Cysteine (symbol Cys or C;[4]/ˈsɪstɪiːn/)[5] is a semiessential[6]proteinogenic amino acid with the formulaHOOC−CH(−NH2)−CH2−SH. The thiol side chain in cysteine often participates in enzymatic reactions as a nucleophile. Cysteine is chiral, but interestingly, both D and L-cysteine are found in nature with D-cysteine having been found in developing brain (see Semenza et al., 2021). Cysteine is named after its discovery in urine, which comes from the urinary bladder or cyst, from kystis "bladder".[7]
The thiol is susceptible to oxidation to give the disulfide derivative cystine, which serves an important structural role in many proteins. In this case, the symbol Cyx is sometimes used.[8][9] The deprotonated form can generally be described by the symbol Cym as well.[9][10]
When used as a food additive, cysteine has the E number E920.
^Weast, Robert C., ed. (1981). CRC Handbook of Chemistry and Physics (62nd ed.). Boca Raton, FL: CRC Press. p. C-259. ISBN0-8493-0462-8..
^"Nomenclature and symbolism for amino acids and peptides (IUPAC-IUB Recommendations 1983)", Pure Appl. Chem., 56 (5): 595–624, 1984, doi:10.1351/pac198456050595