Amino Acids


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Amino acids

Organic compounds possessing one or more basic amino groups and one or more acidic carboxyl groups. Of the more than 80 amino acids which have been found in living organisms, about 20 serve as the building blocks for the proteins.

All the amino acids of proteins, and most of the others which occur naturally, are α-amino acids, meaning that an amino group (—NH2) and a carboxyl group (—COOH) are attached to the same carbon atom. This carbon (the α carbon, being adjacent to the carboxyl group) also carries a hydrogen atom; its fourth valence is satisfied by any of a wide variety of substitutent groups, represented by the letter R in the structural formula below.

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In the simplest amino acid, glycine, R is a hydrogen atom. In all other amino acids, R is an organic radical; for example, in alanine it is a methyl group (—CH3), while in glutamic acid it is an aliphatic chain terminating in a second carboxyl group (—CH2—CH—COOH). Chemically, the amino acids can be considered as falling roughly into nine categories based on the nature of R (see table).

Amino acids of proteins, grouped according to the nature of R
Amino acids R
Glycine Hydrogen
Alanine, valine, leucine, isoleucine Unsubstituted aliphatic chain
Serine, threonine Aliphatic chain bearing a hydroxyl group
Aspartic acid, glutamic acid Aliphatic chain terminating in an acidic carboxyl group
Asparagine, glutamine Aliphatic chain terminating in an amide group
Arginine, lysine Aliphatic chain terminating in a basic amino group
Cysteine, cystine, methionine Sulfur-containing aliphatic chain
Phenylalanine, tyrosine Terminates in an aromatic ring
Tryptophan, proline, histidine Terminates in a heterocyclic ring
*See articles on the individual amino acids listed in the table.

Occurrence

Amino acids occur in living tissues principally in the conjugated form. Most conjugated amino acids are peptides, in which the amino group of one amino acid is linked to the carboxyl group of another. Amino acids are capable of linking together to form chains of various lengths, called polypeptides. Proteins are polypeptides ranging in size from about 50 to many thousand amino acid residues. Although most of the conjugated amino acids in nature are proteins, numerous smaller conjugates occur naturally, many with important biological activity. The line between large peptides and small proteins is difficult to draw, with insulin (molecular weight = 7000; 50 amino acids) usually being considered a small protein and adrenocorticotropic hormone (molecular weight = 5000; 39 amino acids) being considered a large peptide.

Free amino acids are found in living cells, as well as the body fluids of higher animals, in amounts which vary according to the tissue and to the amino acid. The amino acids which play key roles in the incorporation and transfer of ammonia, such as glutamic acid, aspartic acid, and their amides, are often present in relatively high amounts, but the concentrations of the other amino acids of proteins are extremely low, ranging from a fraction of a milligram to several milligrams per 100 g wet weight of tissue. The presence of free amino acids in only trace amounts points to the existence of extraordinarily efficient regulation mechanisms. Each amino acid is ordinarily synthesized at precisely the rate needed for protein synthesis.

General properties

The amino acids are characterized physically by the following: (1) the pK1, or the dissociation constant of the various titratable groups; (2) the isoelectric point, or pH at which a dipolar ion does not migrate in an electric field; (3) the optical rotation, or the rotation imparted to a beam of plane-polarized light (frequently the D line of the sodium spectrum) passing through 1 decimeter of a solution of 100 grams in 100 milliliters; and (4) solubility.

Since all of the amino acids except glycine possess a center of asymmetry at the α carbon atom, they can exist in either of two optically active, mirror-image forms, or enantiomorphs. All of the common amino acids of proteins appear to have the same configuration about the α carbon; this configuration is symbolized by the prefix L-. The opposite, generally unnatural, form is given the prefix D-. Some amino acids, such as isoleucine, threonine, and hydroxyproline, have a second center of asymmetry and can exist in four stereoisomeric forms.

At ordinary temperatures, the amino acids are white crystalline solids; when heated to high temperatures, they decompose rather than melt. They are stable in aqueous solution, and with few exceptions can be heated as high as 120°C (248°F) for short periods without decomposition, even in acid or alkaline solution. Thus, the hydrolysis of proteins can be carried out under such conditions with the complete recovery of most of the constituent free amino acids.

