Electrogenic Protonation of the Secondary Quinone Acceptor Q_B in Spinach Photosystem II Complexes Incorporated into Lipid Vesicles

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M. D. Mamedov^*, A. A. Tyunyatkina, and A. Yu. Semenov

Belozersky Institute of Physico-Chemical Biology, Lomonosov Moscow State University, 119992 Moscow, Russia; fax: (7-095) 939-3181; E-mail: mamedov@genebee.msu.ru

^* To whom correspondence should be addressed.

Received September 9, 2004; Revision received November 4, 2004

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The generation of transmembrane electric potential difference (DeltaPsi) in quinone acceptor complex of proteoliposomes containing core complexes of photosystem II from spinach was studied using for the measurements a direct electrometric technique. Besides the fast increase in the membrane potential associated with the electron transfer between the redox-active tyrosine 161 residue (Y_Z) in D1 polypeptide and the primary quinone acceptor Q_A, an additional electrogenic phase with tau ~ 0.85 msec at pH 7.3 and the maximal relative amplitude of ~11% of the Y_Z^oxQ_A^- phase was observed after the second light flash. The sensitivity of this phase to diuron (an inhibitor of electron transfer between Q_A and the secondary quinone acceptor Q_B), the dependence of its amplitude on the light flash parity, and also a decrease in its rate constant with increase in pH indicated that it was due to dismutation of Q_A^- and Q_B^- with the subsequent protonation of a doubly reduced plastoquinone molecule: Q_A^-Q_B^- + 2H⁺ → Q_AQ_BH₂.

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KEY WORDS: core complex of photosystem II, oxygen evolving complex, secondary quinone acceptor, proteoliposomes, membrane potential, electrogenic phase, direct electrometric technique

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