive isotopes.

Living cells have a unique ability to synthesize L-amino acids using stereospecific enzymes. The stereospecificity of these enzymes is due to the asymmetric nature of their active centers. Below we will see that the characteristic three-dimensional structure of proteins, due to which they exhibit the most different types biological activity, occurs only if all their constituent amino acids belong to the same stereochemical series.

Isomerism of amino acids depending on the position of the amino group

Depending on the position of the amino group relative to the 2nd carbon atom, α-, β-, γ- and other amino acids are isolated.

α- and β-forms of alanine

For the mammalian organism, α-amino acids are the most characteristic.

Isomerism by absolute configuration

According to the absolute configuration of the molecule, D- and L-forms are distinguished. Differences between isomers are associated with mutual arrangement four substituent groups located at the vertices of an imaginary tetrahedron, the center of which is the carbon atom in the α-position. There are only two possible arrangements of chemical groups around it.

The protein of any organism contains only one stereoisomer, for mammals it is L-amino acids.

L- and D-forms of alanine

However, optical isomers can undergo spontaneous non-enzymatic racemization, i.e. L-shape becomes D-shape.

As you know, a tetrahedron is a rather rigid structure in which it is impossible to move the vertices arbitrarily.

In the same way, for molecules built on the basis of a carbon atom, the structure of the glyceraldehyde molecule, established by X-ray diffraction analysis, is taken as the configuration standard. It is accepted that the most highly oxidized a carbon atom (in the diagrams it is placed on top) associated with asymmetrical a carbon atom. Such an oxidized atom in a molecule glyceraldehyde serves as an aldehyde group alanine- UNO group. The hydrogen atom in the asymmetric carbon is arranged in the same way as in glyceraldehyde.

In dentin, the protein of tooth enamel, the rate of L-aspartate racemization is 0.10% per year. When forming a tooth in children, only L-aspartate is used. This feature allows, if desired, to determine the age of centenarians. For fossil remains, along with the radioisotope method, the determination of the racemization of amino acids in a protein is also used.

Division of isomers according to optical activity

By optical activity, amino acids are divided into right - and left-handed.

The presence of an asymmetric α-carbon atom (chiral center) in an amino acid makes possible only two arrangements of chemical groups around it. This leads to a special difference between substances from each other, namely, a change direction of rotation of the plane of polarized light passing through the solution. The angle of rotation is determined using a polarimeter. According to the angle of rotation, dextrorotatory (+) and levorotatory (–) isomers are distinguished.

amino acids

amino acids

amino acids

a class of organic compounds containing carboxyl (-COOH) and amino groups (-NH 2); have the properties of both acids and bases. Participate in the metabolism of nitrogenous substances of all organisms (the initial compound in the biosynthesis of hormones, vitamins, mediators, pigments, purine and pyrimidine bases, alkaloids, etc.). There are more than 150 natural amino acids. About 20 essential amino acids serve as monomeric units from which all proteins are built (the order in which amino acids are included in them is determined by the genetic code). Most microorganisms and plants synthesize the amino acids they need; animals and humans are not capable of forming the so-called essential amino acids obtained from food. The industrial synthesis (chemical and microbiological) of a number of amino acids used to enrich food, feed, as initial products for the production of polyamides, dyes and drugs has been mastered.

