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J. Biol. Chem., Vol. 275, Issue 34, 26121-26127, August 25, 2000

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Kinetic Effects of the Electrochemical Proton Gradient on Plastoquinone Reduction at the Qi Site of the Cytochrome b₆f Complex^*

Romina Paola Barbagallo^§, Cécile Breyton^¶, and Giovanni Finazzi^**

From the  Centro di Studio del CNR sulla Biologia Cellulare e Molecolare delle Piante, via Celoria 26, 20133 Milano, Italy and the ^¶ Max Planck Institute of Biophysics, Department of Structural Biology, Heinrich-Hoffmann-Strasse 7, D-60528 Frankfurt am Main, Germany

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.

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