Energetics of the exchangeable quinone, QB, in Photosystem II

Abstract

Photosystem II (PSII), the light-driven water/plastoquinone photooxidoreductase, is of central importance in the planetary energy cycle. The product of the reaction, plastohydroquinone (PQH2), is released into the membrane from the QBsite, where it is formed. A plastoquinone (PQ) from the membrane pool then binds into the QBsite. Despite their functional importance, the thermodynamic properties of the PQ in the QBsite, QB, in its different redox forms have received relatively little attention. Here we report the midpoint potentials (Em) of QBin PSII fromThermosynechococcus elongatususing electron paramagnetic resonance (EPR) spectroscopy:EmQB/QB•−≈ 90 mV, andEmQB•−/QBH2≈ 40 mV. These data allow the following conclusions: 1) The semiquinone, QB•−, is stabilized thermodynamically; 2) the resultingEmQB/QBH2(∼65 mV) is lower than theEmPQ/PQH2(∼117 mV), and the difference (ΔE ≈ 50 meV) represents the driving force for QBH2release into the pool; 3) PQ is ∼50× more tightly bound than PQH2; and 4) the difference between theEmQB/QB•−measured here and theEmQA/QA•−from the literature is ∼234 meV, in principle corresponding to the driving force for electron transfer from QA•−to QB. The pH dependence of the thermoluminescence associated with QB•−provided a functional estimate for this energy gap and gave a similar value (≥180 meV). These estimates are larger than the generally accepted value (∼70 meV), and this is discussed. The energetics of QBin PSII are comparable to those in the homologous purple bacterial reaction center.