AMINO ACIDS

AMINO ACIDS, organic (carboxylic ( cm. CARBOXIC ACIDS)) acids, which contain an amino group (- NH 2). Participate in the metabolism of proteins and carbohydrates, in the formation of compounds important for organisms (for example, purine ( cm. purine bases) and pyrimidine bases ( cm. PYRIMIDINE BASES), which are an integral part of nucleic acids ( cm. NUCLEIC ACIDS)), are part of hormones ( cm. HORMONES), vitamins ( cm. VITAMINS), alkaloids ( cm. ALKALOIDS), pigments ( cm. PIGMENTS (in biology)), toxins ( cm. TOXINS), antibiotics ( cm. ANTIBIOTICS), etc.; dihydroxyphenylalanine (DOPA) and g-aminobutyric acid serve as mediators in the transmission of nerve impulses ( cm. NERVE IMPULSE). About 300 different amino acids are found in the cells and tissues of living organisms, but only 20 of them serve as links (monomers) from which peptides are built ( cm. PEPTIDES) and proteins ( cm. PROTEINS (organic compounds)) of all organisms (therefore they are called protein amino acids). The sequence of these amino acids in proteins is encoded in the nucleotide sequence ( cm. NUCLEOTIDES) of the corresponding genes (see Genetic code ( cm. GENETIC CODE)). The remaining amino acids are found both in the form of free molecules and in bound form. Many of the amino acids are found only in certain organisms, and there are those that are found in only one of the great many described organisms. History of the discovery of amino acids The first amino acid is asparagine ( cm. ASPARAGIN) - was discovered in 1806, the last of the amino acids found in proteins is threonine ( cm. Threonine) - was identified in 1938. Each amino acid has a trivial (traditional) name, sometimes it is associated with the source of release. For example, asparagine was first discovered in asparagus (asparagus), glutamic acid - in the gluten (from the English gluten - gluten) of wheat, glycine was named so for its sweet taste (from the Greek glykys - sweet). Structure and properties of amino acids The general structural formula of any amino acid can be represented as follows: the carboxyl group (- COOH) and the amino group (- NH 2) are connected to the same a-carbon atom (the count of atoms is from the carboxyl group using the letters of the Greek alphabet - a, b, g, etc.). Amino acids differ in the structure of the side group, or side chain (radical R), which have different sizes, shapes, reactivity, determine the solubility of amino acids in an aqueous medium and their electrical charge. And only at the proline ( cm. PROLINE), the side group is attached not only to the a-carbon atom, but also to the amino group, resulting in a cyclic structure. In a neutral environment and in crystals, -amino acids exist as bipolars, or zwitterions ( cm. ZWITTER IONS). Therefore, for example, the formula of the amino acid glycine - NH 2 -CH 2 -COOH - would be more correct to write as NH 3 + -CH 2 -COO -. Only in the simplest amino acid, glycine, does a hydrogen atom act as a radical. For the remaining amino acids, all four substituents at the a -carbon atom are different (i.e., the a -carbon carbon atom is asymmetric). Therefore, these amino acids have optical activity ( cm. OPTICAL ACTIVITY) (capable of rotating the plane of polarized light) and can exist in the form of two optical isomers - L (left-handed) and D (right-handed). However, all natural amino acids are L-amino acids. Exceptions include D-isomers of glutamic acid ( cm. GLUTAMIC ACID), alanine ( cm. ALANIN), valine ( cm. VALINE), phenylalanine ( cm. phenylalanine), leucine ( cm. LEUCINE) and a number of other amino acids that are found in the cell wall of bacteria; D-conformation amino acids are part of some peptide antibiotics ( cm. ANTIBIOTICS) (including actinomycins, bacitracin, gramicidins ( cm. GRAMICIDINS) A and S), alkaloids ( cm. ALKALOIDS) from ergot, etc. Amino acid classification The amino acids that make up proteins are classified according to the characteristics of their side groups. For example, based on their relationship to water at biological pH values ​​(about pH 7.0), non-polar or hydrophobic amino acids are distinguished from polar or hydrophilic ones. In addition, neutral (uncharged) amino acids are distinguished among polar amino acids; they contain one acidic (carboxyl) and one basic group (amino group). If more than one of the above groups is present in an amino acid, then they are called, respectively, acidic and basic. Most microorganisms and plants create all the amino acids they need from simpler molecules. In contrast, animal organisms cannot synthesize some of the amino acids they need. They should receive such amino acids in finished form, that is, with food. Therefore, based on nutritional value, amino acids are divided into essential and non-essential. Valine is one of the essential amino acids for humans. cm. VALINE), threonine ( cm. THREONINE), tryptophan ( cm. tryptophan), phenylalanine ( cm. phenylalanine), methionine ( cm. METIONINE), lysine ( cm. lysine), leucine ( cm. LEUCINE), isoleucine ( cm. ISOLEUCINE), and histidine is also indispensable for children ( cm. HISTIDINE) and arginine ( cm. arginine). The lack of any of the essential amino acids in the body leads to metabolic disorders, slow growth and development. Rare (non-standard) amino acids are found in individual proteins, which are formed by various chemical transformations of the side groups of ordinary amino acids during protein synthesis on ribosomes or after its completion (the so-called post-translational modification of proteins) (see Proteins ( cm. PROTEINS (organic compounds))). For example, in the composition of collagen ( cm. COLLAGEN) (connective tissue protein) includes hydroxyproline and hydroxylysine, which are derivatives of proline and lysine, respectively; in the muscle protein myosin cm. MYOSIN) methyllysin is present; only in the protein elastin ( cm. ELASTIN) contains a lysine derivative - desmosine. Use of amino acids Amino acids are widely used as food additives ( cm. NUTRITIONAL SUPPLEMENTS). For example, lysine, tryptophan, threonine and methionine enrich the feed of farm animals, the addition of the sodium salt of glutamic acid (monosodium glutamate) gives a meaty taste to a number of products. In a mixture or separately, amino acids are used in medicine, including for metabolic disorders and diseases of the digestive system, for some diseases of the central nervous system (g-aminobutyric and glutamic acids, DOPA). Amino acids are used in the manufacture of medicines, dyes, in the perfume industry, in the production of detergents, synthetic fibers and films, etc. For household and medical needs, amino acids are obtained with the help of microorganisms by the so-called microbiological synthesis ( cm. MICROBIOLOGICAL SYNTHESIS) (lysine, tryptophan, threonine); they are also isolated from hydrolysates of natural proteins (proline ( cm. PROLINE), cysteine ​​( cm. CYSTEIN), arginine ( cm. ARGININE), histidine ( cm. HISTIDINE). But the most promising are mixed methods of obtaining, combining the methods of chemical synthesis and the use of enzymes ( cm. ENZYMES).

