Structural studies on the integral membrane protein, ubiquinol oxidase from Escherichia coli

Sammanfattning: Heme-copper oxidases are redox-driven proton pumps that couple the reduction of molecular oxygen to water with the vectorial translocation of protons across the membrane. The proton gradient generated by heme-copper oxidases and the other members of the aerobic respiratory chain is ultimately used to drive the synthesis of ATP. There are two main branches of the heme-copper oxidases that are characterized by the electron donating substrate; the cytochrome c oxidases, which use cytochrome c as the electron donor, and the ubiquinol oxidases, which use a lipid-soluble molecule, ubiquinol, as their electron donor. These enzymes share important structural and functional features. This thesis presents the procedures that have led to the first crystal structure of a ubiquinol oxidase, cytochrome bo, oxidase from Escherichia coli, at a resolution of 3.5 Å. The overall structure of the enzyme is similar to those of cytochrome c oxidases; however the membrane spanning region of subunit I contains a cluster of polar residues exposed to the interior of the lipid bilayer. No such structural feature is present in cytochrome c oxidases. Mutagenesis studies on residues in this region strongly suggest that this area forms a ubiquinone binding site. A comparison of this region with known ubiquinone binding sites shows remarkable similarities. In light of these findings specific roles for these polar residues is proposed in electron and proton transfer in ubiquinol oxidase. A fusion protein of cytochrome bo3-Protein Z was generated in an attempt to increase the hydrophilic surface of the protein, thus extending protein-protein contacts within the crystal lattice structure. Such an approach can be used to facilitate crystallization.