THE MOLYBDENUM-COFACTOR - A CRYSTALLOGRAPHIC PERSPECTIVE

The molybdenum-cofactor (Mo-co) consists of a mononuclear molybdenum o r tungsten ion coordinated by one or two molybdopterin ligands. Crysta llographic analyses have demonstrated that the molybdopterin ligands a re tricyclic and nonplanar, and that they coordinate the metal through their dithiolene sulfurs. Additional ligands to the metal may be prov ided by amino acid side chains (including serine, cysteine and selenoc ysteine), as well as one or more nonprotein O or S ligands, such as or e, hydroxo, and sulfide. The molybdopterin ligand may participate in t he various electron transfer reactions associated with the catalytic m echanism of these proteins, as suggested by both oxidation state-depen dent changes in the metal coordination environment and the molybdopter in structure, and by the interaction of the molybdopterin with other r edox groups within Mo-co-containing enzymes.