PROTEINS

(proteins), a class of complex nitrogen-containing compounds, the most characteristic and important (along with nucleic acids) components of living matter. Proteins perform 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 tissue, hair and nails. The contractile proteins of muscles have the ability to change their length, using chemical energy to perform mechanical work. Proteins are 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, and among them primarily J. von Liebig, gradually came to the conclusion that proteins are a special class of nitrogenous compounds. The name "proteins" (from the Greek protos - first) was proposed in 1840 by the Dutch chemist G. Mulder. PHYSICAL PROPERTIES Proteins are white in the solid state, but colorless in solution, unless they carry some chromophore (colored) group, such as hemoglobin. The solubility in water of different proteins varies greatly. It also varies with pH and with the concentration of salts in the solution, so that one can choose the conditions under which one protein will selectively precipitate in the presence of other proteins. This "salting out" method is widely used to isolate and purify proteins. The purified protein often precipitates out of solution as crystals. In comparison with other compounds, the molecular weight of proteins is very large - 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 in an electric field. Electrophoresis is based on this - a method used to isolate individual proteins from complex mixtures. Purification of proteins is also carried out by chromatography. CHEMICAL PROPERTIES Structure. Proteins are polymers, i.e. molecules built like chains from repeating monomer units, or subunits, whose role is played by a-amino acids. General formula of amino acids

<="" div="" style="border-style: none;">where R is a hydrogen atom or some organic group. A protein molecule (polypeptide chain) may consist of only a relatively small number of amino acids or several thousand monomer units. The connection of amino acids in the chain is possible because each of them has two different chemical groups: a basic amino group, 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:

<="" div="" style="border-style: none;">After two amino acids have been connected in this way, the chain can be extended by adding a third to the second amino acid, and so on. As can be seen from the above equation, when a 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 cleaved into amino acids with the addition of water. This reaction is called hydrolysis. Hydrolysis proceeds spontaneously, and energy is required to combine amino acids into a polypeptide chain. A carboxyl group and an amide group (or an imide group similar to it - in the case of the amino acid proline) are present in all amino acids, while 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 also 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 highly reactive. Many thousands of different amino acids can be synthesized, and many different amino acids occur 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 ​​may be present as a dimer - cystine). True, there are other amino acids in some proteins, in addition to the regularly occurring twenty, but they are formed as a result of modification of any of the twenty listed after it has been included in the protein. optical activity. All amino acids, with the exception of glycine, have four different groups attached to the a-carbon atom. In terms 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 as an object to its mirror image, i.e. like left hand to right. One configuration is called left, or left-handed (L), and the other is called right-handed, or right-handed (D), since two such isomers differ in the direction of rotation of the plane of polarized light. Only L-amino acids occur in proteins (the exception is glycine; it can only be represented in one form, since two of its four groups 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 the cell wall of bacteria.

An asymmetric carbon atom in an amino acid molecule is depicted here as a ball placed in the center of a tetrahedron. The presented arrangement of the four substituent groups corresponds to the L-configuration, characteristic of all natural amino acids.

