title: The mechanism of ubihydroquinone oxidation at the Qo-site of the cytochrome bc1 complex.
abstract: 1. Recent results suggest that the major flux is carried by a monomeric function, not by an intermonomer electron flow. 2. The bifurcated reaction at the Qo-site involves sequential partial processes, - a rate limiting first electron transfer generating a semiquinone (SQ) intermediate, and a rapid second electron transfer in which the SQ is oxidized by the low potential chain. 3. The rate constant for the first step in a strongly endergonic, proton-first-then-electron mechanism, is given by a Marcus-Brønsted treatment in which a rapid electron transfer is convoluted with a weak occupancy of the proton configuration needed for electron transfer. 4. A rapid second electron transfer pulls the overall reaction over. Mutation of Glu-295 of cyt b shows it to be a key player. 5. In more crippled mutants, electron transfer is severely inhibited and the bell-shaped pH dependence of wildtype is replaced by a dependence on a single pK at ~8.5 favoring electron transfer. Loss of a pK ~6.5 is explained by a change in the rate limiting step from the first to the second electron transfer; the pK ~8.5 may reflect dissociation of QH. 6. A rate constant (<10(3)s(-1)) for oxidation of SQ in the distal domain by heme bL has been determined, which precludes mechanisms for normal flux in which SQ is constrained there. 7. Glu-295 catalyzes proton exit through H(+) transfer from QH, and rotational displacement to deliver the H(+) to exit channel(s). This opens a volume into which Q(-) can move closer to the heme to speed electron transfer. 8. A kinetic model accounts well for the observations, but leaves open the question of gating mechanisms. For the first step we suggest a molecular "escapement"; for the second a molecular ballet choreographed through coulombic interactions. This article is part of a Special Issue entitled: Respiratory complex III and related bc complexes.
authors: Center for Biophysics and Computational Biology, University of Illinois, Urbana, IL 61801, USA; Department of Biochemistry, University of Illinois, Urbana, IL 61801, USA. Electronic address: crofts@illinois.edu.
authors: Antony R
authors: AR
authors: Crofts
authors: Sangjin
authors: S
authors: Hong
authors: Charles
authors: C
authors: Wilson
authors: Rodney
authors: R
authors: Burton
authors: Doreen
authors: D
authors: Victoria
authors: Chris
authors: C
authors: Harrison
authors: Klaus
authors: K
authors: Schulten
mesh_terms: D001665:Binding Sites / Q000235:genetics; D014450:Electron Transport Complex III / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D006418:Heme / Q000737:chemistry / Q000378:metabolism*; D007700:Kinetics; D008958:Models, Molecular; D009154:Mutation; D010084:Oxidation-Reduction; D011487:Protein Conformation; D021122:Protein Subunits / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D014451:Ubiquinone / Q000737:chemistry / Q000378:metabolism*
chemical_list: D021122:Protein Subunits; D014451:Ubiquinone; D006418:Heme; D014450:Electron Transport Complex III
keywords: Bifurcated reaction of Q-cycle; Control and gating; ES(1); ES(2), enzyme–substrate complexes for first and second electron transfers, respectively; H(+) exit pathway; ISP; Iron–sulfur protein; Kinetic model; Q; Q(−); QH; QH(2); ROS; SQ; SU IV; Semiquinone occupancy; anionic semiquinone; neutral semiquinone; reactive oxygen species; semiquinone (dissociation state unspecified); subunit IV; ubihydroquinone-10, ubiquinol, quinol; ubiquinone-10, quinone
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