Cysteine

"Cysteine (abbreviated as Cys or C)^[3] is an α-amino acid with the chemical formula HO₂CCH(NH₂)CH₂SH. It is a non-essential amino acid, which means that it is biosynthesized in humans. Its codons are UGU and UGC. The side chain on cysteine is thiol, which is polar and thus cysteine is usually classified as a hydrophilic amino acid.^[4] The thiol side chain often participates in enzymatic reactions, serving as a nucleophile. The thiol is susceptible to oxidization to give the disulfide derivative cystine, which serves an important structural role in many proteins."