The sequence of amino acids. 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 like you can make up many different texts from the letters of the alphabet. In the past, determining the amino acid sequence of a protein often took several years. 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 derive 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 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 of only amino acids are called simple. Often, however, a metal atom or some chemical compound that is not an amino acid is attached to the polypeptide chain. Such proteins are called complex. An example is hemoglobin: it contains iron porphyrin, which gives it 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: sugars are present in glycoproteins, 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 proteins of the retina, determines its sensitivity to light. Tertiary structure. What is important is not so much the amino acid sequence of the protein (primary structure), but the way it is laid 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 layer (secondary structure). From the combination of such helices and layers, a compact form of the next order arises - the tertiary structure of the protein. Around the bonds that hold the monomeric links of the chain, rotations through small angles are possible. Therefore, from a purely geometric point of view, the number of possible configurations for any polypeptide chain is infinitely large. In reality, each protein normally exists in only one configuration, determined by its amino acid sequence. This structure is not rigid, it seems to "breathe" - it fluctuates around a certain average configuration. The chain is folded into a configuration in which the free energy (the ability to do 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 ​​among amino acids plays a particularly important role. The complexity of the structure of proteins is so great that it is not yet possible to calculate the tertiary structure of a protein, even if its amino acid sequence is known. But if it is possible to obtain protein crystals, then its tertiary structure can be determined by X-ray diffraction. In structural, contractile, and some other proteins, the chains are elongated and several slightly folded chains lying side by side form fibrils; fibrils, in turn, are folded into larger formations - fibers. However, most proteins in solution are globular: the chains are coiled in a globule, like yarn in a ball. Free energy in this configuration is minimal, since hydrophobic ("water-repelling") amino acids are hidden inside the globule, and hydrophilic ("water-attracting") amino acids are located on its surface. Many proteins are complexes of several polypeptide chains. This structure is called the quaternary structure of the protein. The hemoglobin molecule, for example, is made up of four subunits, each of which is a globular protein. Structural proteins, due to their linear configuration, form fibers in which the tensile strength is very high, while the globular configuration allows proteins to enter into specific interactions with other compounds. On the surface of the globule, with the correct laying of chains, cavities of a certain shape appear, in which reactive chemical groups are located. If this protein is an enzyme, then another, usually smaller, molecule of some substance enters such a cavity, just as a key enters a lock; in this case, the configuration of the electron cloud of the molecule changes under the influence of chemical groups located 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 bind and are 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, i.e. their ability to react only with certain compounds. Proteins in different types of organisms. Proteins that perform the same function in different plant and animal species and therefore bear the same name also have a similar configuration. They, however, differ somewhat in their amino acid sequence. As species diverge from a common ancestor, some amino acids in certain positions are replaced by mutations with others. Harmful mutations that cause hereditary diseases are discarded by natural selection, but beneficial or at least neutral ones can be preserved. The closer two biological species are to each other, the less differences are found in their proteins. Some proteins change relatively quickly, others are quite 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 cytochrome c of wheat, only 38% of the amino acids turned out to be different. Even when comparing humans and bacteria, the similarities of cytochromes with (the differences here affect 65% of amino acids) 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 that reflects the evolutionary relationships between different organisms. Denaturation. The synthesized protein molecule, folding, acquires its own configuration. This configuration, however, can be destroyed by heating, by changing the pH, by the action of organic solvents, and even by simply agitating the solution until bubbles appear on its surface. A protein altered 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 able to renature, i.e. reacquire the original configuration. But most of the proteins are simply transformed into a mass of tangled polypeptide chains and do not restore their previous configuration. One of the main difficulties in isolating active proteins is their extreme sensitivity to denaturation. This property of proteins finds useful application in the preservation of food products: high temperature irreversibly denatures the enzymes of microorganisms, and the 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 connected. 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 (similar to how a record is stored on a magnetic tape) in the nucleic acid molecules that make up genes. see also HEREDITY; NUCLEIC ACIDS. Enzyme activation. A polypeptide chain synthesized from amino acids is not always a protein in its final form. Many enzymes are first synthesized as inactive precursors and become active only after another enzyme removes a few amino acids from one end of the chain. Some of the digestive enzymes, such as trypsin, are synthesized in this inactive form; these enzymes are activated in the digestive tract as a result of the removal of the terminal fragment of the chain. The hormone insulin, whose molecule in its active form consists of two short chains, is synthesized in the form of a single chain, the so-called. proinsulin. Then the middle part of this chain is removed, and the remaining fragments bind to each other, forming the active hormone molecule. Complex proteins are formed only after a certain chemical group is attached to the protein, and this attachment often also requires an enzyme. Metabolic circulation. After feeding an animal with amino acids labeled with radioactive isotopes of carbon, nitrogen or hydrogen, the label is quickly incorporated into its proteins. If labeled amino acids cease to enter the body, then the amount of label in proteins begins to decrease. These experiments show that the resulting 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 decomposing to amino acids, and then re-synthesized. Some proteins break down when cells die and are destroyed. This happens all the time, for example, with red blood cells and epithelial cells lining the inner surface of the intestine. In addition, the breakdown and resynthesis of proteins also occur in living