X-ray absorption spectroscopy of a quantitatively Mo(V) dimethyl sulfoxide reductase species.

Molybdenum K-edge X-ray absorption spectroscopy (XAS) has been used to probe the structure of a Mo(V) species that has been suggested to be a catalytic intermediate in the reaction of dimethyl sulfoxide (DMSO) reductase with the alternative substrate trimethylamine N-oxide (Bennet et al. Eur. J. Biochem. 1994, 255, 321-331; Cobb et al. J. Biol. Chem. 2005, 280, 11007-11017; Mtei, et al. J. Am. Chem. Soc. 2011, 133, 9672-9774). The oxidized Mo(VI) state of DMSO reductase has previously been structurally characterized as being six coordinate, with four sulfurs from pyranopterin dithiolene molybdenum cofactors, a terminal oxygen ligand, and an additional oxygen coordination from a serine residue. We find the most plausible structure for the Mo(V) active site is a five-coordinate species with four sulfur donors from the two pyranopterin dithiolene ligands, with an average Mo-S bond-length of 2.35 Å, plus a single oxygen donor at 1.99 Å, very likely from an Mo-OH ligand. Our results thus suggest that the oxygen of the serine residue has dissociated from the metal ion, suggesting hitherto unsuspected flexibility of the active site, and calling into question whether this putative intermediate is catalytically relevant. The relevance to previous Mo(V) electron paramagnetic resonance and other spectroscopic studies on DMSO reductase is discussed. XAS of an extensively studied Mo(V) form of Rhodobacter sphaeroides DMSO reductase (the high-g split species) shows that previously suggested structures for the active site are likely incorrect.