Kinetics of reformation of the S$_0$ state capable of progressing to the S1 state after the O2 release by photosystem II
Résumé
The active site for water oxidation in photosystem II (PSII) comprises a Mn$_4$ CaO$_5$ cluster adjacent to a redox-active tyrosine residue (Tyr Z ). During the water-splitting process, the enzyme transitions through five sequential oxidation states (S$_0$ to S$_4$ ), with O$_2$ evolution occurring during the S$_3$ Tyr$_Z$ • to S$_0$ Tyr$_Z$ 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 S$_3$ Tyr$_Z$ • to S$_0$ Tyr$_Z$ + O$_2$ transition ($t_{1/2}$ ~ 5 ms at room temperature), it was observed that the recovery of a S$_0$ state, defined as the state able to progress to S$_1$ , exhibits similar kinetics ($t_{1/2}$ ~ 5 ms). This suggests that in CaCl-PSII, the reformation of the functional S 0 state directly follows the S$_3$ Tyr$_Z$ • to S$_0$ Tyr$_Z$ + O$_2$ transition, with no additional delay required for the insertion of a new substrate water molecule (O5) and associated protons.
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