Kinetics of reformation of the S0 state capable of progressing to the S1 state after the O2 release by photosystem II.

The active site for water oxidation in photosystem II (PSII) comprises a Mn₄CaO₅ cluster adjacent to a redox-active tyrosine residue (Tyr_Z). During the water-splitting process, the enzyme transitions through five sequential oxidation states (S₀ to S₄), with O₂ evolution occurring during the S₃Tyr_Z· to S₀Tyr_Z 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 S₃Tyr_Z· to S₀Tyr_Z + O₂ transition (t_1/2 ~ 5 ms at room temperature), it was observed that the recovery of a S₀ state, defined as the state able to progress to S₁, exhibits similar kinetics (t_1/2 ~ 5 ms). This suggests that in CaCl-PSII, the reformation of the functional S₀ state directly follows the S₃Tyr_Z· to S₀Tyr_Z + O₂ transition, with no additional delay required for the insertion of a new substrate water molecule (O5) and associated protons.