Chapter 1
Non-Proteinogenic a-Amino Acids, Natural Origin, Biological Functions

a-Amino acids are an important class of physiologically and pharmacologically active compounds. There are more than 1000 different amino acids in microbial cells and plant tissues. However, only 26 of them are found in protein compositions, from which only 20 amino acids can be considered typical components of proteins.

In recent years, the need for significant amounts of a-amino acids has been steadily increasing due to their extensive use in biotechnology, medicine, food, microbiology, and other areas of science and technology [1, 2]. If in the past, the need for most of a-amino acids was met by obtaining them from protein hydrolysates or other natural sources, from the second half of the twentieth century microbiological and synthetic directions of obtaining a-amino acids have been intensively developed.

Selection of a particular method for producing amino acids is mainly determined based on the requirements to chemical and optical purity of the final products and the area of their further use.

Synthetic methods can be considered general only if starting materials necessary for the synthesis are readily available, and reaction conditions and experimental techniques at each stage of the synthesis are similar for all amino acids. However, this is not always possible because the side chains of amino acids can have diverse structures. In addition, the main drawback of the achiral methods of chemical synthesis is the formation of amino acids in the form of racemic mixtures that could be separated on their optically active antipodes by enzymatic or microbiological methods only in the case of protein a-amino acids. In connection with this, achiral chemical methods for amino acid synthesis have found a practical application only for the production of several protein a-amino acids.

Despite this, the current total production of a-amino acids worldwide is about half a million tons per year. A large-scale production of mainly protein amino acids is due to their wide use in medicine, agriculture (growth-stimulating food additives), and food industry (flavoring substances and preservatives). The practical importance of individual amino acids is proved by the scale of their biotechnological and chemical production: tryptophan is produced in the amount of 0.2-0.3 thousand tons, glycine at 7-10 thousand tons, lysine at about 50 thousand tons, methionine at 150-200 thousand tons, glutamic acid at more than 200 thousand tons per year, and so on.

Specifically, methionine is used in medicine for the treatment and prevention of hepatotoxicity and diabetes, while a mixture of methionine and cysteine is used for the treatment of different kinds of poisoning. A mixture of glycine and glutamic acid is used to control gastric acidity. Pure glutamic acid is used for the treatment of CNS disorders (epilepsy, psychosis in children with polio, and mental retardation), and its sodium salt as flavoring and preservative in food. Vitamin B3 (pantothenic acid), which contains a fragment of the nonprotein amino acid ß-alanine (3-aminopropionic acid) is used in polyneurites, dermatoses, bronchitis, venous ulcers. Nonprotein ?-aminobutyric acid, detected in mammalian brain in 1950, acts as a mediator in the transmission of nerve impulses. ?-Aminobutyric acid (GABA) (aminolon, gammalon) is used to treat nervous system disorders, speech disorders, memory loss, cerebral vascular atherosclerosis, and mental retardation in children. 6-Aminohexanoic acid (?-aminocaproic acid) is used in medicine to stop severe bleeding, as it helps in effective blood clotting.

Several oligomers of a-amino acids play an important role in body functions, and some of them are used in medical practice. Thus, methyl ether of L-asparagyl-L-phenylalanine dipeptide (aspartate, aspartame) is used for diabetes as low calorie sugar substitute (150 times sweeter than glucose); a natural antibiotic Gramicidin, S-cyclic decapeptide - [Val-Orn-Leu-(D)-Phe-Pro]2, produced by Bacillus brevis, has bacteriostatic and bactericidal action and is used to treat wounds, burns, and inflammatory diseases. It is also interesting to note that this antimicrobial peptide includes a D-form of phenylalanine. Recently, a number of small natural peptides (of leather tree frogs, snails ganglion, and poison spiders), containing one or two D-amino acids were isolated. It has been found that the D-form of the amino acid moiety in such peptides greatly increases their resistance to hydrolytic action of exo- and endoproteases. This fact is taken into account when oligopeptide drug substances with prolonged action are created [3].

