Li Y F, Zhou W, Blankenship R E, Allen J P
Department of Chemistry and Biochemistry, and Center for The Study of Early Events in Photosynthesis, Arizona State University, Tempe, AZ 85287-1604, USA.
J Mol Biol. 1997 Aug 22;271(3):456-71. doi: 10.1006/jmbi.1997.1189.
The bacteriochlorophyll (BChl) a protein from Chlorobium tepidum, which participates in energy transfer in green photosynthetic bacteria, has been crystallized using the sitting drop method of vapor diffusion. X-ray diffraction data collected from these crystals indicate that the crystals belong to the cubic space group P4132 with cell dimensions of a=b=c=169.5 A. A native X-ray diffraction data set has been collected to a resolution of 2.2 A. The initial solution was determined by using the molecular replacement method using the structure of the previously solved BChl a protein from Prosthecochloris aestuarii. A unique rotation and translation solution was obtained for two monomers in the asymmetric unit giving a pseudo-body centered packing. After rebuilding and refinement the model yields an R factor of 19.0%, a free R-factor of 28.3%, and good geometry with root-mean-square deviations of 0.013 A and 2.1 degrees for the bond lengths and angles, respectively. The structure of the BChl a protein from C. tepidum consists of three identical subunits related by a 3-fold axis of crystallographic symmetry. In each subunit the polypeptide backbone forms large beta-sheets and encloses a central core of seven BChl a molecules. The distances between neighboring bacteriochlorin systems within a subunit range between 4 A to 11 A and that between two bacteriochlorins from different subunits is more than 20 A. The overall structure is comparable with that of P. aestuarii but significant differences are observed for the individual bacteriochlorophyll structures. The surface of the trimer has a hydrophobic region that is modeled as the complex being a peripheral membrane protein partially embedded in the membrane. A general model is presented for the membrane organization with two of the bacteriochlorophyll structures in the membrane and transferring energy to the reaction center complex. In this model these two bacteriochlorophyll structures serve a similar role to the cofactors of integral membrane light-harvesting complexes although the protein structure surrounding the cofactors is significantly different for the BChl a protein compared with the integral membrane complexes.
嗜热绿菌中的细菌叶绿素（BChl）a蛋白参与绿色光合细菌中的能量传递，已采用汽相扩散坐滴法进行了结晶。从这些晶体收集的X射线衍射数据表明，晶体属于立方空间群P4132，晶胞尺寸为a = b = c = 169.5 Å。已收集到分辨率为2.2 Å的天然X射线衍射数据集。初始解是通过分子置换法确定的，使用的是先前解析的来自河口原绿球菌的BChl a蛋白的结构。对于不对称单元中的两个单体，获得了唯一的旋转和平移解，给出了假体心堆积。重建和精修后，模型的R因子为19.0%，自由R因子为28.3%，几何结构良好，键长和键角的均方根偏差分别为0.013 Å和2.1度。嗜热绿菌的BChl a蛋白结构由通过三重晶体学对称轴相关的三个相同亚基组成。在每个亚基中，多肽主链形成大的β折叠片，并包围由七个BChl a分子组成的中心核心。一个亚基内相邻细菌叶绿素系统之间的距离在4 Å至11 Å之间，不同亚基的两个细菌叶绿素之间的距离超过20 Å。整体结构与河口原绿球菌的结构相当，但在单个细菌叶绿素结构上观察到显著差异。三聚体表面有一个疏水区域，该复合物被模拟为部分嵌入膜中的外周膜蛋白。提出了一个膜组织的通用模型，其中两个细菌叶绿素结构位于膜中，并将能量传递到反应中心复合物。在这个模型中，这两个细菌叶绿素结构起到了与整合膜光捕获复合物的辅因子类似的作用，尽管与整合膜复合物相比，BChl a蛋白中围绕辅因子的蛋白质结构有很大不同。
