Mechanism of proton-coupled quinone reduction in Photosystem Ⅱ

摘要

Photosystem II uses light to drive water oxidation and plastoqui-none (PQ) reduction. PQ reduction involves two PQ cofactors, Q_A and Q_B, working in series. Q_A is a one-electron carrier, whereas Q_B undergoes sequential reduction and protonation to form Q_BH_2. Q_BH_2 exchanges with PQ from the pool in the membrane. Based on the atomic coordinates of the Photosystem II crystal structure, we analyzed the proton transfer (PT) energetics adopting a quantum mechanical/molecular mechanical approach. The potential-energy profile suggests that the initial PT to Q_B~(*-) occurs from the protonated, D1-His252 to Q_B~(*-) via D1-Ser264. The second PT is likely to occur from D1-His215 to Q_BH~- via an H-bond with an energy profile with a single well, resulting in the formation of Q_BH_2 and the D1-His215 anion. The pathway for reprotonation of D1-His215 may involve bicarbonate, D1-Tyr246 and water in the QB site. Formate ligation to Fe~(2+) did not significantly affect the protonation of reduced Q_B, suggesting that formate inhibits Q_BH_2 release rather than its formation. The presence of carbonate rather than bicarbonate seems unlikely because the calculations showed that this greatly perturbed the potential of the nonheme iron, stabilizing the Fe~(3+) state in the presence of Q_B~(*-), a situation not encountered experimentally. H-bonding from D1-Tyr246 and D2-Tyr244 to the bicarbonate ligand of the nonheme iron contributes to the stability of the semiquinones. A detailed mechanistic model for Q_B reduction is presented.