Uphaus R A, Fang J Y, Picorel R, Chumanov G, Wang J Y, Cotton T M, Seibert M
Ames Laboratory, Iowa State University, USA.
Photochem Photobiol. 1997 Apr;65(4):673-9. doi: 10.1111/j.1751-1097.1997.tb01910.x.
The photosystem II (PSII) reaction center (RC) is a hydrophobic intrinsic protein complex that drives the water-oxidation process of photosynthesis. Unlike the bacterial RC complex, an X-ray crystal structure of the PSII RC is not available. In order to determine the physical dimensions of the isolated PSII RC complex, we applied Langmuir techniques to determine the cross-sectional area of an isolated RC in a condensed monolayer film. Low-angle X-ray diffraction results obtained by examining Langmuir-Blodgett multilayer films of alternating PSII RC/Cd stearate monolayers were used to determine the length (or height; z-direction, perpendicular to the plane of the original membrane) of the complex. The values obtained for a PSII RC monomer were 26 nm2 and 4.8 nm, respectively, and the structural integrity of the RC in the multilayer film was confirmed by several approaches. Assuming a cylindrical-type RC structure, the above dimensions lead to a predicted volume of about 125 nm3. This value is very close to the expected volume of 118 nm3, calculated from the known molecular weight and partial specific volume of the PSII RC proteins. This same type of comparison was also made with the Rhodobacter sphaeroides RC based on published data, and we conclude that the PSII RC is much shorter in length and has a more regular solid geometric structure than the bacterial RC. Furthermore, the above dimensions of the PSII RC and those of PSII core (RC plus proximal antenna) proteins protruding outside the plane of the PSII membrane into the lumenal space as imaged by scanning tunneling microscopy (Seibert, Aust. J. Pl. Physiol. 22, 161-166, 1995) fit easily into the known dimensions of the PSII core complex visualized by others as electron-density projection maps. From this we conclude that the in situ PSII core complex is a dimeric structure containing two copies of the PSII RC.
光系统II（PSII）反应中心（RC）是一种疏水性内在蛋白复合物，驱动光合作用的水氧化过程。与细菌RC复合物不同，PSII RC的X射线晶体结构尚未可得。为了确定分离的PSII RC复合物的物理尺寸，我们应用朗缪尔技术来确定凝聚单分子层膜中分离的RC的横截面积。通过检查交替的PSII RC/硬脂酸镉单分子层的朗缪尔-布洛杰特多层膜获得的低角度X射线衍射结果，用于确定该复合物的长度（或高度；z方向，垂直于原始膜平面）。PSII RC单体获得的值分别为26 nm²和4.8 nm，并且通过几种方法证实了多层膜中RC的结构完整性。假设为圆柱形RC结构，上述尺寸导致预测体积约为125 nm³。该值与根据PSII RC蛋白的已知分子量和偏比容计算出的预期体积118 nm³非常接近。基于已发表的数据，对球形红细菌RC也进行了同样类型的比较，我们得出结论，PSII RC的长度比细菌RC短得多，并且具有更规则的固体几何结构。此外，通过扫描隧道显微镜成像（Seibert，澳大利亚植物生理学杂志22，161 - 166，1995），PSII RC的上述尺寸以及PSII核心（RC加上近端天线）蛋白突出到PSII膜平面之外进入腔空间的尺寸，很容易与其他人作为电子密度投影图可视化的PSII核心复合物的已知尺寸相匹配。由此我们得出结论，原位PSII核心复合物是一种二聚体结构，包含两个PSII RC拷贝。
