Oxygen as Acceptor

Like most bacteria, Escherichia coli has a flexible and branched respiratory chain that enables the prokaryote to live under a variety of environmental conditions, from highly aerobic to completely anaerobic. In general, the bacterial respiratory chain is composed of dehydrogenases, a quinone pool, and reductases. Substrate specific dehydrogenases transfer reducing equivalents from various donor substrates (NADH, succinate, glycerophoshate, formate, hydrogen, pyruvate, and lactate) to a quinone pool (menaquinone, ubiquinone, and demethylmenoquinone). Then electrons from reduced quinones (quinols) are transferred by terminal reductases to different electron acceptors. Under aerobic growth conditions, the terminal electron acceptor is molecular oxygen. A transfer of electrons from quinol to O2 is served by two major oxidoreductases (oxidases), cytochrome bo3 and cytochrome bd. Terminal oxidases of aerobic respiratory chains of bacteria, which use O2 as the final electron acceptor, can oxidize one of two alternative electron donors, either cytochrome c or quinol. This review compares the effects of different inhibitors on the respiratory activities of cytochrome bo3 and cytochrome bd in E. coli. It also presents a discussion on the genetics and the prosthetic groups of cytochrome bo3 and cytochrome bd. The E. coli membrane contains three types of quinones which all have an octaprenyl side chain (C40). It has been proposed that the bo3 oxidase can have two ubiquinone-binding sites with different affinities. The spectral properties of cytochrome bd-II closely resemble those of cydAB-encoded cytochrome bd.