XAS STUDIES OF THE THIOLATE LIGAND DONORS IN MODELS OF THE SULFITE OXIDASE ACTIV

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This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Sulfite oxidase (SO) is a mononuclear molybdenum enzyme responsible for the conversion of sulfite to sulfate, the terminal step in cysteine and methionine degradation. The consensus structure of the Mo active site derived from both crystallographic and EXAFS studies of the chicken liver SO is that of a five coordinate Mo site consisting of an axial oxo donor and an ene-l,2-dithiolate, a Cys thiolate and a second oxo donor in the equatorial plane. While the mechanism of the sulfite lone-pair attack on the equatorial oxo that is facing the substrate access channel is straight-forward, the role of the thiolate and ene-l,2-dithiolate ligands in facilitating this reaction is unclear. The proposed XAS studies will utilize novel mixed thiolate/selenolate models to assess the covalency contributions of the thiolate and ene-l,2-dithiolate to the Mo center. The results of these experiments will be used in conjunction with ongoing electronic absorption, MCD, resonance Raman, EPR and DFT studies in our laboratory to determine the electronic structure of the SO active site, and how the electronic structure facilitates both the oxidative and reductive half-reactions in SO catalysis.