Coordination chemistry at the molybdenum site of sulfite oxidase: Redox-induced structural changes in the cysteine 207 to serine mutant

The redox chemistry of the molybdenum site of the C207S mutant of recombinant human sulfite oxidase has been studied via potentiometric titrations employing both electron paramagnetic resonance (EPR) spectroscopy and X-ray absorption spectroscopy (XAS) as probes of the active site structure. In earlier EXAFS studies, oxidized Cys207Ser enzyme has been shown to possess a novel tri-oxo active site, in which Ser207 does not appear to be a ligand to Mo [George, G. N.; Garrett, R. M.; Prince, R. C.; Rajagopalan, K. V. J. Am. Chem. Soc. 1996, 118, 8588-8592]. Redox titrations show that the active site is modified under reducing conditions to a mono-oxo Mo(IV) species, probably with Ser207 ligated to the metal. The Mo(IV) species can be reoxidized to a mono-oxo Mo(V) species still coordinated to Ser207, which in turn can be further reoxidized to yield the initial tri-oxo Mo(VI) structure with loss of Ser207 ligation.