TRANSITION-METAL SULFUR CHEMISTRY AND ITS RELEVANCE TO MOLYBDENUM ANDTUNGSTEN ENZYMES

Molybdenum or tungsten is present at the active sites of over 30 disti nct enzymes. The Mo enzyme nitrogenase, with its unique polynuclear me tal sulfide clusters, falls in a class by itself. All other Mo and W e nzymes are mononuclear, with pterin-ene-dithiolate coordination. Recen t x-ray crystallographic results on several of these enzymes confirm t he potential additional presence of oxo (one or two), sulfido, cystein e, selenocysteine, serine, aquo/hydroxo, and/or a second dithiolene in the various metal coordination spheres. The rich chemistry of multisu lfur transition metal systems admits ligand redox, internal electron t ransfer, and 'intermediate' redox states. This redox flexibility may f acilitate coupled proton/electron transfer and/or oxo transfer mechani sms, which are effectively exploited by Mo and W enzymes. In the case of nitrogenase, a hypothesis is put forward that the relatively isolat ed heavy atom grouping at the FeMoco active site allows that site to b ecome locally 'hot' during turnover, thereby facilitating activation o f the recalcitrant dinitrogen molecule.