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serine(sĕr`ēn), organic compound, one of the 20 amino acidsamino acid
, any one of a class of simple organic compounds containing carbon, hydrogen, oxygen, nitrogen, and in certain cases sulfur. These compounds are the building blocks of proteins.
..... Click the link for more information. commonly found in animal proteins. Only the l-stereoisomer appears in mammalian protein. It is not essential to the human diet, since it can be synthesized in the body from other metabolites, including glycineglycine
, organic compound, one of the 20 amino acids commonly found in animal proteins. Glycine is the only one of these amino acids that is not optically active, i.e., it does not have d- and
..... Click the link for more information. . Serine is important in metabolism in that it participates in the biosynthesis of purinespurine,
type of organic base found in the nucleotides and nucleic acids of plant and animal tissue. The German chemist Emil Fischer did much of the basic work on purines and introduced the term into the chemical literature in the early 20th cent.
..... Click the link for more information. and pyrimidinespyrimidine
, type of organic base found in certain coenzymes and in the nucleic acids of plant and animal tissue. The three major pyrimidines of almost universal distribution in living systems are cytosine, thymine, and uracil.
..... Click the link for more information. , cysteinecysteine
, organic compound, one of the 20 amino acids commonly found in animal proteins. Only the l-stereoisomer participates in the biosynthesis of mammalian protein.
..... Click the link for more information. , tryptophantryptophan
, organic compound, one of the 20 amino acids commonly found in animal proteins. Only the l-stereoisomer appears in mammalian protein.
..... Click the link for more information. (in bacteria), and a large number of other metabolites. When incorporated into the structure of enzymes, serine often plays an important role in their catalytic function. It has been shown to occur in the active sites of chymotrypsinchymotrypsin
, proteolytic, or protein-digesting, enzyme active in the mammalian intestinal tract. It catalyzes the hydrolysis of proteins, degrading them into smaller molecules called peptides. Peptides are further split into free amino acids.
..... Click the link for more information. , trypsintrypsin,
enzyme that acts to degrade protein; it is often referred to as a proteolytic enzyme, or proteinase. Trypsin is one of the three principal digestive proteinases, the other two being pepsin and chymotrypsin.
..... Click the link for more information. , and many other enzymes. The so-called nerve gases and many substances used in insecticides have been shown to act by combining with a residue of serine in the active site of acetylcholine esterase, inhibiting the enzyme completely. Without the esterase activity that usually destroys acetylcholineacetylcholine
, a small organic molecule liberated at nerve endings as a neurotransmitter. It is particularly important in the stimulation of muscle tissue. The transmission of an impulse to the end of the nerve causes it to release neurotransmitter molecules onto the surface of
..... Click the link for more information. as soon as it performs its function, dangerously high levels of this neurotransmitter build up, quickly resulting in convulsions and death. Serine was first obtained from silk protein, a particularly rich source, in 1865; its structure was established in 1902.
(also, α-amino-β-hydroxypropionic acid), HOCH2CH(NH2)COOH, a naturally occurring amino acid. Serine exists in two optically active forms, namely, the L and D forms, and in the racemic, or DL, form. Practically all proteins contain L-serine. The proteins of silk are especially rich in serine; fibroin contains up to 16 percent, and sericin up to <0 percent. Serine was isolated from sericin in 1865 by the German chemist E. Cramer. Phosphoesters of serine also enter into the composition of proteins. Serine is a replaceable amino acid; its precursor in biosynthesis by living organisms is D-3-phosphoglyceric acid, an intermediate product of glycolysis. In cells, serine participates in the biosynthesis of glycine, sulfur-containing amino acids (methionine, cysteine), and tryptophan, as well as of ethanolamine and sphingolipids. It serves as a source for a monocarbon fragment (conversion to glycine with the participation of tetrahydrofolic acid), which plays an important role in the biosynthesis of choline and purines:
Serine + Tetrahydrofolic acid → Glycine + N5, N10-methylene-tetrahydrofolic acid
Upon the decomposition of serine in organisms, pyruvic acid is formed, which is introduced into the tricarboxylic acid cycle by means of conversion into acetyl coenzyme A. The catalytic function of a series of enzymes (chymotrypsin, trypsin, bacterial proteases, esterases, phosphorylase, phosphoglucomutase, alkaline phosphatase) derives from the reactivity of the hy-droxyl group of the serine residue, which forms part of the active site of these enzymes. Reactions of the enzymes of the serine group include the hydrolysis of peptides, amides, and the esters of carboxylic acids and the transfer of the residue of phosphoric acid. The antibiotics cycloserine and azaserine are derivatives of serine.
REFERENCELehninger, A. Biokhimiia. Moscow, 1974. (Translated from English.)
E. N. SAFONOVA