High-Resolution EXAFS of the Active Site of Human Sulfite Oxidase:  Comparison with Density Functional Theory and X-ray Crystallographic Results

Much of our knowledge about molybdenum enzymes has originated from EXAFS spectroscopy. This technique provides excellent bond-length accuracy but has only limited bond-length resolution. We have used EXAFS spectroscopy with an extended data range in an attempt to improve bond-length resolution for the molybdenum enzyme sulfite oxidase. The Mo site of sulfite oxidase has two oxygen and three Mo−S ligands (two from cofactor dithiolene plus a cysteine). For the oxidized (MoVI) enzyme, we find that the three Mo−S bond lengths are very similar (within 0.05 Å) at 2.41 Å, as are the MoO ligands at 1.72 Å. Density functional theory shows that this is consistent with the proposed active-site structure. The reduced (MoIV) enzyme shows two Mo−S bond lengths at 2.35 Å and one at 2.41 Å (assigned to cofactor dithiolene and cysteine, respectively, from DFT), together with one MoO at 1.72 Å and one Mo−OH2 at 2.30 Å.