Mechanism of electron transfer in the cytochrome b/f complex of algae: evidence for a semiquinone cycle.

The most widely accepted mechanism of electron and proton transfer within the cytochrome (Cyt) b/f complex derives from the Q-cycle hypothesis originally proposed for the mitochondrial Cyt b/c1 complex by Mitchell [Mitchell, P. (1975) FEBS Lett. 57, 135-137]. In chloroplasts, the Cyt b/f complex catalyzes the oxidation of a plastoquinol at a site, Qo (the plastoquinol binding site), close to the inner aqueous phase and the reduction of a quinone at a site, Qi (the plastoquinone binding site), close to the stromal side of the membrane. In an alternative model, the semiquinone cycle [Wikström, M. & Krab, K. (1986) J. Bioenerg. Biomembr. 18, 181-193], a charged semiquinone formed at site Qo is transferred to site Qi where it is reduced into quinol. Flash-induced kinetics of the redox changes of Cyt b and of the formation of a transmembrane potential have been measured in Chlorella sorokiniana cells incubated in reducing conditions that induce a full reduction of the plastoquinone pool. The experiments were performed in the presence of an uncoupler that collapses the permanent electrochemical proton gradient and thus accelerates the rate of the electrogenic processes. The results show that the electrogenic reaction driven by the Cyt b/f complex precedes the processes of reduction or oxidation of the b-hemes. This electrogenic process is probably due to a transmembrane movement of a charged semiquinone, in agreement with the semiquinone-cycle hypothesis. This mechanism may represent an adaptation to reducing conditions when no oxidized quinone is available at the Qi site.