Miller A F, Brudvig G W
Department of Chemistry, Yale University, New Haven, Connecticut 06511.
Biochemistry. 1990 Feb 13;29(6):1385-92. doi: 10.1021/bi00458a007.
O2-evolution activity and the Mn complex can be reconstituted in photosystem II by a process called photoactivation. We have studied the elementary steps in photoactivation by using electron paramagnetic resonance spectroscopy to probe electron transport in Mn-depleted photosystem II membranes. The electron donation reactions in Mn-depleted photosystem II were found to be identical with those in untreated photosystem II, except that electron donation from the Mn complex was absent and could be replaced by slower electron donation from exogenous Mn2+. Mn2+ photooxidation by Mn-depleted photosystem II membranes correlates with reconstitution of O2-evolution activity. However, photooxidation of Mn2+ occurs in competition with photooxidation of the tyrosine residue YD, and cytochrome b-559. Thus, these two species are excluded from direct participation in the initial steps in the assembly of the Mn complex. Because photooxidation of Mn2+ is slower than photooxidation of the competing electron donors, cytochrome b-559 and chlorophyll, as well as recombination of the charge-separated states chlorophyll+QA- or YZ+QA-, these other reactions dominate in a single photochemical turnover reaction. This provides a molecular basis for both the low yield and low quantum yield of photoactivation. The first photochemical step in the assembly of the Mn complex results in photooxidation of one Mn2+ ion. Therefore, the first intermediate in assembly of the Mn complex contains Mn3+. On the basis of these results and previous kinetic studies [Miller, A.-F., & Brudvig, G. W. (1989) Biochemistry 28, 8181], we conclude that the second intermediate of Mn complex assembly contains Mn2+Mn3+, which is photooxidized to Mn3+2.
通过一种称为光激活的过程，可在光系统II中重建O₂ 释放活性和锰复合物。我们利用电子顺磁共振光谱研究了光激活过程中的基本步骤，以探测锰耗尽的光系统II膜中的电子传输。发现锰耗尽的光系统II中的电子供体反应与未处理的光系统II中的相同，只是锰复合物的电子供体不存在，且可被外源Mn²⁺ 较慢的电子供体所取代。锰耗尽的光系统II膜对Mn²⁺ 的光氧化与O₂ 释放活性的重建相关。然而，Mn²⁺ 的光氧化与酪氨酸残基YD以及细胞色素b - 559的光氧化相互竞争。因此，这两种物质被排除在锰复合物组装初始步骤的直接参与之外。由于Mn²⁺ 的光氧化比竞争性电子供体细胞色素b - 559和叶绿素的光氧化慢，以及电荷分离态叶绿素⁺QA⁻ 或YZ⁺QA⁻ 的复合，这些其他反应在单个光化学反应周转中占主导。这为光激活的低产率和低量子产率提供了分子基础。锰复合物组装中的第一个光化学步骤导致一个Mn²⁺ 离子的光氧化。因此，锰复合物组装中的第一个中间体含有Mn³⁺。基于这些结果和先前的动力学研究[米勒，A.-F.，& 布鲁德维格，G. W.（1989年）《生物化学》28，8181]，我们得出结论，锰复合物组装的第二个中间体含有Mn²⁺Mn³⁺，其被光氧化为Mn³⁺₂。
