Kinetic effects of the electrochemical proton gradient on plastoquinone reduction at the Qi site of the cytochrome b(6)f complex

We have investigated the effects of the light-induced thylakoid transmembra ne potential on the turnover of the b(6)f complex in cells of the unicellul ar green alga Chlamydomonas reinhardtii. The reduction of the potential by either decreasing the light intensity or by adding increasing concentration s of the ionophore carbonylcyanide p-(trifluoromethoxy)phenylhydrazone (FCC P) revealed a marked inhibition of the cytochrome b(6) oxidation rate (10-f old) without substantial modifications of cytochrome f oxidation kinetics. Partial recovery of this inhibition could be obtained in the presence of io nophores provided that the membrane potential was re-established by illumin ation with a train of actinic flashes fired at a frequency higher than its decay. Measurements of isotopic effects on the kinetics of cytochrome b(6), oxidation revealed a synergy between the effects of ionophores and the H2O -D2O exchange. We propose therefore, that protonation events influence the kinetics of cytochrome b(6) oxidation at the Qi site and that these reactio ns are strongly influenced by the light-dependent generation of a transmemb rane potential.