期刊
BIOCHEMISTRY
卷 58, 期 42, 页码 4276-4283出版社
AMER CHEMICAL SOC
DOI: 10.1021/acs.biochem.9b00680
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资金
- JSPS KAKENHI [JP17H06435, JP17H03662, JP17H06433, JP17K07367]
Photosynthetic water oxidation takes place at the Mn4CaO5 cluster in photosystem II via a light-driven cycle of intermediates called S states (S-0-S-4). Clarifying how electron and proton transfer reactions are coupled with each other in the S-2 -> S-3 transition, which occurs just before O-O bond formation, is crucial for understanding the water oxidation mechanism. Here, we investigated the pH dependence of the kinetics of the S-2 -> S-3 transition using time-resolved infrared (TRIR) spectroscopy to identify the proton release phase in this transition. TRIR measurements of Y-D-less PSII core complexes from the D2-Y160F mutant of Thermosynechococcus elongatus showed that the last phase in this transition (tau similar to 350 mu s at pH 6) was strongly dependent on pH, and its time constant at pH .5 was larger than that at pH 8 by a factor of >3. In contrast, the earlier phase with a time constant of similar to 100 ps was virtually independent of pH. These results strongly support the view that proton release is a rate-limiting step of the proton-coupled electron transfer in the last phase of the S-2 -> S-3 transition. This proton release enables electron transfer by removing an excessive positive charge from the catalytic center and hence decreasing its redox potential.
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