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<p>The active site for water oxidation in photosystem II (PSII) comprises a Mn<sub>4</sub>CaO<sub>5</sub> cluster adjacent to a redox-active tyrosine residue (Tyr<sub>Z</sub>). During the water-splitting process, the enzyme transitions through five sequential oxidation states (S<sub>0</sub> to S<sub>4</sub>), with O<sub>2</sub> evolution occurring during the S<sub>3</sub>Tyr<sub>Z</sub>· to S<sub>0</sub>Tyr<sub>Z</sub> transition. Chloride also plays a role in this mechanism. Using PSII from <i>Thermosynechococcus vestitus</i>, where Ca and Cl were replaced with Sr and Br to slow the S<sub>3</sub>Tyr<sub>Z</sub>· to S<sub>0</sub>Tyr<sub>Z</sub> + O<sub>2</sub> transition (<i>t</i><sub><i>1/2</i></sub> ~ 5&#xa0;ms at room temperature), it was observed that the recovery of a S<sub>0</sub> state, defined as the state able to progress to S<sub>1</sub>, exhibits similar kinetics (<i>t</i><sub><i>1/2</i></sub> ~ 5&#xa0;ms). This suggests that in CaCl-PSII, the reformation of the functional S<sub>0</sub> state directly follows the S<sub>3</sub>Tyr<sub>Z</sub>· to S<sub>0</sub>Tyr<sub>Z</sub> + O<sub>2</sub> transition, with no additional delay required for the insertion of a new substrate water molecule (O5) and associated protons.</p>

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Kinetics of reformation of the S0 state capable of progressing to the S1 state after the O2 release by photosystem II

  • 期刊论文

  • 发表日期：

    2025年1月15日

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  • Alain Boussac,
  • Julien Sellés,
  • Miwa Sugiura

摘要

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.