Amino Acids - Aminosan

Japanese: アミノ酸 - あみのさん

A general term for organic compounds that have an amino group and a carboxyl group in one molecule. When proteins are completely hydrolyzed with hydrochloric acid or other agents, ammonia and free amino acids are obtained; amino acids are the basic building blocks of proteins that control all life phenomena. Most amino acids isolated from proteins are α (alpha)-amino acids, in which the amino group and carboxyl group are bonded to the same carbon atom, and have the general formula R-CHNH ₂ -COOH. R represents an aliphatic, aromatic, or heterocyclic substituent. In addition, amino acids are named β (beta)-amino acids, γ (gamma)-amino acids, δ (delta)-amino acids, etc., as the amino group moves successively to the adjacent carbon atom. Generally, amino acids refer to α-amino acids.

The first amino acid discovered was asparagine. In 1806, French chemists Vauclin and Robiquet isolated new crystals from asparagus sprouts and named them asparagine. The first amino acid to be isolated from protein hydrolysates was French chemist H. Braconnot (1780-1855), who in 1820 decomposed glue, meat, wool, etc. with sulfuric acid and isolated glycine from glue and leucine from meat and wool. Over the next 130 years, until threonine was discovered by American biochemists Mayer and WC Rose (1887-1985) in 1936, 22 major amino acids were discovered. In addition to these, various amino acids have been discovered in nature as components of peptides and special proteins of animals, plants, and microorganisms, or as free amino acids, and the total number of amino acids has reached more than 360.

[Hideo Okazaki]

Types and properties

Amino acids generally have optical isomers, except for glycine, but all amino acids in proteins have the same configuration of the carboxyl and amino groups relative to the α-carbon, that is, the L-form. However, D-amino acids also occur naturally and are abundant in the cell walls of certain microorganisms. Many peptides containing D-amino acids exhibit strong antibacterial or toxic effects, such as polypeptide antibiotics such as gramicidin and bacitramin.

The main amino acids that make up proteins are broadly classified into aliphatic amino acids, aromatic amino acids, and heterocyclic amino acids. Among aliphatic amino acids, monoamino monocarboxylic acids include glycine, alanine, valine, leucine, and isoleucine, hydroxymonoamino monocarboxylic acids include serine and threonine, monoamino dicarboxylic acids and amides include aspartic acid, glutamic acid, asparagine, and glutamine, diamino monocarboxylic acids include lysine, hydroxylysine, and arginine, and sulfur-containing amino acids include cysteine, cystine, and methionine. Aromatic amino acids include phenylalanine and tyrosine, and heterocyclic amino acids include tryptophan, histidine, proline, and hydroxyproline.

Other relatively important amino acids found in nature include β-alanine, γ-aminobutyric acid, ornithine, citrulline, homoserine, triiodotyrosine, thyroxine, and dioxyphenylalanine.

Of the approximately 20 major amino acids, the essential amino acids that cannot be synthesized in the body and must be obtained from food are valine, leucine, isoleucine, methionine, threonine, lysine, phenylalanine, and tryptophan for adults, and histidine is also essential for infants, with the rest being non-essential amino acids. Non-essential amino acids can be synthesized in the body from essential amino acids by transaminases, but non-essential amino acids such as glutamine, asparagine, alanine, and proline undergo transamination (a reaction in which an amino group is transferred) much more easily to synthesize other non-essential amino acids.

In general, amino acids are white crystals, relatively stable substances with high melting points, but decomposition makes it difficult to show a clear melting point. Their molecular weights range from 75 for glycine to 204 for tryptophan, with an average molecular weight of 142. In terms of solubility in water, cystine and tyrosine are poorly soluble, while aspartic acid and glutamic acid are somewhat poorly soluble and generally soluble. However, aspartic acid and glutamic acid are both easily soluble in their hydrochloride salts. Amino acids are zwitterions, losing hydrogen ions when alkali is added and gaining hydrogen ions when acid is added. For this reason, aqueous solutions of amino acids have a buffering effect that resists changes in pH and have their own isoelectric points. The isoelectric points of aspartic acid and glutamic acid are about 3, arginine, lysine, and hydroxylysine are 9 to 10, and other amino acids are about 6.

Amino acids exhibit reactions specific to the reactive groups in each molecule, in addition to all reactions specific to the carboxyl and amino groups. The ninhydrin reaction, which is also used to detect and confirm amino acids, is the most important color reaction of the former type, and although some show a yellow color, it usually shows a blue-purple color. In addition, the latter color reactions of specific amino acids that are commonly used as analytical tools include the Millon reaction (tyrosine), the Sakaguchi reaction (arginine), the nitroprusside test (cysteine), the Ehrlich reaction (tryptophan), and the Pauli reaction (histidine, tyrosine).

