Origin of the Reactive and Nonreactive Excited States in the Primary Reaction of Rhodopsins: pH Dependence of Femtosecond Absorption of Light-Driven Sodium Ion Pump Rhodopsin KR2
Origin of the Reactive and Nonreactive Excited States in the Primary Reaction of Rhodopsins: pH Dependence of Femtosecond Absorption of Light-Driven Sodium Ion Pump Rhodopsin KR2
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视紫红质初级反应中反应性和非反应性激发态的起源：光驱动钠离子泵视紫红质 KR2 飞秒吸收的 pH 依赖性
DOI：
10.1021/acs.jpcb.8b01934
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发表时间：
2018
期刊：
影响因子：
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通讯作者：
Tahara Tahei
中科院分区：
文献类型：
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作者：
Tahara Shinya;Takeuchi Satoshi;Abe-Yoshizumi Rei;Inoue Keiichi;Ohtani Hiroyuki;Kandori Hideki;Tahara Tahei
KR2 是第一个被发现的光驱动 Na+ 泵送视紫红质。据报道，KR2的光激发产生多种S1态，即在生理pH下的“反应性”和“非反应性”S1态，但其起源仍不清楚。在本研究中，我们使用飞秒时间分辨吸收光谱在 4 至 11 范围内的不同 pH 值下检查了 KR2 的 S1 态动力学。发现反应性 S1 态主要在 pH > 9 时形成，但其数量随着 pH 值的降低而急剧减少，而非反应性 S1 态的数量则增加。反应性 S1 状态相对数量的 pH 依赖性与 Asp116 的 pH 滴定曲线非常相关，Asp116 是 KR2 中质子化视网膜希夫碱 (PRSB) 的反离子。这强烈表明Asp116的去质子化/质子化与KR2中多个S1态的产生直接相关。对时间分辨吸收数据的定量分析使我们得出结论，KR2 的反应性和非反应性 S1 态分别源自 Asp116 和 PRSB 之间是否有氢键的 KR2 蛋白。换句话说，基态不均匀性是KR2中反应性和非反应性S1态共存的根源。迄今为止，具有不同视紫红质光反应性的多个S1状态的产生主要是通过S1状态中Franck-Condon区域的弛豫通路的分支来解释的。本研究表明，基态结构的不均匀性，特别是氢键网络的结构不均匀性，是反应性和非反应性 S1 态的更合理的起源，这种结构已在各种视紫红质中广泛观察到。
KR2 is the first light-driven Na+-pumping rhodopsin discovered. It was reported that the photoexcitation of KR2 generates multiple S1states, i.e., “reactive” and “nonreactive” S1states at physiological pH, but their origin remained unclear. In this study, we examined the S1state dynamics of KR2 using femtosecond time-resolved absorption spectroscopy at different pH′s in the range from 4 to 11. It was found that the reactive S1state is predominantly formed at pH >9, but its population drastically decreases with decreasing pH while the population of the nonreactive S1state(s) increases. The pH dependence of the relative population of the reactive S1state correlates very well with the pH titration curve of Asp116, which is the counterion of the protonated retinal Schiff base (PRSB) in KR2. This strongly indicates that the deprotonation/protonation of Asp116 is directly related to the generation of the multiple S1states in KR2. The quantitative analysis of the time-resolved absorption data led us to conclude that the reactive and nonreactive S1states of KR2 originate from KR2 proteins having a hydrogen bond between Asp116 and PRSB or not, respectively. In other words, it is the ground-state inhomogeneity that is the origin of the coexistence of the reactive and nonreactive S1states in KR2. So far, the generation of multiple S1states having a different photoreactivity of rhodopsins has been mainly explained with the branching of the relaxation pathway in the Franck–Condon region in the S1state. The present study shows that the structural inhomogeneity in the ground state, in particular that of the hydrogen-bond network, is the more plausible origin of the reactive and nonreactive S1states which have been widely observed for various rhodopsins.