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Ubiquinol-cytochrome c reductase is a redox driven proton pump that utilises the free energy of ubiquinol reduction for the creation of a proton gradient across membranes (periplasma in prokaryotes and intermembrane space in mitochondria) membrane. The proton-motive Q cycle mechanism best explains experimental results on the ET pathway through the four redox centres of the bc1 complex. The mechanism postulates two separate quinone binding sites, one for quinol oxidation (Qo site) and the other for quinone reduction (Qi site), and a bifurcated electron flow at the Qo site where the first electron from the the substrate quinol is transferred sequentially to the ISP domain (containing an iron-sulfur complex) to the cyt. c1 domain (containing a 1c type heme), and eventually to the soluble electron acceptor cyt. c, whereas the second electron is transferred to the b-type hemes bL and bH in sequence, ending at a quinone or a semiquinone anion at the Qi site. The complete Q cycle consumes two molecules of quinol, generates one molecule of quinone and translocates four protons to the positive side of the membrane. The exact order of the steps in this mechanism remains unclear. There is evidence of a long range interaction between both catalytic sites. Although it is not entirely clear is the oxidation/reduction of the quinol/quinone species are concerted or stepwise, evidence supports the stepwise manner shown here. It has been documented [PMID:12885240] that the ubisemiquinone radical produced during this step required an alkaline pH to be stable in the Qi site, consistent with the assumption that both Asp228 and His201 are deprotonated. Furthermore, electron nuclear double resonance experiments showed exchangeable H-bonds bonds to the ubisemiquinone radical, suggesting that the water-mediated H-bonding acts to stabilise the free radical formed.