Mechanistic aspects of molybdenum-containing enzymes

Abstract

In the past several years, the number of enzymes known to possess mononuclear molybdenum centers in their active sites has increased signi¢cantly, and well over 50 such enzymes have been identi¢ed that catalyze a variety of hydroxylation, oxygen atom transfer and other oxidation-reduction reactions. Many of these enzymes have been isolated from either obligate anaerobes or facultative anaerobes grown under anaerobic or microaerobic conditions, and are involved in a variety of anabolic, catabolic and energy-conserving metabolic pathways. There are now several enzymes having known crystal structure, and this new structural information has provided the basis for an increasingly detailed understanding of the mechanisms of action of these enzymes. In the present review, an overview of our present understanding of the mechanism of action of these enzymes will be presented, and those of a few selected enzymes will be considered in greater detail. Several alternative classi¢cation schemes have been suggested for these molybdenum-containing enzymes, but for the purposes of the present discussion they will be considered to fall into three families, based on the structures of their molybdenum centers in their oxidized Mo(VI) state (Fig. 1) [1]. These families include: (i) the molybdenum hydroxylases, a large and broadly dispersed family of enzymes that possess an MoOS unit and catalyze the hydroxylation of a broad range of aldehydes and aromatic heterocycles; (ii) the eukaryotic oxo transferases, a family that at present includes only sul¢te oxidase and the assimilatory nitrate reductases, enzymes which possess an MoO2 unit in their active sites and which catalyze oxygen atom transfer to or from a substrate; and ¢nally (iii) a diverse group of prokaryotic enzymes that catalyze either oxo atom transfer or other oxidation-reduction reactions. As will be seen below, however, some may catalyzesigni¢cantly more complex (and interesting) reactions. Enzymes of this last group have a common molybdenum center structure in which the metal is coordinated by a pair of dithiolene ligands contributed by an unusual pterin cofactor [2]. This cofactor is common to all the mononuclear molybdenum (and tungsten) enzymes, but whereas the ¢rst two families possess only a single equivalent bound to the metal, in members of the third family two equivalents of the cofactor coordinate to the metal. It should be emphasized that this ¢nal group is structurally more diverse than the ¢rst two, and can be subdivided into enzymes that possess an Mo-O(Ser), Mo-S(Cys) or Mo-Se(Se-Cys) group contributed by the polypeptide; in addition, some of these enzymes appear to possess an MoNS group rather than the more commonly encountered MoNO (see [1] for a review).