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The molybdenum-containing enzyme sulfite oxidase catalyzes the conversion of sulfite to sulfate, the terminal step in the oxidative degradation of cysteine and methionine. Deficiency of this enzyme in humans usually leads to major neurological abnormalities and early death. The crystal structure of chicken liver sulfite oxidase at 1.9 A resolution reveals that each monomer of the dimeric enzyme consists of three domains. At the active site, the Mo is penta-coordinated by three sulfur ligands, one oxo group, and one water/hydroxo. A sulfate molecule adjacent to the Mo identifies the substrate binding pocket. Four variants associated with sulfite oxidase deficiency have been identified: two mutations are near the sulfate binding site, while the other mutations occur within the domain mediating dimerization.

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Solution and solid state electronic absorption, magnetic circular dichroism, and resonance Raman spectroscopies have been used to probe in detail the excited state electronic structure of LMoO(bdt) and LMoO(tdt) (L ) hydrotris-(3,5-dimethyl-1-pyrazolyl)borate; bdt ) 1,2-benzenedithiolate; tdt ) 3,4-toluenedithiolate). The observed energies, intensities, and MCD band patterns are found to be characteristic of LMoO(S-S) compounds, where (S-S) is a dithiolate ligand which forms a five-membered chelate ring with Mo. Ab initio calculations on the 1,2-enedithiolate ligand fragment, -SCdCS -, show that the low-energy S f Mo charge transfer transitions result from one-electron promotions originating from an isolated set of four filled dithiolate orbitals that are primarily sulfur in character. Resonance Raman excitation profiles have allowed for the definitive assignment of the ene-dithiolate S in-plane f Mo d xy charge transfer transition. This is a bonding-to-antibonding transition, and its intensity directly probes sulfur covalency contributions to the redox orbital (Mo d xy ). Raman spectroscopy has identified three totally symmetric vibrational modes at 362 cm -1 (S-Mo-S bend), 393 cm -1 (S-Mo-S stretch), and 932 cm -1 (MotO stretch), in contrast to the large number low-frequency modes observed in the resonance Raman spectrum of Rhodobacter sphaeroides DMSO reductase. These results on LMoO(S-S) complexes are interpreted in the context of the mechanism of sulfite oxidase, the modulation of reduction potentials by a coordinated ene-dithiolate (dithiolene), and the orbital pathway for electron transfer regeneration of pyranopterin dithiolate Mo enzyme active sites.