“Amino acids structure, classification, properties, biological role. The amino acids arginine and lysine make up the Impact of peptides on the quality of life of an ordinary person
General characteristics (structure, classification, nomenclature, isomerism).
The main structural unit of proteins are a-amino acids. There are approximately 300 amino acids found in nature. 20 different a-amino acids were found in proteins (one of them, proline, is not amino-, A imino acid). All other amino acids exist in a free state or as part of short peptides, or complexes with other organic substances.
a-Amino acids are derivatives of carboxylic acids in which one hydrogen atom is replaced by an amino group (–NH2) at the a-carbon atom, for example:
Amino acids differ in the structure and properties of the R radical. The radical can represent fatty acid residues, aromatic rings, and heterocycles. Thanks to this, each amino acid is endowed with specific properties that determine the chemical, physical properties and physiological functions of proteins in the body.
It is thanks to amino acid radicals that proteins have a number of unique functions not characteristic of other biopolymers and have chemical individuality.
Amino acids with the b- or g-position of the amino group are much less common in living organisms, for example:
Classification and nomenclature of amino acids.
There are several types of classifications of amino acids that make up proteins.
A) One of the classifications is based on the chemical structure of amino acid radicals. Amino acids are distinguished:
1. Aliphatic - glycine, alanine, valine, leucine, isoleucine:
2. Hydroxyl-containing – serine, threonine:
4. Aromatic – phenylalanine, tyrosine, tryptophan:
5. With anion-forming groups in the side chains - aspartic and glutamic acids:
6. and amides of aspartic and glutamic acids - asparagine, glutamine.
7. The main ones are arginine, histidine, lysine.
8. Imino acid – proline
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B) The second type of classification is based on the polarity of the R groups of amino acids.
Distinguish polar and non-polar amino acids. Nonpolar radicals have nonpolar C–C, C–H bonds; there are eight such amino acids: alanine, valine, leucine, isoleucine, methionine, phenylalanine, tryptophan, proline.
All other amino acids are to the polar(in the R-group there are polar bonds C–O, C–N, –OH, S–H). The more amino acids with polar groups in a protein, the higher its reactivity. The functions of a protein largely depend on its reactivity. Enzymes are characterized by a particularly large number of polar groups. And vice versa, there are very few of them in such a protein as keratin (hair, nails).
B) Amino acids are also classified based on the ionic properties of R-groups(Table 1).
Sour(at pH = 7 the R-group can carry a negative charge) these are aspartic, glutamic acids, cysteine and tyrosine.
Basic(at pH=7 the R-group can carry a positive charge) - these are arginine, lysine, histidine.
All other amino acids belong to neutral (group R is uncharged).
Table 1 – Classification of amino acids based on polarity
R-group.
R-groups
Aspartic acid
Glutamic acid
4. Positively charged
R-groups
Histidine
G) Amino acids are divided according to the number of amine and carboxyl groups:
– to monoamine monocarboxylic containing one carboxyl and one amine group;
– monoaminodicarbonic(two carboxyl and one amine group);
– diaminomonocarboxylic(two amine and one carboxyl group).
E) According to their ability to be synthesized in the human and animal body, all amino acids are divided:
– to replaceable ones,
– irreplaceable,
– partially irreplaceable.
Essential amino acids cannot be synthesized in the human body and animals; they must be supplied with food. There are eight absolutely essential amino acids: valine, leucine, isoleucine, threonine, tryptophan, methionine, lysine, phenylalanine.
Partially essential - synthesized in the body, but in insufficient quantities, so they must be partially supplied with food. These amino acids are arganine, histidine, tyrosine.
Nonessential amino acids are synthesized in the human body in sufficient quantities from other compounds. Plants can synthesize all amino acids.
