Proton translocation by the cytochrome bc1 complexes of phototrophic bacteria: introducing the activated Q-cycle.
The cytochrome bc1 complexes are proton-translocating, dimeric membrane ubiquinol:cytochrome c oxidoreductases that serve as "hubs" in the vast majority of electron transfer chains. After each ubiquinol molecule is oxidized in the catalytic center P at the positively charged membrane side, the two liberated electrons head out, according to the Mitchell's Q-cycle mechanism, to different acceptors. One is taken by the [2Fe-2S] iron-sulfur Rieske protein to be passed further to cytochrome c1. The other electron goes across the membrane, via the low- and high-potential hemes of cytochrome b, to another ubiquinone-binding site N at the opposite membrane side. It has been assumed that two ubiquinol molecules have to be oxidized by center P to yield first a semiquinone in center N and then to reduce this semiquinone to ubiquinol. This review is focused on the operation of cytochrome bc1 complexes in phototrophic purple bacteria. Their membranes provide a unique system where the generation of membrane voltage by light-driven, energy-converting enzymes can be traced via spectral shifts of native carotenoids and correlated with the electron and proton transfer reactions. An "activated Q-cycle" is proposed as a novel mechanism that is consistent with the available experimental data on the electron/proton coupling. Under physiological conditions, the dimeric cytochrome bc1 complex is suggested to be continually primed by prompt oxidation of membrane ubiquinol via center N yielding a bound semiquinone in this center and a reduced, high-potential heme b in the other monomer of the enzyme. Then the oxidation of each ubiquinol molecule in center P is followed by ubiquinol formation in center N, proton translocation and generation of membrane voltage.
Mulkidjanian
Armen Y
AY
A. N. Belozersky Institute of Physico-Chemical Biology, Moscow State University, 119899, Moscow, Russia. amulkid@uos.de
D020130:Bacterial Chromatophores / Q000378:metabolism; D014450:Electron Transport Complex III / Q000378:metabolism*; D004583:Electrons; D008954:Models, Biological; D010084:Oxidation-Reduction; D011522:Protons; D016354:Rhodobacter capsulatus / Q000201:enzymology*; D014451:Ubiquinone / Q000031:analogs & derivatives / Q000378:metabolism
D011522:Protons; D014451:Ubiquinone; D014450:Electron Transport Complex III; C003741:ubiquinol