Our laboratory is primarily engaged in studying membrane-bound metalloproteins that catalyze electron transfer reactions coupled to the generation of both a voltage and ion gradient across the membrane bilayer. Our goal is to determine the catalytic mechanisms and, in particular, the way by which these enzymes generate an electrochemical potential gradient across the membrane. Among the enzymes on which we are working are the respiratory oxidases from Escherichia coli and from Rhodobacter sphaeroides. The proton electrochemical gradients generated by these enzymes are used by the bacteria to provide the energy for active uptake of solutes and for the synthesis of ATP. In addition we are studying the sodium pumping respiratory NADH:ubiquinone oxidoreductase from Vibrio cholerae. This organism uses a sodium electrochemical gradient generated by this enzyme to drive a number of energy requiring processes, such as flagellar rotation required for cell motility. Generally, our research centers around how redox chemistry is coupled to moving ions (protons or sodium) across a membrane.

Techniques we use include classical preparative biochemistry, immunology, genetics, molecular biology, and biophysical methods such as FTIR spectroscopy, electrochemistry and rapid kinetics. In the E. coli system, our efforts are directed at the two terminal oxidases: cytochrome bo3 and cytochrome bd. Both of these enzymes oxidize ubiquinol in the cytoplasmic membrane and reduce O2 to H2O. Both generate a transmembrane voltage during enzyme turnover. Whereas the active site of the heme-copper oxidases contains a heme and a copper atom, the corresponding site in the bd-type oxidases contains two hemes.

Cytochrome bo3 is a member of the heme-copper superfamily of respiratory oxidases and is closely related to the mitocondrial cytochrome oxidase. The aa3-type and the cbb3-type cytochrome c oxidases from Rhodobacter sphaeroides are also members of the heme-copper oxidase superfamily. These enzymes are proton pumps, coupling the oxygen chemistry catalyzed at the active site to the electrogenic movement of protons across the membrane. We are particularly interested in determining the way in which protons are transferred within these heme-copper oxidases and the mechanism by which the proton pump operates. The X-ray structure of cytochrome oxidase provides a framework for extensive structure/function studies. We have identified two functionally important proton-conducting pathways.