Semin B К, Davletshina L N, Seibert M, Rubin A B
Department of Biophysics, Faculty of Biology, Lomonosov Moscow State University, 119234 Moscow, Russia.
Department of Biophysics, Faculty of Biology, Lomonosov Moscow State University, 119234 Moscow, Russia.
J Photochem Photobiol B. 2018 Jan;178:192-200. doi: 10.1016/j.jphotobiol.2017.11.016. Epub 2017 Nov 11.
Extraction of Mn cations from the oxygen-evolving complex (OEC) of Ca-depleted PSII membranes (PSII[-Ca,4Mn]) by reductants like hydroquinone (HQ) occurs with lower efficiency at acidic pH (2Mn/reaction center [RC] are extracted at pH5.7) than at neutral pH (3Mn/RC are extracted at pH6.5) [Semin et al. Photosynth. Res. 125 (2015) 95]. Fe(II) also extracts Mn cations from PSII(-Ca,4Mn), but only 2Mn/RC at pH6.5, forming a heteronuclear 2Mn/2Fe cluster [Semin and Seibert, J. Bioenerg. Biomembr. 48 (2016) 227]. Here we investigated the efficiency of Mn extraction by Fe(II) at acidic pH and found that Fe(II) cations can extract only 1Mn/RC from PSII(-Ca,4Mn) membranes at pH 5.7, forming a 3Mn/1Fe cluster. Also we found that the presence of Fe cations in a heteronuclear cluster (2Mn/2Fe) increases the resistance of the remaining Mn cations to HQ action, since HQ can extract Mn cations from homonuclear Mn clusters of PSII(-Ca,4Mn) and PSII(-Ca,2Mn) membranes but not from the heteronuclear cluster in PSII(-Ca,2Mn,2Fe) membranes. HQ also cannot extract Mn from PSII membranes obtained by incubation of PSII(-Ca,4Mn) membranes with Fe(II) cations at pH5.7, which suggests the formation of a heteronuclear 3Mn/1Fe cluster in the OEC. Functional activity of PSII with a 3Mn/1Fe cluster was investigated. PSII preparations with a 3Mn/1Fe cluster in the OEC are able to photoreduce the exogenous electron acceptor 2,6-dichlorophenolindophenol, possibly due to incomplete oxidation of water molecules as is the case with PSII(-Ca,2Mn,2Fe) samples. However, in the contrast to PSII(-Ca,2Mn,2Fe) samples PSII(-Ca,3Mn,1Fe) membranes can evolve O at a low rate in the presence of exogenous Ca (at about 27% of the rate of O evolution in native PSII membranes). The explanation for this phenomenon (either water splitting and production of molecular O by the 3Mn/1Fe cluster or apparent O evolution due to minor contamination of PSII(3Mn,1Fe) samples with PSII(-Ca,4Mn) membranes) is discussed.
通过对苯二酚（HQ）等还原剂从缺钙的光系统II膜（PSII[-Ca,4Mn]）的析氧复合物（OEC）中提取锰阳离子，在酸性pH值（pH5.7时提取2个锰离子/反应中心[RC]）下的效率低于中性pH值（pH6.5时提取3个锰离子/RC）[塞米恩等人，《光合作用研究》125（2015年）95]。亚铁离子（Fe(II)）也能从PSII(-Ca,4Mn)中提取锰阳离子，但在pH6.5时仅提取2个锰离子/RC，形成异核的2个锰离子/2个铁离子簇[塞米恩和赛伯特，《生物能源与生物膜杂志》48（2016年）227]。在此，我们研究了Fe(II)在酸性pH值下提取锰的效率，发现Fe(II)阳离子在pH5.7时只能从PSII(-Ca,4Mn)膜中提取1个锰离子/RC，形成3个锰离子/1个铁离子簇。我们还发现，异核簇（2个锰离子/2个铁离子）中存在铁阳离子会增加剩余锰阳离子对HQ作用的抗性，因为HQ可以从PSII(-Ca,4Mn)和PSII(-Ca,2Mn)膜的同核锰簇中提取锰阳离子，但不能从PSII(-Ca,2Mn,2Fe)膜中的异核簇中提取。HQ也不能从在pH5.7下用Fe(II)阳离子孵育PSII(-Ca,4Mn)膜得到的PSII膜中提取锰，这表明在OEC中形成了异核的3个锰离子/1个铁离子簇。研究了具有3个锰离子/1个铁离子簇的PSII的功能活性。OEC中具有3个锰离子/1个铁离子簇的PSII制剂能够光还原外源电子受体2,6 - 二氯酚靛酚，这可能是由于水分子不完全氧化，就像PSII(-Ca,2Mn,2Fe)样品的情况一样。然而，与PSII(-Ca,2Mn,2Fe)样品不同的是，PSII(-Ca,3Mn,1Fe)膜在外源钙存在的情况下能够以较低的速率释放氧气（约为天然PSII膜中氧气释放速率的27%）。讨论了对这种现象的解释（要么是由3个锰离子/1个铁离子簇进行水分解并产生分子氧，要么是由于PSII(3Mn,1Fe)样品被PSII(-Ca,4Mn)膜轻微污染而出现明显的氧气释放）。
