Heme-copper terminal oxidase using both cytochrome c and ubiquinol as electron donors

Abstract

The cytochrome c oxidase Cox2 has been purified from native membranes of the hyperthermophilic eubacterium Aquifex aeolicus. It is a cytochrome ba 3 oxidase belonging to the family B of the heme-copper containing terminal oxidases. It consists of three subunits, subunit I (CoxA2, 63.9 kDa), subunit II (CoxB2, 16.8 kDa), and an additional subunit IIa of 5.2 kDa. Surprisingly it is able to oxidize both reduced cytochrome c and ubiquinol in a cyanide sensitive manner. Cox2 is part of a respiratory chain supercomplex. This supercomplex contains the fully assembled cytochrome bc 1 complex and Cox2. Although direct ubiquinol oxidation by Cox2 conserves less energy than ubiquinol oxidation by the cytochrome bc 1complex followed by cytochrome c oxidation by a cytochrome c oxidase, ubiquinol oxidation by Cox2 is of advantage when all ubiquinone would be completely reduced to ubiquinol, e.g., by the sulfide-quinone oxidoreductase, because the cytochrome bc 1complex requires the presence of ubiquinone to function according to the Q-cycle mechanism. In the case that all ubiquinone has been reduced to ubiquinol its reoxidation by Cox2 will enable the cytochrome bc1 complex to resume working.