Isomerism
In the molecules of all natural amino acids (with the exception of glycine), the a-carbon atom has all four valence bonds occupied by various substituents; such a carbon atom is asymmetric, and is called chiral atom. As a result, solutions of amino acids have optical activity - they rotate the plane of plane-polarized light. The number of possible stereoisomers is exactly 2n, where n is the number of asymmetric carbon atoms. For glycine n = 0, for threonine n = 2. All other 17 protein amino acids contain one asymmetric carbon atom; they can exist in the form of two optical isomers.
As a standard when determining L And D- amino acid configurations, the configuration of stereoisomers of glyceraldehyde is used.
The location in the Fischer projection formula of the NH 2 group on the left corresponds to L-configurations, and on the right – D-configurations.
It should be noted that the letters L And D mean that a substance, in its stereochemical configuration, belongs to L or D row, regardless of the direction of rotation.
In addition to the 20 standard amino acids found in almost all proteins, there are also non-standard amino acids that are components of only some types of proteins - these amino acids are also called modified(hydroxyproline and hydroxylysine).
Receipt methods
– Amino acids are of extremely great physiological importance. Proteins and polypeptides are built from amino acid residues.
During hydrolysis of proteins Animal and plant organisms produce amino acids.
Synthetic methods for obtaining amino acids:
– The effect of ammonia on halogenated acids
– α-Amino acids are obtained the effect of ammonia on oxynitriles
It's no secret that in order to maintain vital functions at a high level, a person needs protein - a kind of building material for body tissues; Proteins contain 20 amino acids, the names of which are unlikely to mean anything to the average office worker. Every person, especially if we talk about women, has at least once heard about collagen and keratin - these are proteins that are responsible for the appearance of nails, skin and hair.
Amino acids - what are they?
Amino acids (or aminocarboxylic acids; AMK; peptides) are organic compounds consisting of 16% amines - organic derivatives of ammonium - which distinguishes them from carbohydrates and lipids. They participate in the biosynthesis of protein by the body: in the digestive system, under the influence of enzymes, all proteins supplied with food are destroyed to AMC. In total, there are about 200 peptides in nature, but only 20 basic amino acids are involved in the construction of the human body, which are divided into replaceable and essential; sometimes there is a third type - semi-replaceable (conditionally replaceable).
Nonessential amino acids
Replaceable amino acids are those that are both consumed in food and reproduced directly in the human body from other substances.
- Alanine is a monomer of biological compounds and proteins. It carries out one of the main pathways of glucogenesis, that is, it is converted into glucose in the liver, and vice versa. A highly active participant in metabolic processes in the body.
- Arginine is an amino acid that can be synthesized in the body of an adult, but is not capable of synthesis in the body of a child. Promotes the production of growth hormones and others. The only carrier of nitrogenous compounds in the body. Helps increase muscle mass and reduce fat mass.
- Asparagine is a peptide involved in nitrogen metabolism. During the reaction with the enzyme asparaginase, it splits off ammonia and turns into aspartic acid.
- Aspartic acid - takes part in the creation of immunoglobulin, deactivates ammonia. Necessary for malfunctions of the nervous and cardiovascular systems.
- Histidine - used for the prevention and treatment of gastrointestinal diseases; has positive dynamics in the fight against AIDS. Protects the body from the harmful effects of stress.
- Glycine is a neurotransmitter amino acid. Used as a mild sedative and antidepressant. Enhances the effect of some nootropic drugs.
- Glutamine - in large quantities Activator of tissue repair processes.
- Glutamic acid - has a neurotransmitter effect and also stimulates metabolic processes in the central nervous system.
- Proline is one of the components of almost all proteins. They are especially rich in elastin and collagen, which are responsible for skin elasticity.
- Serine is an amino acid that is found in neurons of the brain and also contributes to the release of large amounts of energy. It is a derivative of glycine.
- Tyrosine is a component of animal and plant tissues. Can be reproduced from phenylalanine by the action of the enzyme phenylalanine hydroxylase; the reverse process does not occur.
- Cysteine is one of the components of keratin, which is responsible for the firmness and elasticity of hair, nails, and skin. It is also an antioxidant. Can be produced from serine.
