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

Abstract

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.