Elcys

Cysteine (symbol Cys or C;[3] /sstin/)[4] is a semiessential[5] proteinogenic amino acid with the formula HOOC-CH-(NH2)-CH2-SH. 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 ... [wikipedia]