Amino acids are heterofunctional compounds that necessarily contain two functional groups: an amino group - NH 2 and a carboxyl group -COOH, linked to a hydrocarbon radical. The general formula of the simplest amino acids can be written as follows:

Since amino acids contain two different functional groups that influence each other, the characteristic reactions differ from the characteristic reactions of carboxylic acids and amines.

Amino acid properties

The amino group - NH 2 determines the basic properties of amino acids, since it is capable of attaching a hydrogen cation to itself by the donor-acceptor mechanism due to the presence of a free electron pair at the nitrogen atom.

The -COOH (carboxyl group) group determines the acidic properties of these compounds. Therefore, amino acids are amphoteric organic compounds. They react with alkalis as acids:

With strong acids - like amine bases:

In addition, the amino group in the amino acid interacts with its constituent carboxyl group, forming an internal salt:

The ionization of amino acid molecules depends on the acidic or alkaline nature of the medium:

Since amino acids in aqueous solutions behave like typical amphoteric compounds, in living organisms they play the role of buffer substances that maintain a certain concentration of hydrogen ions.

Amino acids are colorless crystalline substances that melt with decomposition at temperatures above 200 ° C. They are soluble in water and insoluble in ether. Depending on the radical R-, they can be sweet, bitter or tasteless.

Amino acids are divided into natural (found in living organisms) and synthetic. Among natural amino acids (about 150), proteinogenic amino acids (about 20) are distinguished, which are part of proteins. They are L-shaped. About half of these amino acids belong to irreplaceable, because they are not synthesized in the human body. Indispensable are acids such as valine, leucine, isoleucine, phenylalanine, lysine, threonine, cysteine, methionine, histidine, tryptophan. These substances enter the human body with food. If their quantity in food is insufficient, the normal development and functioning of the human body is disrupted. In some diseases, the body is not able to synthesize some other amino acids. So, with phenylketonuria, tyrosine is not synthesized. The most important property of amino acids is the ability to enter into molecular condensation with the release of water and the formation of an amide group -NH-CO-, for example:

The high molecular weight compounds obtained as a result of such a reaction contain a large number of amide fragments and therefore are called polyamides.

These, in addition to the aforementioned synthetic nylon fiber, include, for example, the enant, which is formed during the polycondensation of aminoenanthic acid. Amino acids with amino and carboxyl groups at the ends of the molecules are suitable for the production of synthetic fibers.

Alpha amino acid polyamides are called peptides... Depending on the number of amino acid residues, dipeptides, tripeptides, polypeptides... In such compounds, the —NH — CO— groups are called peptide groups.

In humans, the main method of deamination is oxidative deamination... There are two variants of oxidative deamination: directand indirect.

Direct oxidative deamination

Direct deamination is catalyzed by a single enzyme, resulting in NH 3 and keto acid. Direct oxidative deamination can take place in the presence of oxygen (aerobic) and does not require oxygen (anaerobic).

1. Aerobic direct oxidative deamination catalyzed by D-amino acid oxidases ( D-oxidase) as a coenzyme using FAD, and L-amino acid oxidases ( L-oxidase) with coenzyme FMN... In the human body, these enzymes are present, but practically inactive.

Reaction catalyzed by D- and L-amino acid oxidases

2. Anaerobic direct oxidative deamination exists only for glutamic acid, catalyzed only glutamate dehydrogenaseconverting glutamate to α-ketoglutarate. The enzyme glutamate dehydrogenase is present in the mitochondria of all cells in the body (except muscle). This type of deamination is closely related to amino acids and forms a process with it transdeamination (see below).

Direct oxidative deamination reaction
glutamic acid

Indirect oxidative deamination (transdeamination)

Indirect oxidative deamination includes 2 stages and actively goes in all cells of the body.

The first stage consists in the reversible transfer of the NH 2 -group from an amino acid to a keto acid with the formation of a new amino acid and a new keto acid with the participation of enzymes aminotransferase... This transfer is called and its mechanism is rather complicated.

As an acceptor keto acid ("keto acid 2") in the body is usually used α-ketoglutaric acidwhich turns into glutamate("amino acid 2").

Scheme of the transamination reaction

As a result of transamination, free amino acids lose their α-NH 2 groups and are converted into the corresponding keto acids. Further, their ketoskeleton catabolizes by specific pathways and is involved in the tricarboxylic acid cycle and tissue respiration, where it burns down to CO2 and H2O.

If necessary (eg fasting), the carbon skeleton of glucogenic amino acids can be used in the liver to synthesize glucose in gluconeogenesis. In this case, the amount of aminotransferases in the hepatocyte increases under the influence of glucocorticoids.

The second stage consists in the cleavage of the amino group from amino acid 2 - deamination.

Because in the body, the collector of all amino acid amino groups is glutamic acid, then only it undergoes oxidative deamination with the formation of ammonia and α-ketoglutaric acid. This stage is carried out glutamate dehydrogenase , which is found in the mitochondria of all cells of the body, except muscle cells.

