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 (PQH(2)), is released into the membrane from the Q(B) site, where it is formed. A plastoquinone (PQ) from the membrane pool then binds into the Q(B) site. Despite their functional importance, the thermodynamic properties of the PQ in the Q(B) site, Q(B), in its different redox forms have received relatively little attention. Here we report the midpoint potentials (E-m) of Q(B) in PSII from Thermosynechococcus elongatus using electron paramagnetic resonance (EPR) spectroscopy: E-m Q(B)/Q(B)(center dot-) approximate to 90 mV, and E-m Q(B)(center dot-)/Q(B)H(2) approximate to 40 mV. These data allow the following conclusions: 1) The semiquinone, Q(B)(center dot-), is stabilized thermodynamically; 2) the resulting E-m Q(B)/Q(B)H(2) (similar to 65 mV) is lower than the E-m PQ/PQH(2) (similar to 117 mV), and the difference (Delta E approximate to 50 meV) represents the driving force for Q(B)H(2) release into the pool; 3) PQ is similar to 50x more tightly bound than PQH(2); and 4) the difference between the E-m Q(B)/Q(B)(center dot-) measured here and the E-m Q(A)/Q(A)(center dot-) from the literature is similar to 234 meV, in principle corresponding to the driving force for electron transfer from Q(A)(center dot-) to Q(B). The pH dependence of the thermoluminescence associated with Q(B)(center dot-) provided a functional estimate for this energy gap and gave a similar value (>= 180 meV). These estimates are larger than the generally accepted value (similar to 70 meV), and this is discussed. The energetics of Q(B) in PSII are comparable to those in the homologous purple bacterial reaction center.