Amino acids that cannot be synthesized in the body are essential
Essential amino acids are those that cannot be generated in the human body and can only be supplied through food.
- Valine is an amino acid found in almost all proteins. Increases muscle coordination and reduces the body's sensitivity to temperature changes. Maintains the hormone serotonin at high levels.
- Isoleucine is a natural anabolic steroid that, through the process of oxidation, saturates muscle and brain tissue with energy.
- Leucine is an amino acid that improves metabolism. It is a kind of “builder” of protein structure.
- These three AMKs are part of the so-called BCAA complex, which is especially in demand among athletes. Substances in this group act as a source for increasing muscle mass, reducing fat mass and maintaining good health during particularly intense physical activity.
- Lysine is a peptide that accelerates tissue regeneration, the production of hormones, enzymes and antibodies. Responsible for the strength of blood vessels, found in muscle protein and collagen.
- Methionine - takes part in the synthesis of choline, the lack of which can lead to increased accumulation of fat in the liver.
- Threonine - gives elasticity and strength to tendons. It has a very positive effect on the heart muscle and tooth enamel.
- Tryptophan - supports emotional state, as it is converted into serotonin in the body. Indispensable for depression and other psychological disorders.
- Phenylalanine - improves the appearance of the skin by normalizing pigmentation. Supports psychological well-being by improving mood and bringing clarity to thinking.
Other methods for classifying peptides
Scientifically, the 20 essential amino acids are divided based on the polarity of their side chains, or radicals. Thus, four groups are distinguished: (but not having a charge), positively charged and negatively charged.
Non-polar are: valine, alanine, leucine, isoleucine, methionine, glycine, tryptophan, phenylalanine, proline. In turn, polar acids that have a negative charge include aspartic and glutamic acids. Polar, having a positive charge, are called arginine, histidine, lysine. Amino acids that have polarity but do not have a charge include cysteine, glutamine, serine, tyrosine, threonine, and asparagine.
20 amino acids: formulas (table)
Amino acid | Abbreviation | |
Asparagine | ||
Aspartic acid | ||
Histidine | ||
Glutamine | ||
Glutamic acid | ||
Isoleucine | ||
Methionine | ||
Tryptophan | ||
Phenylalanine | ||
Based on this, it can be noted that all 20 in the table above) contain carbon, hydrogen, nitrogen and oxygen.
Amino acids: participation in cell activity
Aminocarboxylic acids are involved in the biological synthesis of protein. Protein biosynthesis is the process of modeling a polypeptide (“poly” - many) chain of amino acid residues. The process takes place on the ribosome, an organelle inside the cell that is directly responsible for biosynthesis.
Information is read from a section of the DNA chain according to the principle of complementarity (A-T, C-G); when creating m-RNA (messenger RNA, or i-RNA - information RNA - identical concepts), the nitrogenous base thymine is replaced by uracil. Then, using the same principle, a transporting amino acid molecules to the place of synthesis is created. T-RNA is encoded by triplets (codons) (example: UAU), and if you know what nitrogenous bases a triplet is represented by, you can find out which amino acid it carries.
Food groups with the highest content of AMK
Dairy products and eggs contain important substances such as valine, leucine, isoleucine, arginine, tryptophan, methionine and phenylalanine. Fish and white meat have a high content of valine, leucine, isoleucine, histidine, methionine, lysine, phenylalanine, tryptophan. Legumes, grains and cereals are rich in valine, leucine, isoleucine, tryptophan, methionine, threonine, methionine. Nuts and various seeds will saturate the body with threonine, isoleucine, lysine, arginine and histidine.
Below is the amino acid content of some foods.
The largest amount of tryptophan and methionine can be found in hard cheese, lysine - in rabbit meat, valine, leucine, isoleucine, threonine and phenylalanine - in soy. When creating a diet based on maintaining normal BUN, you should pay attention to squid and peas, while the poorest in terms of peptide content are potatoes and cow's milk.
