Figure 1: Glutathione is a tripeptide: L- -glutamyl-L-cysteinyl-glycine. In its reduced form (a) the N-terminal glutamate and cysteine are linked by the -carboxyl group of glutamate, preventing cleavage by common cellular peptidases and restricting cleavage to -glutamyltranspeptidase. The cysteine residue is the key functional component of glutathione, providing a reactive thiol group that plays an essential role in its functions. Furthermore, cysteine residues form the intermolecular dipeptide bond in the oxidized glutathione molecule (b).