E920 - L-cysteine

Cysteine -symbol Cys or C; - is a semi-essential 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 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.- - Wikipedia