PROJECTION STRUCTURE OF THE CYTOCHROME-BO UBIQUINOL OXIDASE FROM ESCHERICHIA-COLI AT 6-ANGSTROM RESOLUTION

The haem-copper cytochrome oxidases are terminal catalysts of the resp iratory chains in aerobic organisms. These integral membrane protein c omplexes catalyse the reduction of molecular oxygen to water and utili ze the free energy of this reaction to generate a transmembrane proton gradient. Quinol oxidase complexes such as the Escherichia coli cytoc hrome bo belong to this superfamily. To elucidate the similarities as well as differences between ubiquinol and cytochrome c oxidases, we ha ve analysed two-dimensional crystals of cytochrome bo by cryo-electron microscopy. The crystals diffract beyond 5 Angstrom. A projection map was calculated to a resolution of 6 Angstrom. All four subunits can b e identified and single alpha-helices are resolved within the density for the protein complex. The comparison with the three-dimensional str ucture of cytochrome c oxidase shows the clear structural similarity w ithin the common functional core surrounding the metal-binding sites i n subunit I. It also indicates subtle differences which are due to the distinct subunit composition, This study can be extended to a three-d imensional structure analysis of the quinol oxidase complex by electro n image processing of tilted crystals.