Source: https://www.macromol.uni-osnabrueck.de/BC1_complex.php
Timestamp: 2019-04-25 20:24:37+00:00

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The cytochrome bc1 complex (ubiquinol:cytochrome c oxidoreductase) is a key energy converting enzyme that serves as a hub in the vast majority of electron transfer chains. In recent years it draws increasing attention as one of the major sources of hazardous reactive oxygen species (ROS) that are formed, as by-products, when oxygen molecules interact with semiquinone radicals that serve as intermediates in the catalytic cycle of the cytochrome bc1 complex.
The enzyme is unique in its ability to catalyze a bifurcated redox reaction. 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.
In the membranes of phototrophic purple bacteria, the generation of membrane voltage by the cytochrome bc1 complex can be traced � in a single turnover mode in response to a flash of light - via spectral shifts of native carotenoids and correlated with the electron and proton transfer reactions. Using this approach we succeeded to dissect the catalytic cycle of the enzyme into partial steps while studying it in a flash-triggered, single turnover mode. It was concluded that the turnover of the cytochrome bc1 complex proceeds in two steps, at least. During the first step, the FeS domain takes the first electron and the first proton from ubiquinol, while the other electron is transferred to heme bl. The second proton remains in center P. The electron moves then, in an electrically compensated way, from heme bl to heme bh at 1�2 ms. Upon the second, slower step, the FeS domain relocates to re-reduce cytochrome c1, protons are released upon the movement of the FeS domain, ΔΨ is generated, and heme bh is oxidized via center N. It seems that the conformational transmission across the enzyme serves to drive the proton binding in center N by the energetically gainful release of an uncompensated proton from center P [4-6].
Based upon data on flash-induced redox changes of cytochromes b and c1, voltage generation, and proton transfer in membrane vesicles of Rhodobacter capsulatus, we have put forward a scheme of an �activated Q-cycle� for the cytochrome bc1 complex. The scheme suggests that the bc1 dimers, being �activated� by injection of electrons from the membrane ubiquinol pool via centers N, steadily contain two electrons in their cytochrome b moieties under physiological conditions, most likely, as a bound semiquinone in center N of one monomer and a reduced high-potential heme b in the other monomer. Then the oxidation of each ubiquinol molecule in centers P of an activated bc1 should result in a complete catalytic cycle leading to the formation of a ubiquinole molecule in the one of enzyme�s centers N and to voltage generation.
Recently we have suggested that a similar pre-loading by two electrons can explain the available data on flash-induced reactions in cytochrome b6f- complexes of green plants and cyanobacteria .
5. Klishin,S.S. and A.Y.Mulkidjanian. 2005. Proton transfer paths at the quinol oxidizing site of the Rb. capsulatus cytochrome bc complex. In Photosynthesis: Fundamental aspects to global perspectives. A.van der Est and D.Bruce, editors. Allen Press Inc., Lawrence/Montreal, pp. 260-262.
6. Klishin, S.S., W. Junge, and A.Y. Mulkidjanian. 2002. Flash-induced turnover of the cytochrome bc1 complex in chromatophores of Rhodobacter capsulatus: binding of Zn2+ decelerates likewise the oxidation of cytochrome b, the reduction of cytochrome c1 and the voltage generation. Biochim. Biophys. Acta 1553:177-182.
9. Gopta, O.A. and A.Y. Mulkidjanian. 1998. The cytochrome bc1-complex of Rhodobacter capsulatus: does the reaction in the presence of antimycin A correspond to a single turnover of an untreated enzyme? In Photosynthesis: Mechanisms and Effects. G. Garab, editor. Kluwer Academic Publishers, Dordrecht. 1533-1536.
11. Mulkidjanian, A. Y. and W. Junge. 1994. Calibration and time resolution of lumenal pH-transients in chromatophores of Rhodobacter capsulatus following a single turnover flash of light: proton release by the cytochrome-bc1 complex is strongly electrogenic. FEBS Letters. 353:189-193.
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16. Shinkarev, V. P., A. L. Drachev, M. D. Mamedov, A. Y. Mulkidjanian, A. Y. Semenov and M. I. Verkhovsky. 1990. Light-induced electron transfer and electrogenic reactions in the cytochrome-bc1 complex of photosynthetic purple bacterium Rhodobacter sphaeroides. p. 393-400. In Molecular Biology of Membrane-Bound Complexes in Phototrophic Bacteria (G. Drews and E. A. Dawes, eds.). Plenum Press, New York.
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