Lack of amino acids in vegetarianism
It is a myth that there are amino acids that are found exclusively in animal products. Moreover, scientists have found that plant protein is absorbed by the human body better than animal protein. However, when choosing vegetarianism as a lifestyle, it is very important to monitor your diet. The main problem is that one hundred grams of meat and the same amount of beans contain different amounts of BUN in percentage terms. At first, it is necessary to keep track of the amino acid content in the food consumed, then this should become automatic.
How many amino acids should you consume per day?
In the modern world, absolutely all food products contain the nutrients necessary for humans, so there is no need to worry: all 20 protein amino acids are safely supplied from food, and this amount is enough for a person who leads a normal lifestyle and at least slightly monitors his diet.
An athlete’s diet must be saturated with proteins, because without them it is simply impossible to build muscle mass. Physical exercise leads to a colossal consumption of amino acid reserves, so professional bodybuilders are forced to take special supplements. With intensive building of muscle relief, the amount of protein can reach up to one hundred grams of protein per day, but such a diet is not suitable for daily consumption. Any food supplement implies instructions containing different AMKs in doses, which must be read before using the drug.
The influence of peptides on the quality of life of an ordinary person
The need for proteins is present not only among athletes. For example, the proteins elastin, keratin, and collagen affect the appearance of hair, skin, nails, as well as the flexibility and mobility of joints. A number of amino acids affect the body, maintaining fat balance at an optimal level, providing sufficient energy for everyday life. After all, in the process of life, even with the most passive lifestyle, energy is expended, at least for breathing. In addition, cognitive activity is also impossible when there is a lack of certain peptides; maintaining the psycho-emotional state is carried out, among other things, by AMK.
Amino acids and sports
The diet of professional athletes involves a perfectly balanced diet that helps maintain muscle tone. Designed specifically for those athletes who are working on gaining muscle mass, they make life much easier.
As previously written, amino acids are the main building blocks of proteins necessary for muscle growth. They are also able to speed up metabolism and burn fat, which is also important for beautiful muscle definition. When training hard, it is necessary to increase your BUN intake due to the fact that they increase the rate of muscle building and reduce post-workout pain.
The 20 amino acids in proteins can be consumed both as part of aminocarbon complexes and from food. If you choose a balanced diet, then you need to take into account absolutely all grams, which is difficult to implement when the day is very busy.
What happens to the human body when there is a lack or excess of amino acids
The main symptoms of amino acid deficiency are: poor health, lack of appetite, brittle nails, increased fatigue. Even with a lack of BUN alone, a huge number of unpleasant side effects occur, which significantly impair well-being and productivity.
Oversaturation with amino acids can lead to disruptions in the functioning of the cardiovascular and nervous systems, which, in turn, is no less dangerous. In turn, symptoms similar to food poisoning may appear, which also does not entail anything pleasant.
You need to know moderation in everything, so maintaining a healthy lifestyle should not lead to an excess of certain “useful” substances in the body. As the classic wrote, “the best is the enemy of the good.”
In the article we looked at the formulas and names of all 20 amino acids; the table of the content of the main AMAs in products is given above.
Lecture No. 3Topic: “Amino acids - structure, classification, properties, biological role”
Amino acids are 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.
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.
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.
Amino acids are classified according to their structural characteristics.
1. Depending on the relative position of the 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 properties of 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.
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. The total number of naturally occurring amino acids exceeds 100, 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). The general principle of constructing heterocyclic amino acids is the same as 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 in the form of 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 a number of specific properties of 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), esters are obtained in the form of hydrochlorides in good yield. 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
blue copper(11) salts (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 the quantitative analysis of 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 is a reversible process of 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 a mixture 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.
The most important factor determining animal productivity 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. Deficiency of this amino acid also leads to sterility, increased excitability, convulsions, cataract formation, negative nitrogen balance and loss of live weight. Tryptophan, being a precursor (provitamin) of nicotinic acid, prevents the development of pellagra.
