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(sĭs`tēn), 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.
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 commonly found in animal proteinsprotein,
any of the group of highly complex organic compounds found in all living cells and comprising the most abundant class of all biological molecules. Protein comprises approximately 50% of cellular dry weight.
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. Only the l-stereoisomer participates in the biosynthesis of mammalian protein. It is particularly abundant in the proteins of hair, hooves, and the keratinkeratin
, any one of a class of fibrous protein molecules that serve as structural units for various living tissues. The keratins are the major protein components of hair, wool, nails, horn, hoofs, and the quills of feathers.
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 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 insulininsulin,
hormone secreted by the β cells of the islets of Langerhans, specific groups of cells in the pancreas. Insufficiency of insulin in the body results in diabetes. Insulin was one of the first products to be manufactured using genetic engineering.
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, 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 serineserine
, organic compound, one of the 20 amino acids commonly found in animal proteins. Only the l-stereoisomer appears in mammalian protein.
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 and methioninemethionine
, organic compound, one of the 20 amino acids commonly found in animal proteins. Only the L-stereoisomer appears in mammalian protein. It is one of the several essential amino acids needed in the diet; the human body cannot synthesize it from simpler metabolites.
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(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.



C3H7O2NS A crystalline amino acid occurring as a constituent of glutathione and cystine.
References in periodicals archive ?
Myocardial ischemia possibly mediated by cysteinyl leukotrienes.
Staphylococcal enterotoxin B as a nonimmunological mast cell stimulus in primates: the role of endogenous cysteinyl leukotrienes.
The cysteinyl leukotrienes (LTC4, LTD4, LTE4) are products of arachidonic acid metabolism and are released from various cells, including mast cells and eosinophils.
The central component of the apoptotic machinery is a proteolytic system consisting of a family of cysteinyl proteases, termed caspases.
Membrane lipid proteins are identified by a unique signature sequence that is the target for a specific lipoprotein signal peptidase and that allows the cleaved protein product to attach by cysteinyl linkage to a glyceride-fatty acid lipid.
Two other major families of apoptosis effectors under study include members of the TNF/Fas/NGF receptor superfamily and their corresponding ligands, which also includes a number of immune system regulators, and the more recently discovered ICE (interleukin-1-[Beta] converting enzyme) family of cysteinyl proteinases.
Caspases, a family of cysteinyl aspartate-specific proteases, play an essential role in the regulation and the execution of apoptotic cell death (Hengartner 2000; Lavrik et al.
Increase in GSSG will promote oxidation of protein cysteinyl thiols, shifting the equilibrium of thiol-disulfide exchange significantly in the direction of mixed disulfide formation and, changes protein conformation.
17-19) Steinke et al explored the role of cysteinyl leukotrienes produced by eosinophils, mast cells, basophils, and monocytes in asthma and CRS.
Antileukotrienes are antagonists of type 1 cysteinyl leukotriene receptors.
9628409 SINGULAIR(R) (Montelukast Sodium) Tablets, Chewable Tablets, and Oral Granules DESCRIPTION Montelukast sodium, the active ingredient in SINGULAIR*, is a selective and orally active leukotriene receptor antagonist that inhibits the cysteinyl leukotriene CysLT1 receptor.
A series of diacid ezatiostat analogs bearing different substituents on the cysteinyl sulfur were synthesized using computer aided drug design and TRAP, a proprietary drug discovery technology, to improve the inhibitory potency and selectivity for the target enzyme GST P1-1.