Influence of the PsbA1/PsbA3, Ca(2+)/Sr(2+) and Cl(-)/Br(-) exchanges on the redox potential of the primary quinone Q(A) in Photosystem II from Thermosynechococcus elongatus as revealed by spectroelectrochemistry.

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 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 Mn(4)Ca 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 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. +38mV 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. +27mV, 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.

Species-dependence of the redox potential of the primary quinone electron acceptor QA in photosystem II verified by spectroelectrochemistry.

The redox potentials E(m)(Q(A)/Q(A)(-)) of the primary quinone electron acceptor Q(A) in oxygen-evolving photosystem II complexes of three species were determined by spectroelectrochemistry. The E(m)(Q(A)/Q(A)(-)) values were experimentally found to be -162+/-3 mV for a higher plant spinach, -171+/-3 mV for a green alga Chlamydomonas reinhardtii and -104+/-4 mV vs. SHE for a red alga Cyanidioschyzon merolae. On the basis of possible deviations for the experimental values, as estimated to differ by 9-29 mV from each true value, plausible causes for such remarkable species-dependence of E(m)(Q(A)/Q(A)(-)) are discussed, mainly by invoking the effects of extrinsic subunits on the delicate structural environment around Q(A).

Redox potential of the primary plastoquinone electron acceptor Q(A) in photosystem II from Thermosynechococcus elongatus determined by spectroelectrochemistry.

The redox potential of the primary plastoquinone electron acceptor Q(A), E(m)(Q(A)/Q(A)(-)), in an oxygen-evolving photosystem (PS) II complex from a thermophilic cyanobacterium Thermosynechococcus elongatus was determined to be -140 +/- 2 mV vs. SHE by thin-layer cell spectroelectrochemistry for the first time. The E(m)(Q(A)/Q(A)(-)) value obtained here together with the recently determined redox potential of pheophytin (Phe) a [Kato et al. (2009) Proc. Natl. Acad. Sci. U.S.A. 106, 17365-17370] yields -330 to -370 mV for the free energy change by electron transfer from Phe a(-) to Q(A) and provides a renewed picture for the energetics on the electron acceptor side in PS II.

Spectroelectrochemistry of cytochrome b559 in the D1-D2-Cyt b559 complex from spinach.

The redox potential of cytochrome b559 (Cyt b559) in the D1-D2-Cyt b559 complex from spinach has been determined to be +90+/-2mV vs. SHE at pH 6.0, by thin-layer cell spectroelectrochemistry for the first time. The redox potential, corresponding uniquely to the so-called "low-potential form", exhibited a sigmoidal pH-dependence from pH 4.0 to 9.0, ranging from +115 to +50mV. An analysis of the pH-dependence based on model equations suggests that two histidine residues coordinating to the heme iron in the protein subunits may exert electrostatic influence on the redox potential of Cyt b559.