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Membrane Enzymes Involved in Energy Transduction;

Electron Transport Chains in E. coli and R. sphaeroides

Our laboratory studies the structure and function of cytochrome oxidase and other membrane respiratory complexes with the goal to understand how electron transfer is coupled to the generation of a transmembrane proton electrochemical gradient. We are primarily interested in the structure and function of membrane proteins that are proton pumps. Our efforts are directed at several membrane enzymes that are components of bacterial respiratory or photosynthetic electron transport systems. Of principle interest are the members of the large respiratory oxidase superfamily known as the heme-copper oxidases. This superfamily includes the mammalian cytochrome c oxidase and many prokaryotic homologues. The structures of two enzymes in this superfamily have been determined to atomic resolution by X-ray diffraction techniques. The heme-copper oxidases caltalyze the reduction of O2 and utilize the free energy liberated by this reaction to pump protons electrogenically across the membrane bilayer (4 H+ / O2). This generates transmembrane voltage and pH gradients, constituting the protonmotive force. The protonmotive force is used to drive ATP synthesis, active transport of solutes and other reactions. The structure of these enyzmes show two putative proton-conducting channels and we are interested in the roles of residues in these channels during the catalytic cycle.

The bacterial oxidases offer the opportunity to utilize the full array of molecular genetics techniques in combination with spectroscopic methods to address the catalytic mechanism of these enzymes. Single-turnover rapid kinetics techniques are being utilized to examine these questions using site-directed mutations in two members of the heme-copper oxidase superfamily. These are the E. coli cytochrome bo3 ubiquinol oxidase and the Rhodobacter sphaeroides aa3-type cytochrome c oxidase. In addition, we are utilizing FTIR difference spectroscopy to identify specific amino acids directly engaged in the catalytic mechanism.

One structural question that remains is the location of the binding site for ubiquinol to cytochrome bo3. We are approaching this by using a photoreactive analogue of quinol which covalently attaches to cytochrome bo3 upon irradiation. Protein chemistry techniques, including mass spectroscopy, will be used in this project. Another approach to address this question is to use genetics methods by mapping mutants that confer resistance to inhibitors that compete with ubiquinol at a common binding site.

In addition to the studies on the heme-copper oxidases, we are also examining other respiratory enzymes that generate a protonmotive force. These are the cytochrome bd ubiquinol oxidase from E. coli and the cytochrome bc1 complex from Rhodobacter sphaeroides. We are in using techniques aimed at obtaining structural information, including X-ray diffraction, as well as examining aspects of the catalytic mechanisms of these enzymes.



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