Influence of the PsbA1/PsbA3, Ca2+/Sr2+ and Cl−/Br− exchanges on the redox potential of the primary quinone QA in Photosystem II from Thermosynechococcus elongatus as revealed by spectroelectrochemistry

Abstract Ca 2+ and Cl − ions are essential elements for the oxygen evolution activity of photosystem II (PSII). It has been demonstrated that these ions can be exchanged with Sr 2+ and Br − , respectively, and that these ion exchanges modify the kinetics of some electron transfer reactions at the Mn 4 Ca cluster level (Ishida et al. , J. Biol. Chem. 283 (2008) 13330–13340). It has been proposed from thermoluminescence experiments that the kinetic effects arise, at least in part, from a decrease in the free energy level of the Mn 4 Ca cluster in the S 3 state though some changes on the acceptor side were also observed. Therefore, in the present work, by using thin-layer cell spectroelectrochemistry, the effects of the Ca 2+ /Sr 2+ and Cl − /Br − exchanges on the redox potential of the primary quinone electron acceptor Q A , E m (Q A /Q A − ), were investigated. Since the previous studies on the Ca 2+ /Sr 2+ and Cl − /Br − exchanges were performed in PsbA3-containing PSII purified from the thermophilic cyanobacterium Thermosynechococcus elongatus , we first investigated the influences of the PsbA1/PsbA3 exchange on E m (Q A /Q A − ). Here we show that i) the E m (Q A /Q A − ) was up-shifted by ca. + 38 mV in PsbA3-PSII when compared to PsbA1-PSII and ii) the Ca 2+ /Sr 2+ exchange up-shifted the E m (Q A /Q A − ) by ca. + 27 mV, whereas the Cl − /Br − exchange hardly influenced E m (Q A /Q A − ). On the basis of the results of E m (Q A /Q A − ) together with previous thermoluminescence measurements, the ion-exchange effects on the energetics in PSII are discussed.