The mechanism of ubihydroquinone oxidation at the Qo-site of the cytochrome bc1 complex

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 Q(o)-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-Bronsted 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 similar to 8.5 favoring electron transfer. Loss of a pK similar to 6.5 is explained by a change in the rate limiting step from the first to the second electron transfer; the pK similar to 8.5 may reflect dissociation of QH center dot. 6. A rate constant (<10(3) s(-1)) for oxidation of SQ in the distal domain by heme b(L) 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 center dot, and rotational displacement to deliver the H+ to exit channel(s). This opens a volume into which Q center dot(-) 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. (C) 2013 Elsevier B.V. All rights reserved.