98_Qcycle.

98_Qcycle. - FEBS 20531 FEBS Letters 431(1998 291^296 The cytochrome bcI complex of Rhodobacter capsulatus ubiquinol oxidation in a dimeric Q-cycle

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The cytochrome bc 1 complex of Rhodobacter capsulatus : ubiquinol oxidation in a dimeric Q-cycle? Oxana A. Gopta a , Boris A. Feniouk a , Wolfgang Junge b , Armen Y. Mulkidjanian a ; b ; * a A.N. Belozersky Institute of Physico-Chemical Biology, Moscow University, Moscow 119899, Russia b Division of Biophysics, Faculty of Biology and Chemistry, University of Osnabru ºck, D-49069 Osnabru ºck, Germany Received 15 June 1998 Abstract We studied the cytochrome bc 1 complex (hereafter bc ) by flash excitation of Rhodobacter capsulatus chromato- phores. The reduction of the high-potential heme b h of cytochrome b (at 561 nm) and of cytochromes c (at 552 nm) and the electrochromic absorption transients (at 524 nm) were monitored after the first and second flashes of light, respectively. We kept the ubiquinone pool oxidized in the dark and concerned for the ubiquinol formation in the photosynthetic reaction center only after the second flash. Surprisingly, the first flash caused the oxidation of about one ubiquinol per bc dimer. Based on these a n do t h e rd a t aw ep r o p o s ead im e r i cQ - c y c l ew h e r et h e energetically unfavorable oxidation of the first ubiquinol molecule by one of the bc monomers is driven by the energetically favorable oxidation of the second ubiquinol by the other bc monomer resulting in a pairwise oxidation of ubiquinol molecules by the dimeric bc in the dark. The residual unpaired ubiquinol supposedly remains on the enzyme and is then oxidized after the first flash. z 1998 Federation of European Biochemical Societies. Key words: Ubiquinone; Cytochrome bc 1 complex; Electrogenesis; Proton transfer; Electron transfer; Rhodobacter capsulatus 1. Introduction Cytochrome bc 1 complexes of animals and bacteria (here- after denoted bc ) and cytochrome bf complexes of chloro- plasts are membrane quinol:cytochrome c oxidoreductases (see [1^4] for reviews). Mitchell described their operation by a so-called Q-cycle [5]. In this concept, one out of two elec- trons which are released upon quinol oxidation is transferred to cyt c (located at the positively charged p -side of the mem- brane), whereas the second electron crosses the membrane and reduces another quinone close to the negatively charged n -side of the membrane. As a result, bc pumps protons from the n - side of the membrane to the p
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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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98_Qcycle. - FEBS 20531 FEBS Letters 431(1998 291^296 The cytochrome bcI complex of Rhodobacter capsulatus ubiquinol oxidation in a dimeric Q-cycle

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