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Electron transfer by domain movement in cytochrome bc1

Abstract

The cytochrome bc1 is one of the three major respiratory enzyme complexes residing in the inner mitochondrial membrane. Cytochrome bc1 transfers electrons from ubiquinol to cytochrome c and uses the energy thus released to form an electrochemical gradient across the inner membrane. Our X-ray crystal structures of the complex from chicken, cow and rabbit in both the presence and absence of inhibitors of quinone oxidation, reveal two different locations for the extrinsic domain of one component of the enzyme, an iron–sulphur protein. One location is close enough to the supposed quinol oxidation site to allow reduction of the Fe–S protein by ubiquinol. The other site is close enough to cytochrome c1 to allow oxidation of the Fe–S protein by the cytochrome. As neither location will allow both reactions to proceed at a suitable rate, the reaction mechanism must involve movement of the extrinsic domain of the Fe–S component in order to shuttle electrons from ubiquinol to cytochrome c1. Such a mechanism has not previously been observed in redox protein complexes.

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Figure 1: Stereo-view ribbon diagrams of the bc1 complex.
Figure 2: Inhibitor-binding sites.
Figure 3: Structure of the intermembrane (external surface) domains of the chicken bc1 complex.
Figure 4: The structures of cytochrome c1 and cytochrome c.
Figure 5: The Rieske iron–sulphur protein.
Figure 6: Relative positions of the redox centres in the two different conformations of the bc1 complex dimer.
Figure 7: Electron pathway through cytochrome c1 in a hypothetical complex of the bc1 complex with cytochrom.

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Acknowledgements

We thank T. Link and his co-workers for providing coordinates for the soluble domain of the Rieske iron–sulphur protein before the release from the Protein Data Bank; H. Bellamy for performing the XAFS scan and for advice on MAD data collection; S. Hong for preparing coordinate files for the inhibitors; T. Meyer for critically reading the manuscript; and L. Tong for advice on molecular averaging. This work was supported by the NIH (grants to E.A.B. and A.R.C.) and by the Office of Biosciences and Environmental Research, US Department of Energy (grant to S.-H.K.). The work was partially done at SSRL which is operated by the Department of Energy, Division of Chemical/Material Sciences. The SSRL Biotechnology Program is supported by the NIH Biomedical Resource Technology Program, Division of Research Resources.

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Zhang, Z., Huang, L., Shulmeister, V. et al. Electron transfer by domain movement in cytochrome bc1. Nature 392, 677–684 (1998). https://doi.org/10.1038/33612

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