Given the close relationship of both stages, indirect oxidative deamination is called transdeamination.

Scheme of both stages of transdeamination

If the reaction of direct deamination occurs in the mitochondria of the liver, ammonia is used to synthesize urea, which is subsequently removed in the urine. In the epithelium of the renal tubules, the reaction is necessary for the removal of ammonia in the process of ammoniogenesis.

Since NADH is used in the respiratory chain and α-ketoglutarate is involved in the CTX reaction, the reaction is activated when there is a lack of energy and is inhibited excess ATP and NADH.

Role of transamination and transdeamination

Reactions transamination:

• are activated in the liver, muscles and other organs when an excessive amount of certain amino acids enters the cell - in order to optimize their ratio,
• provide the synthesis of nonessential amino acids in the cell in the presence of their carbon skeleton (ketoanalogue),
• begin when the use of amino acids for the synthesis of nitrogen-containing compounds (proteins, creatine, phospholipids, purine and pyrimidine bases) is discontinued - with the aim of further catabolism of their nitrogen-free residue and energy production,
• are necessary for intracellular starvation, for example, with hypoglycemia of various origins - for the use of nitrogen-free amino acid residue in liverfor

23.6.1. Decarboxylation of amino acids - cleavage of the carboxyl group from the amino acid with the formation of CO2. The products of amino acid decarboxylation reactions are biogenic amines participating in the regulation of metabolism and physiological processes in the body (see table 23.1).

Table 23.1

Biogenic amines and their precursors.

Decarboxylation reactions of amino acids and their derivatives catalyze decarboxylase amino acids. Coenzyme - pyridoxal phosphate (a derivative of vitamin B6). The reactions are irreversible.

23.6.2. Examples of decarboxylation reactions. Some amino acids are directly decarboxylated. Decarboxylation reaction histidine :

Histamine has a powerful vasodilating effect, especially of the capillaries in the focus of inflammation; stimulates gastric secretion of both pepsin and hydrochloric acid, and is used to study the secretory function of the stomach.

Decarboxylation reaction glutamate :

GABA - inhibitory mediator in the central nervous system.

A number of amino acids undergo decarboxylation after preoxidation. Hydroxylated product tryptophan turns into serotonin:

Serotonin formed mainly in the cells of the central nervous system, has a vasoconstrictor effect. Participates in the regulation of blood pressure, body temperature, respiration, renal filtration.

Hydroxylated product tyrosine goes into dopamine:

Dopamine serves as a precursor to catecholamines; is an inhibitory type mediator in the central nervous system.

Thiogroup cysteine oxidized to a sulfo group, the product of this reaction is decarboxylated to form taurine:

Taurine formed mainly in the liver; participates in the synthesis of paired bile acids (taurocholic acid).

21.5.3. Biogenic amine catabolism. In organs and tissues, there are special mechanisms that prevent the accumulation of biogenic amines. The main way of inactivation of biogenic amines - oxidative deamination with the formation of ammonia - is catalyzed by mono- and diamine oxidases.

Monoamine oxidase (MAO) - FAD-containing enzyme - carries out the reaction:

The clinic uses MAO inhibitors (nialamide, pyrazidol) to treat depressive conditions.

Redox processes with the participation of amino acids.

These processes take place in the organisms of plants and animals. There are compounds that are capable of either releasing hydrogen or absorbing it (attaching). In biological oxidation, two hydrogen atoms are split off, and in biological reduction, two volumes of hydrogen are added. Consider this with cysteine \u200b\u200band cystine as examples.

HS NH 2 OH -2H S NH 2 OH

HS NH 2 OH + 2H S NH 2 OH

CH 2 - CH - C \u003d O CH 2 - CH - C \u003d O

cysteine \u200b\u200bcystine

reduced form oxidized form

Two molecules of cystine, losing two hydrogen atoms, form an oxidized form - cysteine. This process is reversible, when two hydrogen atoms are attached to cystine, cysteine \u200b\u200bis formed - a reduced form. The redox process proceeds in a similar way using the example of a tripeptide - glutathione, which consists of three amino acids: glutamic, glycine and cysteine.

O \u003d C - NH - CH - CH 2 - SH O \u003d C - NH - CH - CH 2 - S - S -CH 2 - CH - NH - C \u003d O

CH 2 C \u003d O -2Н CH 2 C \u003d O C \u003d O CH 2

CH 2 NH + 2Н CH 2 NH NH CH 2

CH - NH 2 CH 2 glycine CH - NH 2 CH 2 CH 2 CH - NH 2

C \u003d O C \u003d O C \u003d O C \u003d O C \u003d O C \u003d O

OH OH OH OH OH OH

(2 molecules)

tripeptide reduced form hexapeptide - oxidized form

During oxidation, 2 hydrogen atoms are split off and two molecules of glutathione are combined and the tripeptide is converted into a hexapeptide, that is, it is oxidized.

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