Potentiometric analysis of the cytochromes of an Escherichia coli mutant strain lacking the cytochrome d terminal oxidase complex

A combination of potentiometric analysis and electrochemically poised low-temperature difference spectroscopy was used to examine a mutant strain of Escherichia coli that was previously shown by immunological criteria to be lacking the cytochrome d terminal oxidase. It was shown that this strain is missing cytochromes d, a₁, and b₅₅₈ and that the cytochrome composition of the mutant is similar to that of the wild-type strain grown under conditions of high aeration. The data indicate that the high-aeration branch of the respiratory chain contains two cytochrome components, b₅₅₆ (midpoint potential [E(m)] = + 35 mV) and cytochrome o (E(m) = + 165 mV). The latter component binds to CO and apparently has a reduced-minus-oxidized split-α band with peaks at 555 and 562 nm. When the wild-type strain was grown under conditions of low aeration, the components of the cytochrome d terminal oxidase complex were observed: cytochrome d (E(m) = + 260 mV), cytochrome a₁ (E(m) = + 150 mV) and cytochrome b₅₅₈ (E(m) = + 180 mV). All cytochromes appeared to undergo simple one-electron oxidation-reduction reactions. In the absence of CO, cytochromes b₅₅₈ and o have nearly the same E(m) values. In the presence of CO, the E(m) of cytochrome o is raised, thus allowing cytochromes b₅₅₈ and o to be individually quantitated by potentiometric analysis when they are both present.