There have been numerous instances in the recent literature where the properties of ubiquinol and cytochrome c terminal oxidases are compared. Here we specifically examine the cytochrome bo 3-type ubiquinol oxidase from Escherichia coli and the cytochrome aa 3-type cytochrome c oxidases. A second redox-active copper site (CuA) is present only in the cytochrome c oxidases and the physiological electron donors for the two enzymes are different (ubiquinol-8 vs. ferrocytochrome c). In our opinion, these differences are significant and most likely indicate that distinct turnover mechanisms are operative in the two enzymes.