cells. Oddly enough, less is known about the breakdown of proteins than about their synthesis. What is clear, however, is 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 and are not renewed or replaced. Over time, however, some of their properties, in particular elasticity, change, and since they are not renewed, certain age-related changes, such as the appearance of wrinkles on the skin, are the result of this. synthetic proteins. Chemists have long since learned how to polymerize amino acids, but the amino acids are combined randomly, so that the products of such polymerization bear little resemblance 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 whose molecules contain about a hundred amino acids. It is preferable instead to synthesize or isolate the nucleotide sequence of a gene corresponding to the desired amino acid sequence, and then introduce this gene into a bacterium, which will produce by replication a large amount of the desired product. This method, however, also has its drawbacks. see also GENETIC ENGINEERING. PROTEINS AND NUTRITION When proteins in the body are broken down into amino acids, these amino acids can be reused for protein synthesis. At the same time, the amino acids themselves are subject to decay, so that they are not fully utilized. It is also clear that during growth, pregnancy, and wound healing, protein synthesis must exceed degradation. The body continuously loses some proteins; these are the 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 obtain amino acids by eating green plants or other animals. In the digestive tract, the absorbed proteins are broken down into amino acids, the latter are absorbed, and the proteins characteristic of the given organism are built from them. None of the absorbed protein is incorporated into body structures as such. The only exception is that in many mammals, part of maternal antibodies can pass intact through the placenta into the fetal circulation, and through mother's milk (especially in ruminants) be transferred to the newborn immediately after birth. Need for proteins. It is clear that in order to maintain life, the body must receive a certain amount of protein from food. However, the size 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 for the synthesis of their own proteins, but as a source of calories. With prolonged fasting, even your own proteins are spent to meet 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 make up 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 such an indicator as nitrogen balance to assess the quality of protein nutrition, i.e. the difference (in grams) between the amount of nitrogen taken into the body and the amount of nitrogen excreted per day. With normal nutrition in an adult, these amounts are equal. In a growing organism, the amount of excreted nitrogen is less than the amount of incoming, 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 the proteins are completely absent in it, the body saves proteins. At the same time, protein metabolism slows down, and the re-utilization of amino acids in protein synthesis proceeds as efficiently as possible. 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 some additional protein is required to restore the nitrogen balance. If the amount of protein in the diet exceeds what is necessary to maintain nitrogen balance, then there seems to be no harm from this. Excess amino acids are simply used as a source of energy. A particularly striking example is the Eskimo, who consume little carbohydrate and about ten times more protein than is required to maintain nitrogen balance. In most cases, however, using protein as an energy source is not beneficial, since you can get many more calories from a given amount of carbohydrates than from the same amount of protein. In poor countries, the population receives the necessary calories from carbohydrates and consumes a 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. Approximately as much protein is contained in four slices of bread or 0.5 liters of milk. A slightly larger amount is usually considered optimal; recommended from 50 to 70 g. Essential amino acids. Until now, protein has been considered as a whole. Meanwhile, in order for protein synthesis to take place, all the necessary amino acids must be present in the body. Some of the amino acids the body of the animal itself is able to synthesize. They are called interchangeable, since they do not have to be present in the diet - it is only important that, in general, the intake of protein as a source of nitrogen is sufficient; then, with a shortage of non-essential amino acids, the body can synthesize them at the expense of those that are present in excess. The remaining "essential" amino acids cannot be synthesized and must be ingested with food. Essential for humans are valine, leucine, isoleucine, threonine, methionine, phenylalanine, tryptophan, histidine, lysine, and arginine. (Although arginine can be synthesized in the body, it is considered an essential amino acid because it is deficient in newborns and growing children. Dietary intake of some of these amino acids, on the other hand, may become unnecessary for an adult.) This list of essential amino acids 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 a protein is determined by the essential amino acid that is most deficient. Let's illustrate this with an example. The proteins of our body contain an average of 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 a complete one; 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 take place, all amino acids must be present simultaneously, the effect of the intake of essential amino acids can be detected 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 in the human body, so we are unlikely to face amino acid deficiency 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; especially little in them lysine and tryptophan. However, a purely vegetarian diet is by no means unhealthy, unless it consumes a slightly larger amount of vegetable proteins, sufficient to provide the body with essential amino acids. Most protein is found in plants in the seeds, 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 incomplete proteins, such as corn proteins, one can significantly increase the nutritional value of the latter, i.e. thereby increasing the amount of protein consumed. Another possibility is to grow bacteria or yeasts on petroleum hydrocarbons with the addition of nitrates or ammonia as a source of nitrogen. The microbial protein obtained in this way can serve as feed for poultry or livestock, or can be directly consumed by humans. The third, widely used, method uses the physiology of ruminants. In ruminants, in the initial section of the stomach, the so-called. In the rumen, there are special forms of bacteria and protozoa that convert defective plant proteins into more complete microbial proteins, and these, in turn, after digestion and absorption, turn into animal proteins. Urea, a cheap synthetic nitrogen-containing compound, can be added to livestock feed. Microorganisms living in the rumen use urea nitrogen to convert carbohydrates (of which there is much more in the feed) into protein. About a third of all nitrogen in livestock feed can come in the form of urea, which in essence means, to a certain extent, chemical protein synthesis. In the USA, this method plays an important role as one of the ways to obtain protein. LITERATURE

