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Issue 1, 2007
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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.

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

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Publication details

The article was received on 12 Dec 2005, accepted on 07 Nov 2006 and first published on 07 Dec 2006


Article type: Perspective
DOI: 10.1039/B517522D
Citation: Photochem. Photobiol. Sci., 2007,6, 19-34
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    Proton translocation by the cytochrome bc1 complexes of phototrophic bacteria: introducing the activated Q-cycle

    A. Y. Mulkidjanian, Photochem. Photobiol. Sci., 2007, 6, 19
    DOI: 10.1039/B517522D

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