Department of Biochemistry, Arrhenius Laboratories for Natural Sciences, Stockholm University, S-106 91, Stockholm, Sweden.
Photosynth Res. 1993 Jan;38(3):249-63. doi: 10.1007/BF00046750.
Approximately 20 protein subunits are associated with the PS II complex, not counting subunits of peripheral light-harvesting antenna complexes. However, it is not yet established which proteins specifically are involved in the water-oxidation process. Much evidence supports the concept that the D1/D2 reaction center heterodimer not only plays a central role in the primary photochemistry of Photosystem II, but also is involved in electron donation to P680 and in ligation of the manganese cluster. This evidence includes (a) the primary donor to P680 has been shown to be a redox-active tyrosyl residue (Tyr161) in the D1 protein, and (b) site-directed mutagenesis and computer-assisted modeling of the reaction center heterodimer have suggested several sites with a possible function in manganese ligation. These include Asp170, Gln165 and Gln189 of the D1 protein and Glu69 of the D2 protein as well as the C-terminal portion of the mature D1 protein. Also, hydrophilic loops of the chlorophyll-binding protein CP43 that are exposed at the inner thylakoid surface could be essential for the water-splitting process.In photosynthetic eukaryotes, three lumenal extrinsic proteins, PS II-O (33 kDa), PS II-P (23 kDa) and PS II-Q (16 kDa), influence the properties of the manganese cluster without being involved in the actual catalysis of water oxidation. The extrinsic proteins together may have multiple binding sites to the integral portion of PS II, which could be provided by the D1/D2 heterodimer and CP47. A major role for the PS II-O protein is to stabilize the manganese cluster. Most experimental evidence favors a connection of the PS II-P protein with binding of the Cl(-) and Ca(2+) ions required for the water oxidation, while the PS II-Q protein seems to be associated only with the Cl(-) requirement. The two latter proteins are not present in PS II of prokaryotic organisms, where their functions may be replaced by a 10-12 kDa subunit and a newly discovered low-potential cytochrome c-550.
大约有 20 个蛋白质亚基与 PS II 复合物相关，不包括外周光捕获天线复合物的亚基。然而，目前还不清楚哪些蛋白质具体参与了水氧化过程。大量证据支持这样的概念，即 D1/D2 反应中心异二聚体不仅在 PS II 的初级光化学中起着核心作用，而且还参与电子向 P680 的供体以及锰簇的配位。这些证据包括：(a) 已经证明 P680 的初级供体是 D1 蛋白中的一个氧化还原活性的酪氨酸残基（Tyr161），(b) 反应中心异二聚体的定点突变和计算机辅助建模表明，几个位点可能在锰配位中具有功能。这些包括 D1 蛋白中的 Asp170、Gln165 和 Gln189 以及 D2 蛋白中的 Glu69，以及成熟 D1 蛋白的 C 末端部分。此外，在类囊体膜内表面暴露的叶绿素结合蛋白 CP43 的亲水性环可能对水分解过程至关重要。在光合真核生物中，三种腔外的外质蛋白 PS II-O（33 kDa）、PS II-P（23 kDa）和 PS II-Q（16 kDa）影响锰簇的性质，但不参与水氧化的实际催化。外质蛋白一起可能有多个结合位点到 PS II 的整体部分，这可以由 D1/D2 异二聚体和 CP47 提供。PS II-O 蛋白的主要作用是稳定锰簇。大多数实验证据都支持 PS II-P 蛋白与水氧化所需的 Cl(-)和 Ca(2+)离子的结合有关，而 PS II-Q 蛋白似乎只与 Cl(-)的需求有关。后两种蛋白质不存在于原核生物的 PS II 中，它们的功能可能被一个 10-12 kDa 的亚基和一个新发现的低电位细胞色素 c-550 取代。
