Projection structure of the cytochrome bo ubiquinol oxidase from Escherichia coli at 6 resolution

subunits that always include one or more homologues of Ulrich Gohlke, Antony Warne and the mitochondrially coded eukaryotic proteins (Saraste, Matti Saraste 1990). European Molecular Biology Laboratory, Biological Structures The Gram-negative eubacterium Escherichia coli posProgramme, Meyerhofstrasse 1, Postfach 10.2209, sesses a branched respiratory chain. Depending on the D-69012 Heidelberg, Germany growth conditions, ubiquinol-8 is oxidized by molecular oxygen either by the cytochrome bd or the cytochrome The haem–copper cytochrome oxidases are terminal bo complex. Only the cytochrome bo ubiquinol oxidase catalysts of the respiratory chains in aerobic organisms. (hereafter referred to as Cyt bo) is a member of the haem– These integral membrane protein complexes catalyse copper oxidase family. Neither cytochrome c reductase the reduction of molecular oxygen to water and utilize nor cytochrome c oxidase are present in E.coli (Anraku the free energy of this reaction to generate a transmemand Gennis, 1987). Similarly to the mitochondrial brane proton gradient. Quinol oxidase complexes such cytochrome c oxidase, Cyt bo generates a proton motive as the Escherichia coli cytochrome bo belong to this force (Kita et al., 1982) by a combination of a scalar superfamily. To elucidate the similarities as well as mechanism and a proton pump coupled to the redox differences between ubiquinol and cytochrome c oxidchemistry (Puustinen et al., 1989). ases, we have analysed two-dimensional crystals of The genes for Cyt bo are organized in a single operon. cytochrome bo by cryo-electron microscopy. The crysFive open reading frames, cyoABCDE, encode the subunits tals diffract beyond 5 Å. A projection map was calcuII, I, III and IV of the complex and a protohaem IX lated to a resolution of 6 Å. All four subunits can be farnesyltransferase, respectively (Chepuri et al., 1990; identified and single α-helices are resolved within the Saiki et al., 1992). Subunits I–III are homologous (10– density for the protein complex. The comparison with 40% identity) to the major subunits of the mitochondrial the three-dimensional structure of cytochrome c oxidand bacterial cytochrome c oxidases (Chepuri et al., 1990; ase shows the clear structural similarity within the Saraste, 1990), but subunit IV shows no homology to any common functional core surrounding themetal-binding component of the latter (Chepuri et al., 1990; Haltia sites in subunit I. It also indicates subtle differences et al., 1994). which are due to the distinct subunit composition. This Cyt bo can be purified as an intact protein complex, study can be extended to a three-dimensional structure containing all four subunits (Matsushita et al., 1984; analysis of the quinol oxidase complex by electron Georgiou et al., 1988). It contains two haem groups, image processing of tilted crystals. which are haems B and O (Puustinen and Wikström,