Electrogenic events associated with electron and proton transfers within the cytochrome b6/f complex

The kinetics and amplitude of the membrane potential changes associated with electron and proton transfers within the cytochrome b(6)/f(cyt b/f) complex (phase b) are measured in vivo in Chlamydomonas reinhardtii under anaerobic conditions. Upon saturating flash excitation, fast components in the membrane potential decay superimposed on phase b lead to an underestimation of the amplitude of this phase. In the FUD50 mutant strain, which lacks the ATP synthase, the decay of the membrane potential is slowed down compared to the wild type, and the kinetics and amplitude of phase b may be accurately determined. This amplitude corresponds to the transfer of at least 1.5 charges across the membrane per positive charge transferred to photosystem I, whatever the flash energy. This value largely exceeds that predicted by a Q-cycle process. Similar conclusions are reached using the wild type strain in the presence of 9 muM dicyclohexylcarbodiimide, which specifically inhibits the ATP synthase. It is concluded that a proton pumping process is operating in parallel with the Q-cycle, with a yield of similar to 0.5 proton pumped by cyt b/f complex turnover, irrespective of the flash energy. (C) 2001 Elsevier Science B.V. All rights reserved.