The structure of the ubiquinol oxidase from Escherichia coli and its ubiquinone binding site

Cell respiration is catalyzed by the heme-copper oxidase superfamily of enz ymes, which comprises cytochrome c and ubiquinol oxidases, These membrane p roteins utilize different electron donors through dissimilar access mechani sms. We report here the first structure of a ubiquinol oxidase, cytochrome bo(3), from Escherichia coli, The overall structure of the enzyme is simila r to those of cytochrome c oxidases; however, the membrane-spanning region of subunit I contains a cluster of polar residues exposed to the interior o f the lipid bilayer that is not present in the cytochrome c oxidase, Mutage nesis studies on these residues strongly suggest that this region forms a q uinone binding site. A sequence comparison of this region with known quinon e binding sites in other membrane proteins shows remarkable similarities. I n light of these findings we suggest specific roles for these polar residue s in electron and proton transfer in ubiquinol oxidase.