Energetics of the exchangeable quinone, QB, in Photosystem II

Photosystem II (PSII), the light-driven water/plastoquinone photo-oxidoreductase, is of central importance in the planetary energy cycle. The product of the reaction, plastohydroquinone (PQH₂), is released into the membrane from the QB site, where it is formed. A plastoquinone (PQ) from the membrane pool then binds into the QB site. Despite their functional importance, the thermodynamic properties of the PQ in the QB site, QB, in its different redox forms have received relatively little attention. Here we report the midpoint potentials (Em ) of QB in PSII from Thermosynechococcus elongatus using electron paramagnetic resonance (EPR) spectroscopy: Em QB/QB •− ≈ 90 mV, and Em QB •−/QBH₂ ≈ 40 mV. These data allow the following conclusions: 1) The semiquinone, QB •−, is stabilized thermodynamically; 2) the resulting Em QB/QBH₂ (∼65 mV) is lower than the Em PQ/PQH₂ (∼117 mV), and the difference (ΔE ≈ 50 meV) represents the driving force for QBH₂ release into the pool; 3) PQ is ∼50× more tightly bound than PQH₂; and 4) the difference between the Em QB/QB •− measured here and the Em QA/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 QB in PSII are comparable to those in the homologous purple bacterial reaction center.