Organisms can vary greatly in their metabolism because of the differences in their amino acid structure. Lately, researchers are more and more attracted by nonprotein a-amino acids with unusual structures. These include those amino acids that do not exist in the main chains of the proteins due either to the lack of specific tRNA or corresponding triplet codon or to the fact that nonprotein amino acids are not subject to a posttranslational modification. Many of these compounds are the end products of secondary metabolism, others occur as intermediates or as a result of metabolism or detoxification of foreign compounds. Due to the nature of bacterial metabolism, formation of many new compounds is possible by biosynthetic processes by adding the corresponding cell compounds to the substances of nutrient medium. These unusual amino acids can be also obtained synthetically; however, the number of "artificially" obtained amino acids of unusual structure is limited in the literature.

In essence, the nonprotein amino acids are functionally substituted derivatives of protein amino acids (substituted by a-NH2, a-COOH, SH, OH, ß, and ?-COOH, d-NH2, imidazole, guanidine groups, etc.) and C-alkylated analogs (a, ß, ?, etc.) with a variety of aliphatic, aromatic, and heterocyclic substituents.

One of the first isolated and identified nonprotein amino acid is dicysteinyldopa [4]. Study of a major constituent of yellow pigment Tapetum, isolated from a sea pike Lepisosteus, revealed a new sulfur-containing product, which was purified by chromatography (Sephadex L1120, Dowex 50) and identified by physicochemical methods of analysis. Spectral analysis showed the presence of sulfur-containing ortho-diphenyl amino acid. After reductive hydrolysis of the isolated substance in hydrochloric acid, cysteine and dihydroxyphenylalanine (DOPA) in a ratio of 2/1 attached by thioester bond were obtained as the main products (1 H NMR data) (1).

This structure (1) was partially confirmed by biological synthesis. Tyrosinase oxidation of L-DOPA in the presence of an excess L-cysteine resulted in the same amino acid with 5- and 3-S-cysteinyldopa, indicating the substitution in positions 2 and 5 of the aromatic ring. Under the same conditions, catechol and cysteine formed 3,5-cysteinylcatechol and 3,6-S,S-dicysteinylcatechine (2), which is an additional argument in favor of the 3,5-substituted phenyl ring (3,6-S,S-dicysteinylcatechine - symmetric structure of compound 1).

(2 S,21S,211S) - absolute configuration of the isolated product was established by comparing the data of polarimetric measurements of natural and synthetic product samples obtained from L-DOPA and L-cysteine.

The unusual amino acid, 2,4-diamino-3-methylbutyric acid [5], was found in the amino acid composition of root nodules of Lotus plant, which is produced by the bacterial strain of Rhizobium. Chromatographic and spectral analysis (NMR, mass spectroscopy, and chiral Gas-liquid chromatography (GLC) of the fraction isolated from the acid hydrolysate of the ethanol extract of this plant by ion-exchange methods (Amberlite IR 120, Dowex 50) established its (2R,3S)-absolute configuration. In the same plant species, among the protein amino acids, ninhydrin-positive compounds with unusual Rf values were also found.

In general, more than 1000 nonprotein amino acids are found in nature, extracted from plants, microorganisms, and other sources. Complete information on nonprotein amino acids are presented in the book by Barrett [6].

The main sources of known nonprotein amino acids are fauna and microorganisms that are responsible for excreting many compounds into the environment [7]. Many microbiological products show antibiotic properties, and by analogy with the fungi products, contain unusual amino acids included in more complex structures such as depsipeptides [8]. In these structures, D- and L-amino acids of common and unusual nature are connected to each other by peptide as well as by other bonds with components such as carboxylic acids and hydroxy acids. These natural molecules are rich sources of new amino acids with unusual structure.

In higher plants, unusual amino acids are most often found in free state or in the form of low-molecular-weight complexes, such as with glutamic acid. The concentration of these compounds in plant tissues can be very high. Many of the unusual amino acids from plants and animals are components of a number of pigment structures [9, 10].

Bacteria and plants differ from animal organisms by the content and chemical nature of nonprotein amino acids which affect their metabolism. In products of metabolism of animal organisms, there are no secondary by-products...