MOLYBDENUM-COFACTOR-CONTAINING ENZYMES - STRUCTURE AND MECHANISM

Molybdenum-containing enzymes catalyze basic metabolic reactions in th e nitrogen, sulfur, and carbon cycles. With the exception of the nitro genase cofactor, molybdenum is incorporated into proteins as the molyb denum cofactor that contains a mononuclear molybdenum atom coordinated to the sulfur atoms of a pterin derivative named molybdopterin. Certa in microorganisms can also utilize tungsten in a similar fashion. Moly bdenum-cofactor-containing enzymes catalyze the transfer of an oxygen atom, ultimately derived from or incorporated into water, to or from a substrate in a two-electron redox reaction. On the basis of sequence alignments and spectroscopic properties, four families of molybdenum-c ofactor-containing enzymes have been identified. The available crystal lographic structures for members of these families are discussed withi n the framework of the active site structure and catalytic mechanisms of molybdenum-cofactor-containing enzymes. Although the function of th e molybdopterin ligand has not yet been conclusively established, inte ractions of this ligand with the coordinated metal are sensitive to th e oxidation state, indicating that the molybdopterin may be directly i nvolved in the enzymatic mechanism.