The physicochemical and biological properties of proteins are determined by their amino acid composition. Amino acids are amino derivatives of the class of carboxylic acids. Amino acids are not only found in proteins. Many of them perform special functions. Therefore, in living organisms, amino acids are proteinogenic (genetically encoded) and non-proteinogenic (not genetically encoded). There are 20 proteinogenic amino acids. 19 of them are a-amino acids. This means that their amino group is attached to the a-carbon atom of the carboxylic acid of which they are a derivative. The general formula of these amino acids is as follows:

Only one amino acid - proline does not correspond to this general formula. It is classified as an imino acid.

the a-carbon atom of amino acids is asymmetric (the exception is the amino derivative of acetic acid - glycine). This means that each amino acid has at least 2 optically active antipodes. Nature chose the L-form to create proteins. Therefore, natural proteins are built from L-a-amino acids.

In all cases where a carbon atom is bonded to 4 different atoms or functional groups in an organic compound molecule, this atom is asymmetric because it can exist in two isomeric forms, called enantiomers or optical (stereo-) isomers. Compounds with asymmetric "C" atoms occur in the form of two forms (chiral compounds) - left and right, depending on the direction of rotation of the plane of polarization of plane polarized light. All standard amino acids except one (glycine) contain an asymmetric carbon atom in the a-position, to which 4 substituent groups are attached. Therefore, they have optical activity, that is, they are able to rotate the plane

polarization of light in one direction or another.

However, the notation system for stereoisomers is based not on the rotation of the plane of polarization of light, but on the absolute configuration of the stereoisomer molecule. To determine the configuration of optically active amino acids, they are compared with glyceraldehyde, the simplest three-carbon carbohydrate that contains an asymmetric carbon atom. Stereoisomers of all chiral compounds, regardless of the direction of rotation of the plane of polarization of plane polarized light, corresponding in configuration to L-glyceraldehyde, are denoted by the letter L, and those corresponding to D-glyceraldehyde, by the letter D. Thus, the letters L and D refer to the absolute configuration of 4 substituent groups at the chiral atom "C", and not to the direction of rotation of the plane of polarization.

Amino acids are classified according to the structure of their radical. There are different approaches to classification. Most amino acids are aliphatic compounds. 2 amino acids are representatives of the aromatic series and 2 - heterocyclic.

Amino acids can be divided, according to their properties, into basic, neutral and acidic. They differ in the number of amino and carboxyl groups in the molecule. Neutral - contain one amino and one carboxyl group (monoaminomonocarboxylic). Acidic have 2 carboxyl and one amino group (monoaminodicarboxylic), basic - 2 amino groups and one carboxyl (diaminomonocarboxylic).

