Cysteine

Cysteine is a naturally occurring, sulfur-containing amino acid that is a building block to most proteins. Cysteine is unique among the twenty common amino acids because it contains a thiol group. Thiol groups can undergo oxidation; a pair of cysteine residues is oxidised, producing cystine, a disulfide-containing derivative. This reaction is reversible. The disulphide bonds of cystine are crucial to defining the structures of many proteins. Related to its redox behavoir, cysteine is incorporated into many proteins that are redox-active, such as the antioxidant glutathione. Cysteine is named after cystine, which comes from the Greek word kustis meaning bladder − cystine was first isolated from kidney stones.