Also found in: Dictionary, Thesaurus, Medical, Acronyms, Wikipedia.
Related to cysteine: methionine, cytosine


(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.
..... Click the link for more information.
 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.
..... Click the link for more information.
. 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.
..... Click the link for more information.
 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.
..... Click the link for more information.
, 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.
..... Click the link for more information.
 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.
..... Click the link for more information.
The Columbia Electronic Encyclopedia™ Copyright © 2013, Columbia University Press. Licensed from Columbia University Press. All rights reserved.
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.
References in periodicals archive ?
After cells reached approximately 90% to 100% confluency, they were treated with nanoemulsified AAs (methionine and cysteine) at a concentration of 18 [micro]g/mL for 24 h.
During this period, the increasing dietary methionine + cysteine: lysine ratio resulted in a linear increase (P<0.01) of feed intake (FI=16.572+25.07X; [R.sup.2]=0.88; Table 2).
(iv) The electrode was immersed for various periods of time (15, 30, 60, and 1140 min) in 1 x [10 .sup.-2] M cysteine solution.
Minamiyama et al., "S-allyl cysteine prevents CC14-induced acute liver injury in rats," Free Radical Research, vol.
Shi et al., "Growth retardation and cysteine deficiency in gamma-glutamyl transpeptidase-deficient mice," Proceedings of the National Academy of Sciences, vol.
The 5-[HT.sub.7] receptor is dynamically palmitoylated in an agonist-dependent manner, and its C-terminal cysteine residues Cys404 and Cys438/Cys441 were identified as potential palmitoylation sites [75].
The cytotoxicity of the CDCs is sensitive to the modifications in this region, especially mutations of the tryptophan or the cysteine residues highly affect its binding capacity, for example, intermedilysin (ILY) secreted by Streptococcus intermedius has an alanine residue instead of cysteine of the conserved region, resulting in an oxidation-resistant toxin and a proline residue is present in place of second conserved tryptophan [24].
PSF structure is the structure after the sulfur transfer from cysteine to ToCDS and release of alanine product from ToCDS.
This was true for glutathione and [alpha]-tocopherol, as well as for all three oxidative stress biomarkers (protein carbonyls, 3-nitrotyrosine, and malondialdehyde), although lycopene (P <0.069) and cysteine (P <0.072) also reached the borderline of significance.
Functional components in garlic of neuroprotective effects included allicin, S-methylcysteine, S-allyl cysteine, S-allyl-L-cysteine, diallyl disulfide, diallyl trisulfide, N-[alpha]-(1-deoxy-D-fructose-1-yl)-L-arginine, and Zajoene; nevertheless, major functional components in onion of neuroprotective effects contained flavonoids and quercetin (see Table 1).
CatL, a member of cysteine Cat family, exhibits strong extracellular matrix degradation activity and participates in photoaging.[sup][12] We demonstrated that a single UVA irradiation increased CatL expression and activity in fibroblasts for three consecutive days.