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cysteine (sĭsˈtēn), organic compound, one of the 20 amino acids commonly found in animal proteins. Only the L-stereoisomer participates in the biosynthesis of mammalian protein. It is particularly abundant in the proteins of hair, hooves, and the keratin of the skin. Cysteine's importance is related to the presence of a sulfur-containing thiol group in its side chain. This group participates in the catalytic reactions of certain enzymes, such as that of papain, the enzyme from papaya latex used to make commercial meat tenderizers. The thiol group of one cysteine residue is capable of combining with the thiol group of another to form a disulfide bridge, either linking two peptide chains together, as in the case of insulin, or causing a single peptide chain to fold back on itself, making a loop. This latter effect on the secondary structure of proteins is evidently of great importance in maintaining the proper configuration of both structural proteins and enzymes. Two cysteine molecules linked together by a disulfide linkage make up the amino acid cystine, often occurring as a separate entry in lists of common amino acids. A major complication of cystinuria, an inherited metabolic disease, one of whose symptoms is a twentyfold to thirtyfold increase in urinary excretion of cystine, is the precipitation of this relatively insoluble amino acid in the kidney, impairing its function. A similar sort of renal failure often accompanies cystinosis, another inherited disease. Cystine was isolated from a urinary calculus in 1810 and from horn tissue in 1899. The reduction of cystine to cysteine was reported in 1884, and the structures of the two amino acids were proved by chemical synthesis in 1903–4. Neither cysteine nor cystine is essential to the diet of man; cystine and cysteine are interconvertible, and cysteine is made in the body from serine and methionine.

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The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.



(also known as α-amino-β-thiopropionic acid), HSCH2CH(NH2)COOH, a sulfur-containing amino acid. Cysteine occurs in the optically active L- and D- forms as well as the racemic DL- form. L-cysteine is a component of glutathione and almost all natural proteins.

In the hydrolysis of proteins, cystine is converted into cystine, from which cysteine may be obtained by reduction. Cysteine is a replaceable amino acid. In mammals it is synthesized from the amino acids methionine and serine with the participation of adenosine triphosphate (ATP); in the process, cystathionine is formed as an intermediate. In some microorganisms and plants, cysteine is formed from serine and hydrogen sulfide with the participation of acetyl-coenzyme A and is used in the synthesis of methionine.

Because of the high reactivity of the sulfhydryl (—SH) groups, cysteine plays an important role in metabolism. The sulfhydryl groups of cysteine radicals at the active center of thiol enzymes either establish the enzyme-cofactor bond or participate directly in the enzymatic reaction—that is, in the formation and decomposition of the enzyme-substrate complex. Cysteine radicals located outside the active center give the enzyme its catalytically active conformation.

Cysteine plays a protective role in the body by binding toxic heavy metal ions (with the formation of mercaptides), arsenic compounds, cyanides (with the formation of thiazolidines), and aromatic hydrocarbons (with the formation of mercapturic acid). Cysteine and its decarboxylation product, cystamine, are used as radioprotectors. Cysteine is also used for the treatment of cataracts in early stages.


Young, L., and G. Maw. Metabolizm soedinenii sery. Moscow, 1961. (Translated from English.)
Meister, A. Biokhimiia aminokislot. Moscow, 1961. (Translated from English.)
Torchinskii, Iu. M. Sul’fgidril’nye i disul’fidnye gruppy belkov. Moscow, 1971.


The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.


C3H7O2NS A crystalline amino acid occurring as a constituent of glutathione and cystine.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
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