底物水在光系统ⅡS_2态锰簇中的结合方式(英文)

The Substrate Water Binding Mode in the S_2-state Mn_4O_5Ca Cluster of Photosystem Ⅱ

在绿色植物的放氧光合作用过程中,每个反应周期(即S态循环)都有两个底物水分子被氧化,释放出四个质子和四个电子,并最终释放一个氧气分子,这种化学反应机制维持着地球上的各种需氧生命活动。目前,人们已经清楚地知道,这个生物催化过程是由光系统Ⅱ(photosystemⅡ,PSⅡ)中一个无机簇状结构(即锰簇Mn4O5Ca)催化完成的。然而,这两个底物水分子是在此反应同期的什么时候、什么地方、以什么方式结合到锰簇上的,目前仍旧是一个谜团。在探讨此问题的很多实验研究中,人们常用脉冲电子顺磁共振(electron paramagnetic resonance,EPR)波谱学方法,探测底物水分子在反应中间氧化态S2态锰簇中的结合方式和结合位点,即依据超精细耦合常数的大小来判断未成对电子与磁性核的距离。基于此前的实验数据和文献报道,EPR理论模拟的结果显示,中间S2态锰簇上仅仅可容纳一个可发生置换的底物水,并且已经发生完全去质子化。这个结合水可能就是研究人员使用时间分辨薄膜进样质谱(membrane-inlet mass spectrometry,MIMS)检测到的强磁性耦合氧。此结果排除了源自强磁性耦合14N信号的干扰。因此,理论和实验结果均说明,另一个底物水是在锰簇由S2态氧化到更高中间态S3态的过程中结合到锰簇上的,并最终参与了O2分子的形成。

It is well-known that in oxygenic photosynthesis,two substrate water molecules are oxidized to release one molecule oxygen into the ambient environment,contributing the ~20% O2 in the nowadays atmosphere which sustains the overall life on the Earth.However,it is still an enigma as to precisely when,where,and how the two substrate water molecules bind to the Mn4O5Ca cluster of photosystem Ⅱ（PS Ⅱ） despite the progress in many spectroscopic efforts.In a series of experiments,the pulsed electron paramagnetic resonance（EPR） spectroscopy was used to probe the bound substrate water molecules within the S2-state Mn4O5Ca cluster of PSII prepared from higher plant spinach.The 17O-and 2H-hyper ne sublevel correlation（HYSCORE） spectroscopy with the H217O/H216O and D2O/H2O substitutes have been preliminarily interpreted in consequence.In present study,the 17O HYSCORE was simulated,and only one exchangeable 17O nucleus within the S2-state Mn4O5Ca cluster was clarified with respect to the data measured in different PSII preparations（D2O enrichment,and the site-specific damage of the Mn4O5Ca cluster by NH2OH）.The strong coupled 17O was proposed to be the slow-exchanging water observed by time-resolved membrane-inlet mass spectrometry（MIMS）.The result is also clarified from the possible overlapping by the strong coupled 14N originating from the other paramagnetic cofactors of PSII.It is also proposed that the other substrate water binds to the Mn4O5Ca cluster during the S2→S3 transition.