Identity of the Exchangeable Sulfur-Containing Ligand at the Mo(V) Center of R160Q Human Sulfite Oxidase

In our previous study of the fatal R160Q mutant of human sulfite oxidase (hSO) at low pH (Astashkin et al. J. Am. Chem. Soc. 2008, 130, 8471-8480), a new Mo(V) species, denoted species 1, was observed at low pH values. Species 1 was ascribed to a six-coordinate Mo(V) center with an exchangeable terminal oxo ligand and an equatorial sulfate group on the basis of pulsed EPR spectroscopy and S-33 and O-17 labeling. Here we report new results for species 1 of R160Q based on substitution of the sulfur-containing ligand by a phosphate group, pulsed EPR spectroscopy in K-a- and W-bands, and extensive density functional theory (DFT) calculations applied to large, more realistic molecular models of the enzyme active site. The combined results unambiguously show that species 1 has an equatorial sulfite as the only exchangeable ligand. The two types of O-17 signals that are observed arise from the coordinated and remote oxygen atoms of the sulfite ligand. A typical five-coordinate Mo(V) site is compatible with the observed and calculated EPR parameters.