ABSTRACT
Catalytic and single-turnover experiments with the R. sphaeroides DMSO reductase, 180-labeled DMSO, and PTA (l,3,5-triaza-7-phosphatricyclo[3.3.1.11decane) as an oxygen atom acceptor have been used to demonstrate that the enzyme is an oxotransferase. No transfer of 180 occurred in the absence of the enzyme but, in the presence of the enzyme, catalytic and quantitative transfer of 180 from DMSO to PTA occurs [250]. Complementary resonance Raman studies have been interpreted
on the basis of a direct oxygen-atom transfer mechanism, with the active site cycling between mono-oxo-Mo(VI) and des-oxo-Mo(IV) forms via a DMSO-bound Mo(IV) intermediate and both MPT dithiolene ligands staying firmly attached throughout the catalytic cycle [251]. Physiologically, a cytochrome would then reduce the Mo(VI) state of the enzyme to Mo(IV) by two one-electron steps. Reliable activity measure ments on the enzyme have proved problematic, but assays in both the forward (enzyme oxidation) and reverse (enzyme reduction) direction have been developed [252].
