Molybdenum Enzymes, Cofactors, and Model Systems

Abstract

Molybdenum enzymes, which are present in plants, animals, and microorganisms, play key roles in the biogeochemical cycles of nitrogen, sulfur, and carbon. A variety of molybdenum enzymes contain the molybdenum cofactor, Moco, which consists of a pterin-enedithiolate organic ligand (molybdopterin) and a single Mo atom. These enzymes include nitrate reductase, xanthine oxidase, sulfite oxidase, and DMSO reductase. Their Mo active sites engage in oxygen atom and/or proton-electron transfer reactions. The second main class of Mo enzymes consists of the molybdenum nitrogenases which are responsible for dinitrogen reduction to ammonia. The nitrogenase FeMo proteins contain Fe8S8 (P cluster) centers and iron-molybdenum cofactor (FeMoco) centers, the latter consisting of a Fe7MoS8-9 cluster and the homocitrate ligand. P clusters and FeMoco have unique structures, which have yet to be achieved synthetically. Nitrogenase model systems include structural mimics of the metal-sulfur clusters, reactivity analogs for dihydrogen, dinitrogen and reduced intermediates, and simple (chemical) dinitrogen reducing systems. Studies on tungsten enzymes from thermophilic organisms promise a rich comparative biochemistry with molybdenum.