The molybdenum cofactor (Moco) has been found to be associated with a diverse set of redox enzymes and contains a mononuclear molybdenum or tungsten ion co-ordinated by the dithiolene sulfurs of one or two molybdopterin {a pterin [2-amino-4(1H)-pteridinone] derivative} ligands. The remaining co-ordination sites on the metal are occupied by non-protein oxygen or sulfur species and, occasionally, amino acid side chains. The molybdopterin ligand can exhibit oxidation-state-dependent changes in structure and metal co-ordination, and may also interact with other redox groups in the enzyme. These observations suggest that the molybdopterin may participate in the various electron-transfer reactions associated with the catalytic mechanism of Moco containing enzymes.