Effects of pH and detergent on the kinetic and electrochemical properties of the purified cytochrome d terminal oxidase complex of Escherichia coli

The cytochrome d terminal oxidase complex is one of the two terminal oxidases in the aerobic respiratory chain of Escherichia coli. In this paper, effects of pH and detergent on the electrochemical and kinetic properties of the enzyme are investigated. There are two significant conclusions. (1) The oxidation-reduction midpoint potential of the cytochrome b-558 component is sensitive to the detergent used to solubilize the complex. In particular, it is shown that octylglucoside and cholate cause a large decrease in the midpoint potential of cytochrome b-558, while they also result in the reversible inactivation of the oxidase. (2) The midpoint potentials of the cytochrome b-558, a₁ and d components are sensitive to pH. More acidic solutions result in stabilizing the reduced forms of the redox-active groups, i.e., raising their midpoint potentials. This may be significant in view of the fact that it has been demonstrated that this enzyme catalyzes an electrogenic reaction and appears to function as a proton pump.