文摘
Ca2+ 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 Sr2+ and Br?, respectively, and that these ion exchanges modify the kinetics of some electron transfer reactions at the Mn4Ca 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 Mn4Ca cluster in the S3 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 Ca2+/Sr2+ and Cl?/Br? exchanges on the redox potential of the primary quinone electron acceptor QA, Em(QA/QA?), were investigated. Since the previous studies on the Ca2+/Sr2+ 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 Em(QA/QA?). Here we show that i) the Em(QA/QA?) was up-shifted by ca. + 38 mV in PsbA3-PSII when compared to PsbA1-PSII and ii) the Ca2+/Sr2+ exchange up-shifted the Em(QA/QA?) by ca. + 27 mV, whereas the Cl?/Br? exchange hardly influenced Em(QA/QA?). On the basis of the results of Em(QA/QA?) together with previous thermoluminescence measurements, the ion-exchange effects on the energetics in PSII are discussed.
