The molybdenum-cofactor: a crystallographic perspective

The molybdenum-cofactor (Mo-co) consists of a mononuclear molybdenum or tungsten ion coordinated by one or two molybdopterin ligands. Crystallographic analyses have demonstrated that the molybdopterin ligands are tricyclic and nonplanar, and that they coordinate the metal through their dithiolene sulfurs. Additional ligands to the metal may be provided by amino acid side chains (including serine, cysteine and selenocysteine), as well as one or more nonprotein O or S ligands, such as ore, hydroxo, and sulfide. The molybdopterin ligand may participate in the various electron transfer reactions associated with the catalytic mechanism of these proteins, as suggested by both oxidation state-dependent changes in the metal coordination environment and the molybdopterin structure, and by the interaction of the molybdopterin with other redox groups within Mo-co-containing enzymes.