Common reactions of amino acids include reactions with nitrous acid, ninhydrin, hydrogen peroxide, and glycerin to produce oxyacids, aldehydes, ketoacids, and amines, respectively. Although amino acids are not susceptible to reducing agents, their esters can be easily reduced with sodium amalgam or hydrogen to the corresponding aldehydes or alcohols.

In addition to the somewhat classical method of applying the principles of ninhydrin reaction and ion exchange chromatography, the analytical and quantitative analysis of amino acids has recently become possible by combining fluorescent labeling and high performance liquid chromatography, making it possible to perform microanalysis on the order of picomole (10 ^-12 mol), and this method has come to be widely used in clinical diagnosis. Significant increases or decreases in amino acid levels in urine or blood are often associated with abnormal brain development, and as many as 40 types of amino acid metabolic disorders have been reported.

[Hideo Okazaki]

Manufacturing method

In addition to the method of hydrolyzing natural proteins, chemical synthesis, fermentation, and enzyme methods are currently used. Recently, amino acids, like vitamins, are widely used in nutritional supplements, seasonings, and animal feed enrichment, and the demand for amino acids has been remarkable. In Japan, in particular, the amino acid manufacturing industry has made unique developments ahead of Europe and the United States. In 1908 (Meiji 41), Kikunae Ikeda discovered that the umami component of kelp is glutamic acid, and it was initially produced by hydrolyzing gluten or soybean protein, but in 1957 (Showa 32), Kinoshita Shukuro et al. announced a microorganism that produces large amounts of L-glutamic acid directly from glucose, and various amino acids other than glutamic acid have been produced by fermentation. All amino acids obtained by this hydrolysis and fermentation are L-amino acids, but amino acids obtained by synthesis are a mixture of D- and L-amino acids, and research on optical resolution and racemization has been actively promoted. In other words, the enzymatic method involves carrying out this racemization using a microbial enzyme (racemase), making it possible to produce large quantities of L-amino acids.

Currently, with the exception of a few special amino acids, all are produced industrially using chemical synthesis, fermentation, and enzymatic methods. As of 2005, the world's monosodium glutamate production capacity was 1.67 million tons, of which Japan's production capacity was only about 30,000 tons, with a total of 77,000 tons imported, mainly from Vietnam, Indonesia, South Korea, Taiwan, etc.

[Hideo Okazaki]

Relationship with the human body

There are about 20 types of amino acids obtained from food proteins, and they are α-amino acids with an amino group on the carbon next to the carboxyl group. Amino acids are classified as optical isomers of D-type and L-type, but naturally occurring amino acids are L-type. Proteins are broken down into amino acids in the digestive tract by digestive enzymes such as pepsin, trypsin, and chymotrypsin. Most are absorbed in the form of amino acids, but a small portion is absorbed as small peptides. Amino acids absorbed from the intestinal tract enter the liver through the portal vein, where they are synthesized into plasma proteins. Some amino acids enter the blood as they are, and together with plasma proteins, they become a source of tissue proteins. Amino acids supplied from food and free amino acids supplied from body tissues mix together to form an amino acid pool, which constantly renews the body's functions by repeatedly synthesizing and decomposing body proteins. Body proteins are synthesized from amino acids, and the amino acids that are particularly important in this process are called essential amino acids. As mentioned above, there are eight types of essential amino acids for adults and nine types for infants. The ratio of these amino acids is important, and it is desirable to supply them in a nutritionally appropriate ratio, and the required amount is indicated. This ratio of essential amino acids is called the amino acid balance. Some food proteins, especially plant proteins, lack essential amino acids, and the nutritional value is improved by adding lysine, methionine, tryptophan, etc. This is called the amino acid supplement effect.

Amino acids are industrially produced by synthesis and fermentation, and are added to livestock feed to increase its nutritional value. It is also effective to enrich bread with lysine. Amino acids are used in food processing to improve taste and aroma.

[Miyazaki Motoyoshi]

[Reference item] | Amino group | Carboxy group

Structures and Abbreviations of Major Protein-Constituent Amino Acids - Aliphatic Amino Acids (1)
Note: Red indicates the position of the amino group. From "Biochemical Data Book 1" edited by the Japanese Biochemical Society ©Shogakukan