Active Site Structure and Chemistry in Enzymatic Sulfite Reduction

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With support from the Inorganic, Bioinorganic and Organometallic Chemistry Program and the Biochemistry Program Dr. James A. Cowan and Dr. Richard P. Swenson will investigate the active site structure and mechanism of a metalloprotein from the bacterium Desulfovibrio vulgaris which mediates the reduction of sulfite. This enzyme is involved in the overall reduction of sulfate to hydrogen sulfide, an extremely important natural process, particularly in the ecology of wetlands. The development of a clear understanding of structure/function relationships for this relatively small multielectron redox enzyme is expected to also provide better insight into the operation of more complex and experimentally less tenable systems, such as the human respiratory enzymes. A variety of physical methods, including NMR spectroscopy, will be used to characterize the structural environment of the enzyme active site, in terms of the ligands coordinating to the prosthetic groups and of neighboring residues that might be involved in catalysis. Sulfite labeled with S-35 and O-18 will be used to investigate the mechanism of the enzyme. Exchange interactions and the coordination and redox chemistry of the coupled siroheme-Fe4S4 complex in both the native enzyme and with bound exogenous ligands will be studied. A cloning program will be developed for the dual purpose of providing larger quantities of the enzyme and, by mutagenesis, to clarify the roles of specific residues in the function of the enzyme.