Amino acids

Amino acids are organic carboxylic acid, containing amine (-NH2) group. Depending on the provisions of the amine group on carboxyl (2nd, 3rd, 4th, etc. carbon atoms) distinguish respectively alpha-amino acids, b-A., '-A. and so on
Amino acids are contained in all cells of the body as in a free state, and in the form of a number of compounds, mainly in the composition of proteins.
Amino acid - a colorless crystalline substance. Most of them are well soluble in water; tyrosine, cystine, tryptophan are dissolved badly. Glycine and alanine have a sweet taste; leucine, isoleucine, tyrosine, tryptophan, valine - bitter; glutamic acid has characterized meat taste. All amino acids due to the presence of acid (carboxyl) and main (hydrated amine) groups are characterized by amphoteric properties and are in solution in the form of bipolar ions (empioni or zwitterionic):
At a certain pH called isoelectric point (see Ampholytes), not the same for different amino acids, dissociation carboxylic groups equal to the dissociation amine groups and the particle as a whole electrically neutral.
The greatest biological value are amino acids that comprise proteins. Such A. - over 20 . All but two (containing imine group-NH-) - Proline and hydroxyproline, are α A., i.e. contain amino group from neighboring with carboxyl group of atoms of carbon and have the General formula:

where R is the radical, not the same for different amino acids.
All natural A. except glycine, optically active [asymmetric alpha-carbon atom is indicated in the formula (1) an asterisk] and, as a rule, belong to the L-series (see Isomerism). However, as part of some natural products, especially bacterial origin, including antibiotics, are amino acids D-series. Amino acids D-series and L-series is almost not visible on chemical and physico-chemical properties. However, biologically against the enzymes they behave differently; as a rule, amino acids D-series differ from the amino acid L-number of more sweet taste.

Classification of amino acids is based on the composition and properties of the radical [see formula (1)]. There are A. aliphatic, aromatic and heterocyclic. In addition, there are monoaminooksidasy amino acids, containing one amino and carboxyl groups, monoaminooksidasy, amine containing one and two carboxyl group, and diaminomethylene containing two amino and carboxyl groups. Monoaminooksidasy A. have isoelectric point at pH of about 6. Monoaminooksidasy A. prevails acid dissociation. Diaminomethylene A., on the contrary, are mainly alkaline properties. Diaminoxydasy arginine and lysine with heterocyclic A. - histidine, also having the main character, sometimes called exonomy grounds (because of the basic properties and the availability of six carbon atoms each of these amino acids).
Depending on the presence of other radical groups or atoms, except amino and carboxyl, distinguish hydroxy-acids, sulfur-containing A. and amides monoaminooksidasy acids glutamine and asparagine. Classification, chemical composition and the most important properties of the 22 amino acids commonly found in proteins, shown in the table.
In addition to those listed in the table in organisms found other A., meeting in the composition of some proteins or other natural compounds or play a role in metabolism. So, derived lysine - oxidizing
isolated from hydrolyzed gelatin. N-acetylised and N-metallized recently discovered in the composition of the nuclear protein histone. From soy flour obtained canavanine (alpha-amino-V-guanidine-hydroxy-N-butyric acid). Canavanine similar to the properties of arginine and split the enzyme arginase with the formation of urea. The splitting of arginine along with urea is produced ornithine. Ornithine, adding one-balance and the amino group, constitutes another diaminophenoxyethanol acid - citrulline, which is further converted to arginine. These transformations are called ornitogalum cycle, play a critical role in the formation of urea in humans and mammals. In the free state and in the content of some natural products meet amino acids, containing methyl group attached to the amine nitrogen. Among them-known Sarkozy (N-methylglycyl), mitigation, methyltrithion, metallizing. Such A., as Norvaline and norlain obtained synthetically, but not found in proteins. From hydrolysates wool and from antibiotic of subtilin allocated serosoderjashchaya the amino acid lantinen [β,?'-thiodi-(alpha-aminopropionic) acid]:
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Amino acids react with nitrous acid, allocating free nitrogen and forming corresponding to the hydroxy acids. This reaction is used for the quantitative determination of nitrogen amino acids (amino nitrogen) in the blood and other biological fluids. Another important reagent in A. - ninhydrin (see) - gives when heated with a blue colouring.
Shade of color depends on the nature of amino acids and varies from blue to purple. Amino acids give yellow staining. Reaction with ninhydrin very sensitive and can detect A. both qualitatively and quantitatively. This reaction is widely used to determine the amino acid composition of proteins and free amino acids in biological material using chromatography on paper or on a column with synthetic resins. For the analysis of amino acids is also applied the method of gas chromatography - division derivatives A. in the form of gaseous products.
Individual A. give the typical reactions due to the radicals. Some of these reactions are used for detection of proteins. Such, for example, the reaction of Damkevica on tryptophan (violet color when heated with concentrated sulphuric acid in the presence of acetic acid). In this reaction is valid aldehyde group Glyoxylic acid in acetic acid in the form of impurity.
Very sensitive Sakaguchi reaction on arginine (red color in alkaline medium with hypobromite or hypochlorite and II-naphthol).
Phenolic group tyrosine gives red color when heated with salts of mercury in the presence of nitrous acid (reaction Millon).
The amino acids are Central in the nitrate exchange (see) and face many transformations related to their disintegration and use for the synthesis of proteins and other substances. A. subject to deamination (see), Tsaplya amine group, and decarboxylation (see), forming the corresponding amines (see).
A very important role in the exchange of amino acids plays the reactions of transamination (open A. E. Brownstein), consisting in the transfer of the amino group A. to ketoacid, resulting in another A. a Necessary component of the reactions of transamination taking place under the action of enzymes of aminotrans-time (minieres)are dicarboxylic amino acids. Due to this reaction dicarboxylic A. and their amides actively participate in the exchange of A. and form the link between the exchange A. and oxidative transformations of carbohydrates and fats. So, aspartic acid under the action of aspartate it ammonia and forms fumaric acid.
These amino acids and their amides are major sources of nitrogen in the synthesis of a number of biologically important substances in the formation of the final products of nitrogen metabolism, particularly urea.
Sulfur-containing A. in its currency is closely connected with hydroxy-amino acids. Sulfgidrile group share with o si groups in the process of peresushivaya. So, reacting with ' -hydroxy-alpha-minamas-lanai acid (homoserine), cysteine transfers its sulfhydryl group, resulting in homocysteine ('tio-alpha-amino-butyric acid):
As an intermediate product of the reaction is formed cystathionine. Homocysteine next attaches metal group and forms methionine. The reaction goes in both directions, and methionine is the main source of metal groups in methylation processes in the body.
The part of amino acids can be synthesized in the body, using nitrogen A. other Such A. call interchangeable. A., which are not synthesized in the body and to sustain life must come with food, called indispensable. An indispensable valine, isoleucine, leucine, lysine, methionine, threonine, tryptophan and phenylalanine. The need for each of these amino acids varies, but on average for an adult about 1 g every irreplaceable A. a day. The notion of irreplaceability A. to some extent conditional. So, for example, tyrosine is formed in the body only of phenylalanine and therefore becomes indispensable for lack of phenylalanine. On the other hand, the presence of tyrosine in the diet reduces the need for phenylalanine. Similar relationships exist between methionine and cysteine (cysteine).
Since the main source of amino acids in the diet are proteins, nitrogen balance, and intakes of essential A. in food are determined by the number (3) and amino acid composition of food proteins. Practically the most common lack of food three indispensable A. - tryptophan, lysine and methionine. The need for dietary protein can be completely covered by a mixture of amino acids. In medical practice it is of great importance at parenteral nutrition. Along with General impact on the growth and health status lack A. individual can give specific symptoms. So, the lack of methionine causes obesity and cirrhosis of the liver, lack tryptophan at low concentrations in food nicotinic acid (as tryptophan can turn into the last) lead to pellagra. The lack of valine animals causes neurotic symptoms, increased sensitivity to touch.

