Boussac, A.; Selles, J.; Sugiura, M.

The active site for water oxidation in Photosystem II (PSII ) consists of a Mn4CaO5 cluster close to a redox-active tyrosine residue (TyrZ). The enzyme cycles through five sequential oxidation states, from S0 to S4, in the water splitting process. O2 evolution occurs in the final S3TyrZ dot to S0TyrZ transition. Chloride is also involved in this mechanism. By using PSII from Thermosynechococcus elongatus in which both Ca and Cl have been substituted for Sr and Br, in order to slow down the S3TyrZdot to S0TyrZ + O2 transition, with a t1/2 ~ 5 ms at room temperature, it is shown that the kinetics of the recovery of a functional S0 has a tfrac12 also close to 5 ms. It is suggested that, similarly, the reformation of a functional S0 state follows the S3TyrZdot to S0TyrZ + O2 transition transition in CaCl-PSII and that the insertion of a new substrate molecule of water (O5) and protons does not require further delay.