Physiological functions of the respiratory cytochrome bd and the CuA nitric oxide reductase (CuANor)

Abstract

Biological membranes form a binding platform for a variety of proteins vital to the cell. The respiratory chain consists mostly of membrane-bound enzymes. These enzymes form a functional chain in which electrons derived from nutrition successively flow from low potential to high potential redox couples. During this process, the chemical energy is transformed to potential energy in the form of a transmembrane electrochemical gradient, which is subsequently stored in the form of ATP. The operation of this electron transfer chain and the mechanisms whereby a transmembrane electrochemical gradient is formed and utilized for ATP production are reviewed (Chapter 2). Cytochrome bd is a terminal oxidase of the aerobic respiratory pathway that catalyzes the electrogenic reduction of oxygen to water using ubiquinol as electron donor. Reports have shown that disruption of cytochrome bd in bacteria leads to increased sensitivity to hydrogen peroxide (H2O2), accumulation of H2O2 and decreased virulence. Here we show that besides its oxidase activity, cytochrome bd from Escherichia coli is a genuine quinol peroxidase that reduces hydrogen peroxide to water (Chapter 3). The observation that cytochrome bd is a quinol peroxidase, provides a biochemical basis for its role in detoxification of exogenous peroxide such as encountered in phagosomes or generated by competing bacteria in the natural environment. Another respiratory enzyme, nitric oxide reductase (Nor), an enzyme of the denitrification pathway, reduces nitric oxide (NO) to nitrous oxide (N2O). The cytochrome-c-dependent cNor was shown to be non-electrogenic, i.e. unable to conserve energy. Based on its structure, the quinol-dependent qNor, was suggested to be an electrogenic enzyme and is implicated in virulence, as it can detoxify the NO produced in phagocytes. A third type of Nor is the copper-A-Nor (CuANor) from Bacillus azotoformans (Chapters 4 and 5). This enzyme is highly homologous to the proton-pumping cytochrome ba3 oxidase. Homology modeling indicated that CuANor contains the proton entry and proton exit pathways implicated in proton pumping by the cytochrome ba3 oxidase. Using proteoliposomes, we show here that CuANor is electrogenic and thus couples NO reduction directly to the formation of a proton-dependent electochemical gradient. By employing CuANor bacilli generate more ATP than organisms that employ cNOR. More insight into the interplay between respiration for bioenergetic ends and “respiration” with detoxification as a goal can aid in the design of bacteriocidal drugs to combat human pathogens.