1. Actually aliphatic can be called 5 amino acids. Glycine or glycocol (Gly),

when working with a computer - (G), - a-aminoacetic acid. It is the only optically inactive amino acid. Glycine is used for more than just protein synthesis. Its atoms are part of nucleotides, heme, it is part of an important tripeptide - glutathione.

Alanine (Ala), when working with a computer - (A) - a-aminopropionic acid. Alanine is often used in the body to synthesize glucose.

According to the structure, all amino acids, with the exception of glycine, can be considered as derivatives of alanine, in which one or more hydrogen atoms in the radical are replaced by various functional groups.

Valine (Val), when working with a computer (V) - aminoisovaleric acid. Leucine (Leu, L) - aminoisocaproic acid. Isoleucine (Ile, I) - a-amino-b-ethyl-b-methylpropionic acid. These three amino acids, having pronounced hydrophobic properties, play an important role in the formation of the spatial structure of the protein molecule.

2. Hydroxyamino acids. Serine (Ser, S) - a-amino-b-hydroxypropionic acid and threonine (Tre, T) - a-amino-b-hydroxybutyric acid play an important role in the processes of covalent modification of the protein structure. Their hydroxyl group easily interacts with phosphoric acid, which is necessary to change the functional activity of proteins.

3. Sulfur-containing amino acids. Cysteine ​​(Cis, C) - a-amino-b-thiopropionic acid. A special property of cysteine ​​is the ability to oxidize (in the presence of oxygen) and interact with another cysteine ​​molecule to form a disulfide bond and a new compound, cystine. This amino acid, due to the active -SH group, easily enters into redox reactions, protecting the cell from the action of oxidizing agents, participates in the formation of disulfide bridges that stabilize the structure of proteins, and is part of the active center of enzymes.

Methionine (Met, M) -a-amino-b-thiomethylbutyric acid. Performs the function of a donor of a mobile methyl group, necessary for the synthesis of biologically active compounds: choline, nucleotides, etc. It is a hydrophobic amino acid.

4. Dicarboxylic amino acids. Glutamic (Glu, E) - a-aminoglutaric acid and aspartic acid (Asp, D) - a-aminosuccinic acid. These are the most common amino acids in animal proteins. With additional carboxyl group in the radical, these amino acids contribute to ionic interaction, give a charge to the protein molecule. These amino acids can form amides.

5. Amides of dicarboxylic amino acids. Glutamine (Gln, Q) and asparagine (Asn, N). These amino acids perform an important function in the neutralization and transport of ammonia in the body. The amide bond in their composition partially has a double character. Due to this, the amide group has a partial positive charge and will not dissociate.

6. Cyclic amino acids have an aromatic or heterocyclic ring in their radical. Phenylalanine (Phen, F) - a-amino-b-phenylpropionic acid. Tyrosine (Tyr, Y) - a-amino-b-paraoxyphenyl-propionic acid. These 2 amino acids form an interconnected pair that perform important functions in the body, among which should be noted their use by cells for the synthesis of a number of biologically active substances (adrenaline, thyroxine).

Tryptophan (Three, W) -a-amino-b-indolylpropionic acid. Used for the synthesis of vitamin PP, serotonin, pineal hormones.

Histidine (His, H) - a-amino-b-imidazolylpropionic acid. It can be used in the formation of histamine, which regulates the permeability of tissues and manifests its effect in allergies.

7. Diaminomonocarboxylic amino acids. Lysine (Liz, K) - diaminocaproic acid. Arginine (Arg, R) - a-amino-b-guanidine-valeric acid. These amino acids have an additional amino group that gives basic properties to proteins containing many of these amino acids. The formation of arginine is part of the metabolic pathway for the detoxification of ammonia (urea synthesis).

8. Imino acid - proline (Pro, P). It differs from other amino acids in structure. Its radical forms a single cyclic structure with the a-amino group. Due to this feature, no rotation is possible around the bond between the a-amino group and the a-carbon atom. All other amino acids have the ability to rotate around this bond. In addition, proline contains a secondary amino group (only one hydrogen atom is bonded to the nitrogen nitrogen), which differs in its chemical characteristics from the primary amino group (-NH 2) as part of other amino acids. A special place is given to this amino acid in the structure of collagen, where proline, in the process of collagen synthesis, can turn into hydroxyproline.

