Cysteine is an amino acid, C3H7O2NS, derived from cystine and found in most proteins. It is the only amino acid to contain a thiol (SH) group. In intracellular enzymes the unique reactivity of this group is frequently exploited at the catalytic site. In extracellular proteins found only as cysteine in disulphide bridges or fatty acylates.
Amino acid: An organic compound containing an amino group (NH2), a carboxylic acid group (COOH), and any of various side groups, especially any of the 20 compounds that have the basic formula NH2CHRCOOH, and that link together by peptide bonds to form proteins or that function as chemical messengers and as intermediates in metabolism. (Source: Answer.com)