Biochemistry

The main physiological role of another sulfur-containing amino acid, methionine, is associated with the presence of a labile methyl group. Giving the -- CH3 group through the formation of S-adenosylmethionine, methionine is involved in the synthesis of glycerophospholipids. The physiological role of the amino acid serine that like threonine, in the esterified form with the participation of phosphoric acid that it is a large part of complex proteins - phosphoproteins (milk casein and egg vitellin). Serine, in turn, is synthesized from glycine by the action of an enzyme containing tetrahydrofolic acid (THF). The essential amino acid tryptophan serves as a precursor of nicotinic acid, NAD and NADP, serotonin and indolylacetic acid - plant growth hormone. Serotonin, with a vasoconstrictor effect, is synthesized in the cells of the intestine and nerve tissue. Tyrosine is responsible for the coloring of hair, skin, eyes, for the dark color of food products (for example, rye bread), since with its participation dark-colored pigments are synthesized - melanins. The reaction mechanism is not fully understood, but the first stages of their synthesis are known. Under the action of the copper-containing enzyme, tyrosine is converted into dioxiphenylalanine (DOPA) that is further oxidized, cyclized, forming indolequinone. Polymerization of the latter leads to the synthesis of melanins. The formation of melanins is enhanced by the influence of ultraviolet rays (with tanning) and can be the cause of malignant neoplasms. With a hereditary disease - albinism, characterized by the absence of the tyrosinase enzyme, on the contrary, pigmentation of the skin and hair is not observed, but there is a fear of light. Tyrosine is formed from phenylalanine. From tyrosine and tryptophan contained in food, with the participation of microbial enzymes in the intestine, poisonous products are formed - cresol, phenol, scatol, indole, which are neutralized in the liver by binding to sulfuric or glucuronic acid with the formation of non-toxic (paired) acids, for example phenol sulfuric acid. As a result of decarboxylation of amino acids, some important biogenic amines are formed in the body. Decarboxylation of aspartic acid provides the synthesis of /З -alanine that is an integral part of the biologically active compounds - CoA and APB, and decarboxylation of lysine and ornithine under the influence of intestinal microflora enzymes leads to the formation of poisonous diamines - cadaverine and putrescine. In a healthy body, both amines are completely neutralized in the intestinal mucosa. Some amino acids play the role of mediators - substances involved in the transmission of nerve impulses from one nerve cell to another. With irritation of nerve fibers, mediators react with a specific receptor and provide the corresponding physiological function: regulation of sleep, wakefulness, 48