Activated Q-cycle as a common mechanism for cytochrome bc1 and cytochrome b6f complexes

Cytochrome bc(1) complexes of animals and bacteria ( hereafter bc(1)) as well as related cytochrome b(6)f complexes of plants and cyanobacteria (hereafter bf) are dimeric quinol cytochrome c/plastocyanin oxidoreductases capable of translocating protons across energy-converting membranes The commonly accepted Q-cycle mechanism suggests that these enzymes oxidize two quinol molecules in their catalytic centers P to yield one quinol molecule in another catalytic center N Earlier based upon data on flash-induced redox changes of cytochromes b and c(1) voltage generation and proton transfer in membrane vesicles of Rhodobacter capsulatus we have put forward a scheme of an activated Q-cycle for the bc(1) The scheme suggests that the bc(1) 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 bc(1) 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 Here It is argued that a similar pre-loading by two electrons can explain the available data on flash-induced reactions in cytochrome b(6)f-complexes of green plants and cyanobacteria (C) 2010 Elsevier B V All rights reserved