Department of Biophysics, Faculty of Biology, M.V. Lomonosov Moscow State University, 119234, Moscow, Russia.
A.N. Belozersky Institute of Physico-Chemical Biology, M.V. Lomonosov Moscow State University, 119234, Moscow, Russia.
Photosynth Res. 2018 Apr;136(1):83-91. doi: 10.1007/s11120-017-0441-4. Epub 2017 Sep 11.
Lumenal extrinsic proteins PsbO, PsbP, and PsbQ of photosystem II (PSII) protect the catalytic cluster MnCaO of oxygen-evolving complex (OEC) from the bulk solution and from soluble compounds in the surrounding medium. Extraction of PsbP and PsbQ proteins by NaCl-washing together with chelator EGTA is followed also by the depletion of Ca cation from OEC. In this study, the effects of PsbP and PsbQ proteins, as well as Ca extraction from OEC on the kinetics of the reduced primary electron acceptor (Q) oxidation, have been studied by fluorescence decay kinetics measurements in PSII membrane fragments. We found that in addition to the impairment of OEC, removal of PsbP and PsbQ significantly slows the rate of electron transfer from Q to the secondary quinone acceptor Q. Electron transfer from Q to Q in photosystem II membranes with an occupied Q site was slowed down by a factor of 8. However, addition of EGTA or CaCl to NaCl-washed PSII did not change the kinetics of fluorescence decay. Moreover, the kinetics of Q oxidation by Q in Ca-depleted PSII membranes obtained by treatment with citrate buffer at pH 3.0 (such treatment keeps all extrinsic proteins in PSII but extracts Ca from OEC) was not changed. The results obtained indicate that the effect of NaCl-washing on the Q to Q electron transport is due to PsbP and PsbQ extrinsic proteins extraction, but not due to Ca depletion.
类囊体腔内在蛋白 PsbO、PsbP 和 PsbQ 可以保护光系统 II（PSII）的放氧复合酶（OEC）的催化簇 MnCaO 免受本体溶液和周围介质中可溶性化合物的影响。用 NaCl 洗涤和螯合剂 EGTA 一起提取 PsbP 和 PsbQ 蛋白，也会导致 OEC 中的 Ca 阳离子耗尽。在这项研究中，通过 PSII 膜片段的荧光衰减动力学测量，研究了 PsbP 和 PsbQ 蛋白以及 OEC 中 Ca 的提取对还原的初级电子受体（Q）氧化动力学的影响。我们发现，除了 OEC 的损伤之外，去除 PsbP 和 PsbQ 还显著减缓了 Q 向次级醌受体 Q 的电子转移速率。在占据 Q 位点的 PSII 膜中，Q 向 Q 的电子转移被减缓了 8 倍。然而，向 NaCl 洗涤后的 PSII 中添加 EGTA 或 CaCl 并没有改变荧光衰减的动力学。此外，用柠檬酸缓冲液在 pH 3.0 处理（这种处理方式保持 PSII 中的所有外在蛋白，但从 OEC 中提取 Ca）得到的 Ca 耗尽的 PSII 膜中 Q 氧化的 Q 动力学也没有改变。所得结果表明，NaCl 洗涤对 Q 到 Q 电子传递的影响是由于 PsbP 和 PsbQ 外在蛋白的提取，而不是由于 Ca 的耗尽。
