Faculty of Science, Ibaraki University, 2-1-1 Bunkyo, Mito 310-8512, Japan.
J Mol Biol. 2010 Apr 16;397(5):1175-87. doi: 10.1016/j.jmb.2010.02.011. Epub 2010 Feb 12.
In green sulfur photosynthetic bacteria, the cytochrome c(z) (cyt c(z)) subunit in the reaction center complex mediates electron transfer mainly from menaquinol/cytochrome c oxidoreductase to the special pair (P840) of the reaction center. The cyt c(z) subunit consists of an N-terminal transmembrane domain and a C-terminal soluble domain that binds a single heme group. The periplasmic soluble domain has been proposed to be highly mobile and to fluctuate between oxidoreductase and P840 during photosynthetic electron transfer. We have determined the crystal structure of the oxidized form of the C-terminal functional domain of the cyt c(z) subunit (C-cyt c(z)) from thermophilic green sulfur bacterium Chlorobium tepidum at 1.3-A resolution. The overall fold of C-cyt c(z) consists of four alpha-helices and is similar to that of class I cytochrome c proteins despite the low similarity in their amino acid sequences. The N-terminal structure of C-cyt c(z) supports the swinging mechanism previously proposed in relation with electron transfer, and the surface properties provide useful information on possible interaction sites with its electron transfer partners. Several characteristic features are observed for the heme environment: These include orientation of the axial ligands with respect to the heme plane, surface-exposed area of the heme, positions of water molecules, and hydrogen-bond network involving heme propionate groups. These structural features are essential for elucidating the mechanism for regulating the redox state of cyt c(z).
在绿色硫光合细菌中,反应中心复合物中的细胞色素 c(z)(cyt c(z))亚基主要介导电子从menaquinol/cytochrome c 氧化还原酶向反应中心的特殊对(P840)转移。cyt c(z)亚基由一个 N 端跨膜结构域和一个 C 端可溶性结构域组成,该结构域结合一个单一的血红素基团。推测周质可溶性结构域具有高度的流动性,并在光合作用电子转移过程中在氧化还原酶和 P840 之间波动。我们已经确定了嗜热绿色硫细菌 Chlorobium tepidum 中细胞色素 c(z)亚基 C 末端功能域(C-cyt c(z))氧化形式的晶体结构,分辨率为 1.3-A。C-cyt c(z)的整体折叠由四个α-螺旋组成,尽管其氨基酸序列的相似性较低,但与 I 类细胞色素 c 蛋白的结构相似。C-cyt c(z)的 N 端结构支持与电子转移有关的摆动机制,表面性质为与电子转移伙伴相互作用的可能位点提供了有用的信息。观察到血红素环境的几个特征:这些包括轴向配体相对于血红素平面的取向、血红素暴露面积、水分子的位置以及涉及血红素丙酸基团的氢键网络。这些结构特征对于阐明调节 cyt c(z)氧化还原状态的机制至关重要。