Energetics of the exchangeable quinone, Q, in Photosystem II.

Photosystem II (PSII), the light-driven water/plastoquinone photooxidoreductase, is of central importance in the planetary energy cycle. The product of the reaction, plastohydroquinone (PQH), is released into the membrane from the Q site, where it is formed. A plastoquinone (PQ) from the membrane pool then binds into the Q site. Despite their functional importance, the thermodynamic properties of the PQ in the Q site, Q, in its different redox forms have received relatively little attention. Here we report the midpoint potentials ( ) of Q in PSII from using electron paramagnetic resonance (EPR) spectroscopy: Q/Q ≈ 90 mV, and Q/QH ≈ 40 mV. These data allow the following conclusions: 1) The semiquinone, Q, is stabilized thermodynamically; 2) the resulting Q/QH (∼65 mV) is lower than the PQ/PQH (∼117 mV), and the difference (ΔE ≈ 50 meV) represents the driving force for QH release into the pool; 3) PQ is ∼50× more tightly bound than PQH; and 4) the difference between the Q/Q measured here and the Q/Q from the literature is ∼234 meV, in principle corresponding to the driving force for electron transfer from Q to Q The pH dependence of the thermoluminescence associated with Q 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 Q in PSII are comparable to those in the homologous purple bacterial reaction center.