Biosynthesis

Since amino acids, as precursors of proteins, are essential to all organisms, all cells must be able to synthesize those they cannot obtain from their environment. The selective advantage of being able rapidly to shift from endogenous to exogenous sources of these compounds has led to the evolution of very complex and precise methods of adjusting the rate of synthesis to the available level of the compound. An immediately effective control is that of feedback inhibition. The biosynthesis of amino acids usually requires at least three enzymatic steps. In most cases so far examined, the amino acid end product of the biosynthetic pathway inhibits the first enzyme to catalyze a reaction specific to the biosynthesis of that amino acid. This inhibition is extremely specific; the enzymes involved have special sites for binding the inhibitor. This inhibition functions to shut off the pathway in the presence of transient high levels of the product, thus saving both carbon and energy for other biosynthetic reactions. When the level of the product decreases, the pathway begins to function once more.

The metabolic pathways by which amino acids are synthesized generally are found to be the same in all living cells investigated, whether microbial or animal. Biosynthetic mechanisms thus appear to have developed soon after the origin of life and to have remained unchanged through the divergent evolution of modern organisms.

Biosynthetic pathway diagrams reveal only one quantitatively important reaction by which organic nitrogen enters the amino groups of amino acids: the reductive amination of α-ketoglutaric acid to glutamic acid by the enzyme glutamic acid dehydrogenase. All other amino acids are formed either by transamination (transfer of an amino group, ultimately from glutamic acid) or by a modification of an existing amino acid. An example of the former is the formation of valine by transfer of the amino group from glutamic acid to α-ketoisovaleric acid; an example of the latter is the reduction and cyclization of glutamic acid to form proline.

Importance in nutrition

The nutritional requirement for the amino acids of protein can vary from zero, in the case of an organism which synthesizes them all, to the complete list, in the case of an organism in which all the biosynthetic pathways are blocked. There are 8 or 10 amino acids required by certain mammals; most plants synthesize all of their amino acids, while microorganisms vary from types which synthesize all, to others (such as certain lactic acid bacteria) which require as many as 18 different amino acids. See Protein metabolism

Amino Acids

 

a class of organic compounds with the combined properties of acids and amines—that is, containing an amino group (NH2) along with a carboxyl group (COOH). The various amino acids (α, β, γ, and others) are distinguished according to the position of the amino group in relation to the carboxyl group.

Amino acids play a large role in the life of organisms, since all protein substances are constructed of amino acids. With complete hydrolysis (splitting with the addition of water), all proteins decompose into free amino acids which play the role of monomers in the polymeric protein molecule. In the biosynthesis of protein the order and sequence of amino acids are assigned a genetic code which is recorded in the chemical structure of desoxyribonucleic acid. The 20 most important amino acids that enter into the composition of proteins correspond to the general formula RCH(NH2)COOH and belong to the α-amino acids. The β-amimo cids—RCH(NH2)CH2COOH—are also found in nature—for example, β-alanine, CH2(NH2)CH2COOH, which is a component of pantothenic acid. Amino acids may contain one NH2 group and one COOH group (monoaminocarboxylic acids), one NH2 group and two COOH groups (monoaminodicarboxylic acids), or two NH 2groups and one COOH group (diaminomonocarboxylic acids).

Some monoaminocarboxylic acids

glycine—NH2CH2COOH

alanine-CH3CH(NH2)COOH

cysteine—CH2(SH)CH(NH2)COOH

methionine—CH2(SCH3)CH2CH(NH2)COOH

valine—(CH3)2CHCH(NH2)COOH

Some monoaminodicarboxylic acids

aspartic acid—HOOC CH2CH(NH2)COOH

glutamic acid—HOOC(CH2)2CH(NH2)COOH

Some diaminomonocarboxylic acids

lysine—NH2CH2(CH2)3CH(NH2)COOH

arginine—NH2C (= NH)NH(CH2)3CH(NH2)COOH

Amino acids are colorless, crystalline substances, soluble in water; melting points are 220°-315°C. The high melting points of amino acids are related to the fact that their molecules are composed mainly of amphoteric (doubly charged) ions. For example, the structure of the simplest amino acid, glycine, may be expressed by the formula NH3CH2COO (not NH2CH2COOH). All natural amino acids except glycine contain asymmetrical carbon atoms, exist in optically active modifications, and, as a rule, belong to the L-series. Amino acids of the D-series are contained only in several antibiotics and in bacteria capsules.

