Cysteine (symbol Cys or C; ) is a semiessential proteinogenic amino acid with the formula HO2CCH(NH2)CH2SH. The thiol side chain in cysteine often participates in enzymatic reactions, as a nucleophile. The thiol is susceptible to oxidation to give the disulfide derivative cystine, which serves an important structural role in many proteins. When used as a food additive, it has the E number E920. It is encoded by the codons UGU and UGC.
Cysteine has the same structure as serine, but with one of its ...
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