Crofts-2 - Biochemistry 1999 38 15791-15806 15791 Mechanism of Ubiquinol Oxidation by the bc1 Complex Role of the Iron Sulfur Protein and Its

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Mechanism of Ubiquinol Oxidation by the bc 1 Complex: Role of the Iron Sulfur Protein and Its Mobility ² Antony R. Crofts,* ,‡ Mariana Guergova-Kuras, LiShar Huang, § Richard Kuras, Zhaolei Zhang, § and Edward A. Berry* Center for Biophysics and Computational Biology, Uni V ersity of Illinois at Urbana - Champaign, Urbana, Illinois 61801, and Lawrence Berkeley National Laboratory, Uni V ersity of California, Berkeley, California 94720 Recei V ed April 27, 1999; Re V ised Manuscript Recei V ed August 16, 1999 ABSTRACT : Native structures of ubihydroquinone:cytochrome c oxidoreductase ( bc 1 complex) from different sources, and structures with inhibitors in place, show a 16 - 22 Å displacement of the [2Fe-2S] cluster and the position of the C-terminal extrinsic domain of the iron sulfur protein. None of the structures shows a static configuration that would allow catalysis of all partial reactions of quinol oxidation. We have suggested that the different conformations reflect a movement of the subunit necessary for catalysis. The displacement from an interface with cytochrome c 1 in native crystals to an interface with cytochrome b is induced by stigmatellin or 5- n -undecyl-6-hydroxy-4,7-dioxobenzothiazole (UHDBT) and involves ligand formation between His-161 of the [2Fe-2S] binding cluster and the inhibitor. The movement is a rotational displacement, so that the same conserved docking surface on the iron sulfur protein interacts with cytochrome c 1 and with cytochrome b . The mobile extrinsic domain retains essentially the same tertiary structure, and the anchoring N-terminal tail remains in the same position. The movement occurs through an extension of a helical segment in the short linking span. We report details of the protein structure for the two main configurations in the chicken heart mitochondrial complex and discuss insights into mechanism provided by the structures and by mutant strains in which the docking at the cytochrome b interface is impaired. The movement of the iron sulfur protein represents a novel mechanism of electron transfer, in which a tethered mobile head allows electron transfer through a distance without the entropic loss from free diffusion. The ubihydroquinone:cytochrome c oxidoreductase ( bc 1 complex) 1 (E.C. 1.10.2.2) and the related b 6 f complex of oxygenic photosynthesis are central components of the major electron-transfer chains that carry the energy flux of the biosphere. The bc 1 complex of the mitochondrial respiratory chain transfers electrons from ubihydroquinone (quinol) to cytochrome (cyt) c in the aqueous phase and catalyzes the coupled transfer of protons across the membrane. Our understanding of the function of these enzymes at the atomic level has been greatly aided by structures recently solved by X-ray crystallography. These include cyt f (the equivalent of cyt c 1 in the b 6 f complex) from chloroplasts ( 1, 2 ) and the Rieske iron sulfur protein (ISP) from the beef heart mitochondrial complex ( 3 ) and chloroplasts ( 4 ), all crystal-
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This note was uploaded on 04/27/2011 for the course PHYS 2202 taught by Professor Mackie during the Spring '11 term at Temple.

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Crofts-2 - Biochemistry 1999 38 15791-15806 15791 Mechanism of Ubiquinol Oxidation by the bc1 Complex Role of the Iron Sulfur Protein and Its

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