The active site for water oxidation in photosystem II (PSII) comprises a MnCaO cluster adjacent to a redox-active tyrosine residue (Tyr). During the water-splitting process, the enzyme transitions through five sequential oxidation states (S to S), with O evolution occurring during the STyr· to STyr transition. Chloride also plays a role in this mechanism. Using PSII from Thermosynechococcus vestitus, where Ca and Cl were replaced with Sr and Br to slow the STyr· to STyr + O transition (t ~ 5 ms at room temperature), it was observed that the recovery of a S state, defined as the state able to progress to S, exhibits similar kinetics (t ~ 5 ms). This suggests that in CaCl-PSII, the reformation of the functional S state directly follows the STyr· to STyr + O transition, with no additional delay required for the insertion of a new substrate water molecule (O5) and associated protons.
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