Untangling the sequence of events during the S ₂ → S ₃ transition in photosystem II and implications for the water oxidation mechanism

Abstract

In oxygenic photosynthesis, light-driven oxidation of water to molecular oxygen is carried out by the oxygen-evolving complex (OEC) in photosystem II (PS II). Recently, we reported the room-temperature structures of PS II in the four (semi)stable S-states, S₁, S₂, S₃, and S₀, showing that a water molecule is inserted during the S₂ → S₃ transition, as a new bridging O(H)-ligand between Mn1 and Ca. To understand the sequence of events leading to the formation of this last stable intermediate state before O₂ formation, we recorded diffraction and Mn X-ray emission spectroscopy (XES) data at several time points during the S₂ → S₃ transition. At the electron acceptor site, changes due to the two-electron redox chemistry at the quinones, Q_A and Q_B, are observed. At the donor site, tyrosine Y_Z and His190 H-bonded to it move by 50 µs after the second flash, and Glu189 moves away from Ca. This is followed by Mn1 and Mn4 moving apart, and the insertion of O_X(H) at the open coordination site of Mn1. This water, possibly a ligand of Ca, could be supplied via a "water wheel"-like arrangement of five waters next to the OEC that is connected by a large channel to the bulk solvent. XES spectra show that Mn oxidation (τ of ∼350 µs) during the S₂ → S₃ transition mirrors the appearance of O_X electron density. This indicates that the oxidation state change and the insertion of water as a bridging atom between Mn1 and Ca are highly correlated.