Kinetic effects of the electrochemical proton gradient on plastoquione reduction at the Qi site of the cytochrome b6f complex

Abstract

We have investigated the effects of the light-induced thylakoid transmembrane potential on the turnover of the b6f complex in cells of the unicellular green alga Chlamydomonas reinhardtii. The reduction of the potential by either decreasing the light intensity or by adding increasing concentrations of the ionophore carbonylcyanide p-(trifluoromethoxy)phenylhydrazone (FCCP) revealed a marked inhibition of the cytochrome b6 oxidation rate (10-fold) without substantial modifications of cytochrome f oxidation kinetics. Partial recovery of this inhibition could be obtained in the presence of ionophores provided that the membrane potential was re-established by illumination with a train of actinic flashes fired at a frequency higher than its decay. Measurements of isotopic effects on the kinetics of cytochrome b6 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 b6 oxidation at the Qi site and that these reactions are strongly influenced by the light-dependent generation of a transmembrane potential.