Elucidation of the Molecular Mechanisms of Photosystem II Complex Based on Its Crystal Structure Analysis

光系统II （PSII）是一种超分子膜蛋白复合物，由14-17个跨膜亚基和3个膜外亚基组成，总分子质量为350 kDa。本研究旨在分析PSII的晶体结构，并在此基础上阐明PSII中发生的电子转移、水裂解和出氧反应的分子机制。为此，我们从嗜热蓝藻热共生球菌（Thermosynechococcus vulcanus）中分离出PSII复合体，并在3.7Å分辨率下对其晶体结构进行了分析，其中基于当前分辨率下可见的一些残基大侧链的电子密度图，我们确定了70-80%的PSII大亚基CP47、CP43、D1、D2的残基。我们构建了所有3种参与氧演化的外源蛋白的结构，其中12 kDa蛋白的结构为首次报道。整个结构还包含14个跨膜螺旋，其中一些被分配到一些低分子质量亚基上，包括细胞色素b559的α和β-亚基，其他螺旋未被识别。在反应中心4个叶绿素中，我们发现“特殊二聚体”PD1-PD2与两个“副叶绿素”的距离较短，说明PSII反应中心不是一个均匀的“四聚体”。我们在D2和细胞色素b559之间分配了两个β-胡萝卜素，这表明PSII的二次电子转移途径是由细胞色素b559或chlzd2串联两个β-胡萝卜素，再到ChlD_2，PD_2,PD_1。我们还获得了先前报道的y形或“3+1”模型的mn团簇的电子密度。我们进一步将PSII晶体分辨率提高到3.5Å，并对原有的PSII结构模型进行了修改，并基于修改后的结构对PSII的功能进行了更详细的分析。

Photosystem II (PSII) is a supra-molecular membrane-protein complex consisting of 14-17 membrane-spanning subunits and 3 membrane-peripheral (extrinsic) subunits with a total molecular mass of 350 kDa. This research aimed to analyze the crystal structure of PSII and, on the basis of this, to elucidate the molecular mechanisms of electron transfer, water-splitting and oxygen-evolving reactions taken place in PSII. For this purpose, we crystallized the PSII complex from a thermophilic cyanobacterium Thermosynechococcus vulcanus and analyzed its crystal structure at 3.7Å resolution in which, we assigned 70-80% residues of PSII large subunits CP47,CP43,D1,D2 based on the electron density maps of some residue's large side chains which were visible at the current resolution. We built the structures of all the 3 extrinsic proteins involved in oxygen evolution, of which, the structure of 12 kDa protein was reported for the first time. The whole structure contained in addition 14 trans-membrane helices, some of which were assigned to some low-molecular mass subunits including the α and β-subunits of cytochrome b559,and other helices were not identified. In the reaction center 4 chlorophylls, we identified that the "special dimer" PD1-PD2 has a shorter distance than those between them and the two "accessory chlorophylls", suggesting that the PSII reaction center is not a homogenous "tetramer". We assigned two β-carotenes between the region of D2 and cytochrome b559, thus implied that the secondary electron transfer pathway in PSII is from cytochrome b559 or ChlZD_2 via the two β-carotenes in series and then to ChlD_2,PD_2,PD_1. We also obtained the electron density for the Mn-cluster which is a Y-shaped or "3+1" model as reported previously. We further improved the PSII crystal resolution to 3.5Å, modified our original PSII structure model, and analyzed the PSII functions in more details based on the modified structure.