Rhodobacter capsulatus Contains a Novel cb-Type Cytochrome c Oxidase without a Cu_A Center

The facultative phototrophic bacterium Rhodobacter capsulatus is capable of growth in a wide range of environmental conditions using a highly branched electron-transfer chain. During respiratory growth of this organism reducing equivalents are conveyed to oxygen via two terminal oxidases, previously called “cyt b₄₁₀” (cytochrome c oxidase) and “cyt b₂₆₀” (quinol oxidase). The cytochrome c oxidase was purified to homogeneity from a semiaerobically grown R. capsulatus strain. The purified enzyme consumes oxygen at a rate of 600 s⁻¹, oxidizes reduced equine cyt c and R. capsulatus cyt c₂, and has high sensitivity to cyanide. The complex is composed of three major polypeptides of apparent molecular masses 45, 32, and 28 kDa on SDS—PAGE. The 32- and 28-kDa proteins also stain with tetramethylbenzidine, indicating that they are c-type cytochromes. Partial amino acid sequences obtained from each of the subunits reveal significant homology to the fixN,fixO, and fixP gene products of Bradyrhizobium japonicum and Rhizobium meliloti. The reduced enzyme has an optical absorption spectrum with distinct features near 550 and 560 nm and an asymmetric Soret band centered at 418 nm, indicating the presence of both c- and b-type cytochromes. Two electrochemically distinct cyt c are apparent, with redox midpoint potentials (E_m7) of 265 and 320 mV, while the low-spin cyt b has an E_m7 value of 385 mV. The enzyme binds carbon monoxide, and the CO difference spectrum indicates that CO binds to a high-spin cyt b. Pyridine hemochrome and HPLC analyses suggest that the complex contains 1 mol of heme C to 1 mol of protoheme and that neither heme O nor heme A is present. Further, the R. capsulatus mutant M7 and its derivatives known to lack “cyt b₄₁₀” also lack the 32-kDa cyt c subunit, and redox titrations of the mutant membranes indicate that the 320-mV cyt c correlates with the remaining 28-kDa cyt c. Electron paramagnetic resonance spectroscopy of the purified enzyme performed at 10 and 77 K shows the presence of both low-spin and high-spin ferricytochromes, but in contrast to all other previously characterized cyt c oxidases, no indication for a Cu_A-like signal was found at either temperature. The overall data therefore indicate that R. capsulatus contains a novel cb-type cyt c oxidase without a Cu_A center.