Abbreviations for amino acids are given in brackets, which are formed from the first three letters of their trivial name. IN Lately single-letter symbols are also used to record the primary structure, which is important when using a computer in working with proteins.

Amino acids (AA) - organic molecules that consist of a basic amino group (-NH 2), an acidic carboxyl group (-COOH), and an organic R radical (or side chain) that is unique to each AA

Amino acid structure

Functions of amino acids in the body

Examples of biological properties of AA. Although there are more than 200 different AAs found in nature, only about one tenth of them are incorporated into proteins, others have other biological functions:

• They are the building blocks of proteins and peptides
• Precursors of many biologically important molecules derived from AA. For example, tyrosine is a precursor of the hormone thyroxin and the skin pigment melanin, tyrosine is also a precursor of the compound DOPA (dioxy-phenylalanine). It is a neurotransmitter for impulse transmission nervous system. Tryptophan is a precursor of vitamin B3 - nicotinic acid
• Sources of sulfur - sulfur-containing AK.
• AA are involved in many metabolic pathways, such as gluconeogenesis - the synthesis of glucose in the body, the synthesis of fatty acids, etc.

Depending on the position of the amino group relative to the carboxyl group, AA can be alpha, α-, beta, β- and gamma, γ.

The alpha amino group is attached to the carbon adjacent to the carboxyl group:

The beta-amino group is located on the 2nd carbon from the carboxyl group

Gamma - amino group on the 3rd carbon from the carboxyl group

Only alpha-AA is included in the composition of proteins

General properties of alpha-AA proteins

1 - Optical activity - a property of amino acids

All AAs, with the exception of glycine, exhibit optical activity, since contain at least one asymmetric carbon atom (chiral atom).

What is an asymmetric carbon atom? This is a carbon atom that has four different chemical substituents attached to it. Why does glycine not exhibit optical activity? Its radical has only three different substituents, i.e. alpha carbon is not asymmetric.

What does optical activity mean? This means that AA in solution can be present in two isomers. Dextrorotatory isomer (+), which has the ability to rotate the plane of polarized light to the right. Left-handed isomer (-), which has the ability to rotate the plane of polarization of light to the left. Both isomers can rotate the plane of polarization of light by the same amount, but in the opposite direction.

2 - Acid-base properties

As a result of their ability to ionize, the following equilibrium of this reaction can be written:

R-COOH<------->R-C00-+H+

R- NH 2<--------->R-NH3+

Since these reactions are reversible, this means that they can act as acids (forward reaction) or as bases (reverse reaction), which explains the amphoteric properties of amino acids.

Zwitter ion - AK property

All neutral amino acids at a physiological pH value (about 7.4) are present as zwitterions - a non-protonated carboxyl group and a protonated amino group (Fig. 2). In solutions more basic than the amino acid isoelectric point (IEP), the amino group -NH3 + in AA donates a proton. In a solution more acidic than IET AA, the carboxyl group -COO - in AA accepts a proton. Thus, AA sometimes behaves like an acid, at other times like a base, depending on the pH of the solution.

Polarity as a general property of amino acids

At physiological pH, AAs are present as zwitter ions. The positive charge is carried by the alpha-amino group, and the negative charge is carboxylic. Thus, two opposite charges are created at both ends of the AA molecule, the molecule has polar properties.

The presence of an isoelectric point (IEP) is a property of amino acids

The pH value at which the net electrical charge of an amino acid is zero and therefore cannot move in an electric field is called IEP.

The ability to absorb in ultraviolet light is a property of aromatic amino acids

Phenylalanine, histidine, tyrosine and tryptophan absorb at 280 nm. On fig. the values ​​of the molar extinction coefficient (ε) of these AAs are shown. In the visible part of the spectrum, amino acids do not absorb, therefore, they are colorless.

AA can be present in two variants of isomers: L-isomer and D- isomers that are mirror images and differ in the arrangement of chemical groups around the α-carbon atom.

All amino acids in proteins are in the L-configuration, L-amino acids.

Physical properties of the amino acid

Amino acids are mostly water soluble due to their polarity and the presence of charged groups. They are soluble in polar and insoluble in non-polar solvents.

AAs have a high melting point, reflecting the presence of strong bonds that support their crystal lattice.

General properties of AK is common to all AK and in many cases are determined by the alpha-amino group and alpha-carboxyl group. AAs also have specific properties that are dictated by their unique side chain.