Jegerschöld C, Styring S
Department of Biochemistry, Arrheniuslaboratories for Natural Sciences, Stockholm University, Sweden.
Biochemistry. 1996 Jun 18;35(24):7794-801. doi: 10.1021/bi960083p.
The reaction center protein D1 in photosystem II shows a high turnover during illumination. The degradation of the D1 protein is preceded by photoinhibition of the electron transport in photosystem II. There are two distinct mechanisms for this: acceptor-side- and donor-side-induced photoinhibition. Here, donor-side-induced photoinhibition was studied in photosystem II membranes after Cl- depletion or washing with tris(hydroxymethyl)aminomethane (Tris) which destroys water oxidation, reversibly or irreversibly, respectively. Photoinhibition after these treatments leads to fast degradation of the D1 protein, and the mechanism behind this was investigated. Illumination of Cl- depleted photosystem II membranes resulted in a rapid and simultaneous inhibition of Cl(-)-reconstitutable oxygen evolution, loss of 2 Mn ions per photosystem II center, increase in the electron transfer between the electron donor diphenylcarbazide and electron acceptor 2,6-dichlorophenolindophenol, and an increase in the EPR signal IIfast from tyrosine-Zox. The destruction of the Mn cluster leads to the loss of oxygen evolution and to an increased accessibility for diphenylcarbazide to donate electrons to Tyr-Zox. The increase in the EPR signal from Tyr-Zox can be explained by slower reduction kinetics of Tyr-Zox due to the Mn release. On a longer photoinhibition time scale, a decrease in the amplitude of Tyr-Zox and inhibition of the electron transport from diphenylcarbazide to 2,6-dichlorophenolindophenol occurred simultaneously in both Cl(-)-depleted and Tris-washed photosystem II membranes. These slower photoinhibition reactions were then studied in detail in Tris-washed photosystem II membranes. Compared to photoinhibition of Tyr-Zox, the EPR signal from tyrosine-Dox decreased much slower. Tyr-Dox was photoinhibited in parallel with the EPRsignals from reduced QA, reduced pheophytin, and an oxidized chlorophyll radical (chlorophyllz). This shows that the acceptor side components and the primary charge separation reaction (P680+ pheophytin-) were operational although Tyr-Z was inactivated. The amount of the D1 protein also declined in parallel with Tyr-Dox, which shows that the D1 protein is not damaged until long after the Mn complex and Tyr-Z have become inactivated. Instead, it is likely that the strongly oxidizing P680+ is responsible for the damage to the D1 protein.
光系统II中的反应中心蛋白D1在光照期间表现出高周转率。D1蛋白的降解之前是光系统II中电子传递的光抑制。对此有两种不同的机制：受体侧诱导和供体侧诱导的光抑制。在此，分别在通过去除Cl⁻或用三（羟甲基）氨基甲烷（Tris）洗涤（分别可逆或不可逆地破坏水氧化）后的光系统II膜中研究了供体侧诱导的光抑制。这些处理后的光抑制导致D1蛋白快速降解，并对其背后的机制进行了研究。对去除Cl⁻的光系统II膜进行光照导致Cl⁻可重构的氧气释放迅速且同时受到抑制，每个光系统II中心损失2个锰离子，电子供体二苯卡巴肼与电子受体2,6 - 二氯酚靛酚之间的电子转移增加，以及来自酪氨酸 - Zox的EPR信号IIfast增加。锰簇的破坏导致氧气释放丧失，并使二苯卡巴肼向Tyr - Zox供电子的可及性增加。来自Tyr - Zox的EPR信号增加可以用由于锰释放导致的Tyr - Zox还原动力学减慢来解释。在更长的光抑制时间尺度上，在去除Cl⁻和用Tris洗涤的光系统II膜中，Tyr - Zox的振幅降低和从二苯卡巴肼到2,6 - 二氯酚靛酚的电子传递抑制同时发生。然后在经Tris洗涤的光系统II膜中详细研究了这些较慢的光抑制反应。与Tyr - Zox的光抑制相比，来自酪氨酸 - Dox的EPR信号下降要慢得多。Tyr - Dox与来自还原态QA、还原态去镁叶绿素和氧化态叶绿素自由基（叶绿素z）的EPR信号同时受到光抑制。这表明尽管Tyr - Z失活，但受体侧成分和初级电荷分离反应（P680⁺ 去镁叶绿素⁻）仍在运行。D1蛋白的量也与Tyr - Dox同时下降，这表明直到锰复合物和Tyr - Z失活很久之后D1蛋白才受到损伤。相反，很可能是强氧化性的P680⁺导致了D1蛋白的损伤。
