Observation of Ligand-Based Redox Chemistry at the Active Site of a Molybdenum Enzyme

Abstract

The mononuclear molybdenum enzymes all possess one or two molybdopterin cofactors coordinated to the molybdenum through the ditholene motif. Despite this common feature, they exhibit quite diverse functionality. The molybdenum enzymes previously have been described as all involving two-electron redox chemistry at molybdenum, coupled with the transfer of an oxygen atom from water via molybdenum to substrate, or the reverse. While these rules still appear to hold for most molybdenum enzymes, and for their close relatives the tungsten enzymes, it now seems that there are at least some exceptions. The recently discovered tungsten enzyme acetylene hydratase catalyzes a net hydration reaction, rather than a redox one. Very recently it has been shown that formate oxidation to CO{sub 2} by Eschericia coli formate dehydrogenase H (FDH{sub H}) does not involve oxygen atom transfer. This enzyme has also been shown to possess a potentially redox-active selenosulfide ligand to molybdenum, with the selenosulfide sulfur probably being one of the sulfurs of the cofactor dithiolene. The authors present an extended X-ray absorption fine structure (EXAFS) spectroscopic study of the molybdenum site of Desulfovibrio desulfuricans ATCC 27774 formate dehydrogenase (FDH) and show that under reducing conditions the selenosulfide group can be reduced. This is themore » first observation of ligand-based redox chemistry in a molybdenum enzyme.« less