Spectroscopic and genetic evidence for two heme-Cu-containing oxidases in Rhodobacter sphaeroides

It has recently become evident that many bacterial respiratory oxidases are members of a superfamily that is related to the eukaryotic cytochrome c oxidase. These oxidases catalyze the reduction of oxygen to water at a heme-copper binuclear center. Fourier transform infrared (FTIR) spectroscopy has been used to examine the heme-copper-containing respiratory oxidases of Rhodobacter sphaeroides Ga. This technique monitors the stretching frequency of CO bound at the oxygen binding site and can be used to characterize the oxidases in situ with membrane preparations. Oxidases that have a heme-copper binuclear center are recognizable by FTIR spectroscopy because the bound CO moves from the heme iron to the nearby copper upon photolysis at low temperature, where it exhibits a diagnostic spectrum. The FTIR spectra indicate that the binuclear center of the R. sphaeroides aa3-type cytochrome c oxidase is remarkably similar to that of the bovine mitochondrial oxidase. Upon deletion of the ctaD gene, encoding subunit I of the aa3-type oxidase, substantial cytochrome c oxidase remains in the membranes of aerobically grown R. sphaeroides. This correlates with a second heme-copper oxidase in these membranes revealed by FTIR spectroscopy. When wild-type R. sphaeroides is grown photosynthetically, the chromatophore membranes lack the aa3-type oxidase but have this second heme-copper oxidase. Subunit I of the heme-copper oxidase superfamily contains the binuclear center. Amino acid sequence alignments show that this subunit is structurally very highly conserved among both eukaryoti