Many of A. used in veterinary medicine, technology, medicine and scientific research. Industrial obtaining amino acids based on their chemical synthesis, the allocation of protein hydrolysates and getting them with the help of microorganisms.
The addition of amino acids to the diet improves the quality, accessibility and gustatory properties. Especially often used glutamic acid, which gives the food a pleasant meat taste.
In blood the number of aminatta (nitrogen amino groups, free amino acids) is fairly constant and is the person in norm about 6-8 mg%. In erythrocytes content aminatta higher and varies in wide range. In plasma, it does not exceed 4-6 mg%. The definition of aminatta blood used in clinical practice. The content of aminatta in blood in violation of the functions of the liver kidneys. His sharp increase (hyperaminoacidaemia) is observed in toxicosis of pregnancy and with the increased protein breakdown (cachexia). Due to the ability of cells to actively focus amino acids. their content in tissues sometimes several times higher than in the blood. Blood plasma contains almost no peptides. Free A. dominated by glutamine, alanine, valine, leucine, glycine, tyrosine, and Proline, tryptophan and tyrosine.
Urine man distinguishes a rate of about 1 g of free amino acids per day. Large quantities A. in urine (aminoaciduria may occur when hyperaminoacidaemia, as well as some (usually due to heredity) violations transformations separate A. as persons with cystinuria poorly soluble cystine appears in the urine in the form of crystals. With specific violation of turning phenylalanine to tyrosine associated dementia (vinilpirrolidona mental retardation). Until recently, this hereditary disease was considered incurable. However, if the age of eating regularly add tyrosine, the lack of exchange are fully compensated and patients grow up normal. Increased allocation of histidine with urine (histidine) is often observed during pregnancy. Glutamic acid is used for therapeutic purposes in psychiatry. Its therapeutic effect is based on strengthening exchange and mostly on the ammonia by associating it and the formation of glutamine. Methionine also gives good results in obesity, cirrhosis and other liver diseases. Histidine is applied at stomach ulcers and other diseases of the gastrointestinal tract.
Too much of some amino acids with a lack of other A. can give toxic effects because of unbalanced protein nutrition. Cm. also Proteins.