Research in my group involves using biophysical, molecular biological and synthetic model approaches to elucidate the structure of transition metal sites in proteins and enzymes, and investigate how these sites function in biology. We have focused our efforts on nickel biochemistry and redox systems involving S-donor ligands. Systems that are of current interest in my group include proteins involved in nickel trafficking, and several enzymes including hydrogenase, superoxide dismutase and cysteine dioxygenase.
The wide variety of techniques that are used to investigate bioinorganic systems appeal to chemists earning degrees in biological, inorganic, organic and physical chemistry. Our research on Ni-containing hydrogenases illustrates this point. Hydrogenases are enzymes that catalyze the reversible two-electron oxidation of H2. As such, they are key enzymes in anaerobic microbial metabolism and have generated much interest as a possible means for producing and utilizing an alternative fuel. EPR studies of the hydrogenase isolated from the purple photosynthetic bacterium Thiocapsa roseopersicina demonstrated the presence an unusual redox-active Ni center that is involved in activating H2. X-ray absorption spectroscopy on the enzyme in various redox states reveals that the Ni is ligated by a combination of S- and N- or O-donor ligands. These studies also indicate that, although the EPR spectrum of the Ni site disappears and then reappears upon exposure to H2, the charge on the Ni atom does not change. Model studies directed at investigating the redox chemistry of Ni thiolate complexes suggest that much of the redox chemistry observed in hydrogenase involve the S-donor ligands (Figure). Sulfur-centered chemistry occurs naturally, and oxidation by O2 is the reaction that is catalyzed by cysteine dioxygenase in the conversion of cysteine to cysteine sulfinic acid. Hydrogenase must acquire nickel for activity, and this is accomplished via nickel-specific importers, metallochaperones and exporters that are regulated by nickel-specific transcription factors (Figure). How these trafficking proteins discriminate between metals and generate metal-specific biological responses is another aspect of the research.