Transition metal sulfur chemistry and its relevance to molybdenum and tungsten enzymes

Abstract: Molybdenum or tungsten is present at the active sites of over 30 distinct enzymes. The Mo enzyme nitrogenase, with its unique polynuclear metal sulfide clusters, falls in a class by itself All other Mo and W enzymes are mononuclear, with pterin-ene-dithiolate coordination. Recent x-ray crystallographic results on several of these enzymes confirm the potential additional presence of 0x0 (one or two), sulfido, cysteine, selenocysteine, serine, aquohydroxo, and/or a second dithiolene in the various metal coordination spheres. The rich chemistry of multi-sulhr transition metal systems admits ligand redox, internal electron transfer, and 'intermediate ' redox states. This redox flexibility may facilitate coupled protodelectron transfer and/or 0x0 transfer mechanisms, which are effectively exploited by Mo and W enzymes. In the case of nitrogenase, a hypothesis is put forward that the relatively isolated heavy atom grouping at the FeMoco active site allows that site to become locally 'hot ' during turnover, thereby facilitating activation of the recalcitrant dinitrogen molecule. MOLYBDENUM AND TUNGSTEN ENZYME