Concerted Mechanism of Water Insertion and O₂ Release during the S₄ to S₀ Transition of the Oxygen-Evolving Complex in Photosystem II

The O₂ release of the oxygen-evolving complex of the photosystem II (PSII) is one of the essential processes responsible for the highly efficient O₂ production. Despite its importance, the detailed molecular mechanism is still unsolved. In the present study, we show that the O₂ release is directly coupled with water insertion into the Mn cluster based on the quantum mechanics/molecular mechanics (QM/MM) calculations. In this mechanism, the O₂ molecule first dissociates from the Mn sites in order, that is, the O atom coordinating to the Mn3 (O5a) first dissociates, then the other O atom coordinating to the Mn1 (O5d) dissociates in the next step in the late S₄ state (1 → 2). Next, the O₂ migrates to a space surrounded by the Val185 and His332 side chains as one water molecule coordinating to the Ca²⁺ ion (W3) comes into the O₂ bonded site (2 → 3). Finally, a pre-S₀ state (4) is formed after a proton transfer from the inserted water to the other proton acceptor site (W2) (3 → 4). The highest activation barrier during these reactions was found at the O₂ release step (2 → 3) that only requires E^† = 12.7 kcal mol^-1 (G^† = 10.4 kcal mol^-1). A series of the reactions (2 → 3) look like a chain crash of billiard balls because the W3 is inserted into the catalytic center from the water-abundant side (Ca²⁺ ion side), and then the O₂ moiety is pushed out to the opposite side (Val185 side). The hydrophobic residue of Val185 covers the active O5 site and forms an O₂-specific permeation tunnel. The present sequential reactions clearly demonstrate the efficient removal of the toxic O₂ from the catalytic center and implications of the essential roles of Val185, Ca²⁺ ions and water molecules, which are all present in the active site of PSII as the indispensable constituents.