The molybdenum site of sulfite oxidase: A comparison of wild-type and the cysteine 207 to serine mutant using X-ray absorption spectroscopy

X-ray absorption spectroscopy at the molybdenum and sulfur K-edges has been used to probe the active site of wild-type and cysteine 207 --> serine mutant human sulfite oxidases. We compare the active site structures in the Mo(VI) oxidation states: the wild-type enzyme possesses two Mo=O ligands at 1.71 Angstrom and three Mo-S ligands at 2.41 Angstrom. The mutant molybdenum site is a novel trioxo site with Mo=O bond lengths of 1.74 Angstrom, with two Mo-S ligands at 2.47 Angstrom. We conclude that cysteine 207 is a ligand of molybdenum in wild-type human sulfite oxidase, and that, in the mutant, the Mo is ligated to an extra oxo group rather than the hydroxyl of the substituent serine 207.