Abstract
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 Mn4CaO5 (PSII/Ca) or a Mn4SrO5 (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 S2-state is in a low-spin configuration (S2LS) whereas in PSII/Sr at pH 7.0, the S2-state is in a high-spin configuration (S2HS) 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 PD1 at 440 nm. The fittings of the oscillations with a period of four indicate that one proton is released in the S1 to S2HS transition in PSII/Sr at pH 7.0. It was earlier suggested that the proton released in the S2LS to S3 transition is released in a S2LSTyrZ● → S2HSTyrZ● transition before the electron transfer from the cluster to TyrZ● occurs. The release of a proton in the S1TyrZ● → S2HSTyrZ transition would logically imply that this proton release is missing in the S2HSTyrZ● to S3TyrZ transition. Instead, the proton release in the S1 to S2HS transition in PSII/Sr at pH 7.0 was mainly done at the expense of the proton release in the S3 to S0 and S0 to S1 transitions. However, at pH 7.0, the electrochromism of PD1 seems larger in PSII/Sr when compared to PSII/Ca in the S3 state. This observation points to the complex link between proton movements in and immediately around the Mn4 cluster and the mechanism leading to the release of protons into the bulk.
Competing Interest Statement
The authors have declared no competing interest.
Footnotes
The authors declare that they have no conflict of interest.
Abbreviations: Chl, chlorophyll; ChlD1/ChlD2, accessory Chl’s on the D1 or D2 side, respectively; PSII, Photosystem II; MES, 2–(N–morpholino) ethanesulfonic acid; HEPES, 4-(2-hydroxyethyl)-1-piperazine ethane sulfonic acid. P680, primary electron donor; PD1 and PD2; Chl monomer of P680 on the D1 or D2 side, respectively, PheD1 and PheD2, pheophytin on the D1 or D2 side, respectively; QA, primary quinone acceptor; QB, secondary quinone acceptor; TyrZ, redox active tyrosine 161 of D1; WT*3, T. elongatus mutant strain containing only the psbA3 gene and a His6-tag on the C-terminus of CP43. EPR, Electron Paramagnetic Resonance spectroscopy; EDNMR, ELDOR-detected Nuclear Magnetic Resonance; ELDOR, Electron-Electron Double Resonance; SQR, sum of the squares of the residues.