Many plants and bacteria are able to synthesize all the amino acids they need from simple inorganic compounds. Most amino acids are synthesized in the bodies of humans and animals from ordinary nitrogen-free metabolic products and assimilated nitrogen. However, eight amino acids (valine, isoleucine, leucine, lysine, methionine, threonine, tryptophan, and phenylalanine) are essential—that is, they cannot be synthesized in the organisms of animals and man and must be obtained from food. The average daily adult requirement for each of the essential amino acids is approximately 1 g. When there is an insufficiency of these amino acids (most often tryptophan, lysine, and methionine) or if even one of them is lacking from the diet, the synthesis of proteins and of many other biologically important substances necessary for life is impossible. Histidine and arginine are synthesized in animal organisms, but only in limited, sometimes insufficient, quantities. Cysteine and tyrosine are formed only from their precursors, methionine and phenylalanine, respectively, and may become essential when there is a lack of these amino acids. Some amino acids may be synthesized in animal organisms from nitrogen-free precursors by the process of transamination, the transfer of an amino group from one amino acid to another. Amino acids are constantly used in the body for the synthesis and resynthesis of proteins and other substances—hormones, amines, alkaloids, coenzymes, pigments, and others. Excess amino acids undergo decomposition to the end products of metabolism (in humans and mammals to urea, carbon dioxide, and water), in the course of which is generated the energy necessary to the organism for its life processes. An intermediate step of this decomposition is usually deamination (most often oxidizing).

Amino acid derivatives of great practical interest include the lactam of €-norleucine—the stock for the manufacture of kapron.

Many methods are known for synthesizing amino acids—for example, the action of ammonia on halogen-displaced carboxylic acids:

RCHC1COOH + 2NH3 → RCHNH2COOH + NH4C1

the reduction of oximes and hydrazones by keto or aldehyde acids:

RC( = NOH)COOH → RCHNH2COOH

Several amino acids yield amino-rich proteins from the products of hydrolysis by means of adsorption on ion-exchange resins; in this way they produce glutamic acid from casein and the gluten of cereals, tyrosine from the fibroin of silk, arginine from the gelatin, and histidine from blood proteins. Several amino acids are produced synthetically—for example, methionine, lysine, and glutamic acid. Amino acids are also obtained in large quantities by microbiological synthesis. Body intake of essential amino acids is determined by the quantity and amino acid composition of food proteins. This must be taken into account in organizing proper public nutrition and in establishing allowances for various age and professional groups in the population. The requirement for dietary protein may be fully met through a mixture of amino acids. This is used in therapeutic nutrition.

Amino acids are used in medicine for the parenteral feeding of patients (that is, bypassing the gastrointestinal tract) with disorders of the digestive and other organs and also for treating liver disease, anemia, burns (methionine), and stomach ulcers (histidine); in neuropsychic illnesses (glutamic acid and such); in animal husbandry and in veterinary medicine for feeding (see below) and treatment of animals; and also in the microbiological, medical, and food industries.

Studies of the amino acid content of proteins and of amino acid metabolism are conducted by means of a number of color reactions (for example, the Ninhydrin reaction) and also by means of chromatography and special automatic amino acid analyzers.

Amino acids in the feeding of farm animals Rations for farm animals must contain all amino acids necessary to the animal, especially the essential acids. Therefore, present-day practice in organizing feeding has begun to take into account not only the overall quantity of protein in the fodder, as was done previously, but also the quantity of essential amino acids. Amino acid requirements. for various species of animals are not the same. In ruminants the microflora of the rumen are capable of synthesizing all the necessary amino acids from the ammonia produced in the breakdown of protein or nonprotein nitrogenous compounds—for example, urea. For these animals normalization of amino acids is not performed. However, in order to supplement the animals’ rations with nonprotein nitrogenous substances, urea is used. Young stock with as yet insufficiently developed rumina have some need for the essential amino acids. Swine and poultry rations must have balanced amino acid content. For this reason fodders are selected that complement each other in amino acid composition; synthetic amino acids produced by industry are also used. Synthetic amino acids are fed in a mixture with concentrates; it is more expedient to add them to a commercially prepared combined fodder. Excess amino acids have a negative influence on animal organisms.

REFERENCES

Meister, A. Biokhimiia aminokislot. Moscow, 1961. (Translated from English.)
Aminokislotnoe pitanie svinei i ptitsy. Moscow, 1963.
Zbarskii, B. I., I. I. Ivanov, and S. R. Mardashev. Biologicheskaia khimiia, 4th ed. Leningrad, 1965.
Popov, I. S. Aminokislotnyi sostav kormov, 2nd ed. Moscow, 1965.
“Obmen aminokislot.” Materialy Vsesoiuznoi konferentsii [13–17 okt. 1965]. Tbilisi, 1967.
Kretovich, V. L. Osnovy biokhimii rastenii, 4th ed. Moscow, 1964.

I. B. ZBARSKII and IA. F. KOMISSAROV

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