A derivative of the amino acid cysteine
, formed by the oxidative linkage of two cysteine molecules through a disulphide bridge. The formation of sulphide bridges is important in the maintenance of the tertiary structure of many globular proteins. Cysteine residues in different parts of a protein chain may cross-link so causing folding of the chain. Cystine is also necessary for the synthesis of thiamin pyrophosphate, a coenzyme in many decarboxylation reactions.