Division of Material Science, Graduate School of Science, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8602, Japan.
Division of Material Science, Graduate School of Science, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8602, Japan.
Biochim Biophys Acta Bioenerg. 2019 Dec 1;1860(12):148082. doi: 10.1016/j.bbabio.2019.148082. Epub 2019 Oct 25.
Redox titration using fluorescence measurements of photosystem II (PSII) has long shown that impairment of the water-oxidizing MnCaO cluster upshifts the redox potential (E) of the primary quinone electron acceptor Q by more than 100 mV, which has been proposed as a photoprotection mechanism of PSII. However, the molecular mechanism of this long-distance interaction between the MnCaO cluster and Q in PSII remains unresolved. In this study, we reinvestigated the effect of depletion of the MnCaO cluster on E(Q/Q) using Fourier transform infrared (FTIR) spectroelectrochemistry, which can directly monitor the redox state of Q at an intended potential. Light-induced FTIR difference measurements at a series of electrode potentials for intact and Mn-depleted PSII preparations from spinach and Thermosynechococcus elongatus showed that depletion of the MnCaO cluster hardly affected the E(Q/Q) values. In contrast, fluorescence spectroelectrochemical measurement using the same PSII sample, electrochemical cell, and redox mediators reproduced a large upshift of apparent E upon Mn depletion, whereas a smaller shift was observed when weaker visible light was used for fluorescence excitation. Thus, the possibility was suggested that the measuring light for fluorescence disturbed the titration curve in Mn-depleted PSII, in contrast to no interference of infrared light with the PSII reactions in FTIR measurements. From these results, it was concluded that the MnCaO cluster does not directly regulate E(Q/Q) to control the redox reactions on the electron acceptor side of PSII.
使用荧光测量法对光系统 II（PSII）进行的氧化还原滴定法长期以来表明，水氧化 MnCaO 簇的损伤会使初级醌电子受体 Q 的氧化还原电位（E）升高超过 100 mV，这被提议为 PSII 的光保护机制。然而，PSII 中 MnCaO 簇和 Q 之间这种长程相互作用的分子机制仍未解决。在这项研究中，我们使用傅里叶变换红外（FTIR）光谱电化学重新研究了 MnCaO 簇耗竭对 E(Q/Q)的影响，该技术可以直接在预期电位下监测 Q 的氧化还原状态。在一系列电极电位下，对来自菠菜和伸长高温球菌的完整和 Mn 耗尽 PSII 制剂进行光诱导 FTIR 差示测量，结果表明 MnCaO 簇的耗竭几乎不会影响 E(Q/Q)值。相比之下，使用相同 PSII 样品、电化学电池和氧化还原介体进行荧光光谱电化学测量，在 Mn 耗尽时重现了明显 E 的大幅上移，而当使用较弱的可见光进行荧光激发时，观察到较小的偏移。因此，提出了这样一种可能性，即用于荧光的测量光会干扰 Mn 耗尽 PSII 中的滴定曲线，而与 FTIR 测量中 PSII 反应的红外光没有干扰形成对比。根据这些结果得出的结论是，MnCaO 簇不会直接调节 E(Q/Q)来控制 PSII 电子受体侧的氧化还原反应。
