Probing the proton release by Photosystem II in the S to S high-spin transition.

The stoichiometry and kinetics of the proton release were investigated during each transition of the S-state cycle in Photosystem II (PSII) from Thermosynechococcus elongatus containing either a MnCaO (PSII/Ca) or a MnSrO (PSII/Sr) cluster. The measurements were done at pH 6.0 and pH 7.0 knowing that, in PSII/Ca at pH 6.0 and pH 7.0 and in PSII/Sr at pH 6.0, the flash-induced S-state is in a low-spin configuration (S) whereas in PSII/Sr at pH 7.0, the S-state is in a high-spin configuration (S) in half of the centers. Two measurements were done; the time-resolved flash dependent i) absorption of either bromocresol purple at pH 6.0 or neutral red at pH 7.0 and ii) electrochromism in the Soret band of P at 440 nm. The fittings of the oscillations with a period of four indicate that one proton is released in the S to S transition in PSII/Sr at pH 7.0. It has previously been suggested that the proton released in the S to S transition would be released in a STyr → STyr transition before the electron transfer from the cluster to Tyr occurs. The release of a proton in the STyr → STyr transition would logically imply that this proton release is missing in the STyr to STyr transition. Instead, the proton release in the S to S transition in PSII/Sr at pH 7.0 was mainly done at the expense of the proton release in the S to S and S to S transitions. However, at pH 7.0, the electrochromism of P seems larger in PSII/Sr when compared to PSII/Ca in the S state. This points to the complex link between proton movements in and immediately around the Mn cluster and the mechanism leading to the release of protons into the bulk.