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Proton translocation by the cytochrome bc1 complexes of phototrophic bacteria: introducing the activated Q-cycle

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Abstract

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.

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  1. A. N. Belozersky Institute of Physico-Chemical Biology, Moscow State University, 119899, Moscow, Russia

    Armen Y. Mulkidjanian

  2. School of Physics, University of Osnabrück, D-49069, Osnabrück, Germany

    Armen Y. Mulkidjanian

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Submitted as part of the special issue ‘Proton Transfer in Biological Systems’, Photochem. Photobiol. Sci., 2006, 5(6).

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Mulkidjanian, A.Y. Proton translocation by the cytochrome bc1 complexes of phototrophic bacteria: introducing the activated Q-cycle. Photochem Photobiol Sci 6, 19–34 (2007). https://doi.org/10.1039/b517522d

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