Kinetics of reformation of an S0 state capable of progressing to an S1 state after the O2 release by Photosystem II
Boussac, A.; Selles, J.; Sugiura, M.
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The active site for water oxidation in Photosystem II (PSII) comprises a Mn4CaO5 cluster adjacent to a redox-active tyrosine residue (TyrZ). During the water-splitting process, the enzyme transitions through five sequential oxidation states (S0 to S4), with O2 evolution occurring during the S3TyrZ* to S0TyrZ 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 S3TyrZ* to S0TyrZ + O2 transition (t1/2 [~] 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 (t1/2 [~] 5 ms). This suggests that in CaCl-PSII, the reformation of the functional S0 state directly follows the S3TyrZ* to S0TyrZ + O2 transition, with no additional delay required for the insertion of a new substrate water molecule (O5) and associated protons.
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