Cysteine (symbol Cys or C) () is a semi-essential proteinogenic amino acid with the formula HO2CCH(NH2)CH2SH. It is encoded by the codons UGU and UGC. 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.
Cysteine has the same structure as serine, but with one of its oxygen atoms replaced by sulfur; replacing it with selenium gives selenocysteine. (Like other natural proteinogenic amino acids cysteine has (L) chirality in the older D/L notation based on homology to D and L glyceraldehyde. In the newer R/S system of designating chirality, based on the atomic numbers of atoms near the asymmetric carbon, cysteine (and selenocysteine) have R chirality, because of the presence of sulfur (resp. selenium) as a second neighbor to the asymmetric carbon. The remaining chiral amino acids, having lighter atoms in that position, have S chirality.)

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