The cytochrome bc1 complex (ubiquinol:cytochrome c oxidoreductase) is a key energy converting enzyme that serves as a hub in the vast majority of electron transfer chains. In recent years it draws increasing attention as one of the major sources of hazardous reactive oxygen species (ROS) that are formed, as by-products, when oxygen molecules interact with semiquinone radicals that serve as intermediates in the catalytic cycle of the cytochrome bc1 complex.
The enzyme is unique in its ability to catalyze a bifurcated redox reaction. After each ubiquinol molecule is oxidized in the catalytic center P at the positively charged membrane side, the two liberated electrons head out, according to the Mitchell's Q-cycle mechanism, to different acceptors. One is taken by the [2Fe-2S] iron-sulfur Rieske protein to be passed further to cytochrome c1. The other electron goes across the membrane, via the low- and high-potential hemes of cytochrome b, to another ubiquinone-binding site N at the opposite membrane side. It has been assumed that two ubiquinol molecules have to be oxidized by center P to yield first a semiquinone in center N and then to reduce this semiquinone to ubiquinol.
Overview of structure and function of the cytochrome bc1 complex. Figure taken from 
Partial steps of the enzyme turnover and electrical silence of the transmembrane electron transfer
In the membranes of phototrophic purple bacteria, the generation of membrane voltage by the cytochrome bc1 complex can be traced – in a single turnover mode in response to a flash of light - via spectral shifts of native carotenoids and correlated with the electron and proton transfer reactions. Using this approach we succeeded to dissect the catalytic cycle of the enzyme into partial steps while studying it in a flash-triggered, single turnover mode. It was concluded that the turnover of the cytochrome bc1 complex proceeds in two steps, at least. During the first step, the FeS domain takes the first electron and the first proton from ubiquinol, while the other electron is transferred to heme bl. The second proton remains in center P. The electron moves then, in an electrically compensated way, from heme bl to heme bh at 1–2 ms. Upon the second, slower step, the FeS domain relocates to re-reduce cytochrome c1, protons are released upon the movement of the FeS domain, ΔΨ is generated, and heme bh is oxidized via center N. It seems that the conformational transmission across the enzyme serves to drive the proton binding in center N by the energetically gainful release of an uncompensated proton from center P [4-6].
Tentative scheme of electron and proton transfer during the initial steps of bc1 turnover. Figure taken from 
Based upon data on flash-induced redox changes of cytochromes b and c1, voltage generation, and proton transfer in membrane vesicles of Rhodobacter capsulatus, we have put forward a scheme of an “activated Q-cycle” for the cytochrome bc1 complex. The scheme suggests that the bc1 dimers, being “activated” by injection of electrons from the membrane ubiquinol pool via centers N, steadily contain two electrons in their cytochrome b moieties under physiological conditions, most likely, as a bound semiquinone in center N of one monomer and a reduced high-potential heme b in the other monomer. Then the oxidation of each ubiquinol molecule in centers P of an activated bc1 should result in a complete catalytic cycle leading to the formation of a ubiquinole molecule in the one of enzyme’s centers N and to voltage generation.
Activated Q-cycle in the cytochrome bc1 complex. Thin black arrows, electron transfer steps; thick red arrows, proton transfer steps; thick gray arrows, quinone/quinol exchange reactions. Blue “lightning signs“ indicate those steps that can be triggered by flashes of light in the bc1 of phototrophic bacteria. Protons are depicted as red circled crosses. The cartoon in the middle shows a tentative structure of an activated bc1 with a QN•- semiquinone and a reduced heme bh present in different monomers (colored in magenta and red, respectively, the PDB entry 1EZV was used). The figure is taken taken from 
Recently we have suggested that a similar pre-loading by two electrons can explain the available data on flash-induced reactions in cytochrome b6f- complexes of green plants and cyanobacteria .
Activated Q-cycle in the cytochrome bf complex (tentative scheme). Electrons at the cn/bh heme pair are shown as circled minus signs (magenta). Because of space limitations, plastoquinone is denoted as Q and not PQ. Otherwise the notation is similar to that in previous Fig. A, the activated state of the bf: the FeS clusters and cytochromes f are pre-reduced and one electron is shared by each cn/bh heme pair. Transition A → B, fast oxidation of a plastoquinol molecule in center P: an electron vacancy migrates from the photosystem I to the bf complex after the flash; the oxidized FeS domain docks to cytochrome b6 and binds a plastoquinol molecule in center P; oxidation of this plastoquinol molecule is accompanied by a delivery of an electron and a proton to the FeS cluster leaving a semiquinone in center P; the second electron goes from the semiquinone, via heme bl, to the pre-reduced cn/bh heme pair; the FeS domain remains docked to cytochrome b6 and plugs the proton outlet from center P. Transition B → C, slow formation of a plastoquinol molecule in center N and voltage generation: a two-electron oxidation of the cn/bh heme pair generates a plastoquinol molecule in center N and causes electrogenic binding of two protons from the n-side of the membrane; the formation of this plastoquinol molecule (or its release from center N) enables undocking of a reduced FeS domain and its movement towards cytochrome f; the oxidation of the reduced and protonated FeS domain by cytochrome f is accompanied by proton release into the water phase; concomitantly protons leave center P via the outlet that is now open; displacement of all these protons across the membrane dielectric, as well as the re-orientation of the intra-membrane dipoles, account for a large electrogenic reaction. Transition C → D and D → E describe the oxidation of the second plastoquinol molecule in the other center P and are essentially similar to the transitions A → B and B → C, respectively. Blue dashed arrows indicate that the bf can be activated (transition F → A) either via the reduction of heme cn by ferredoxin, or via the light-induced oxidation of two plastoquinol molecules in center P, or through the dark oxidation of a plastoquinol molecule via center N. The figure is taken taken from 
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