Xu Qiang, Axelrod Herbert L, Abresch Edward C, Paddock Mark L, Okamura Melvin Y, Feher George
Department of Physics, University of California, San Diego, 9500 Gilman Drive, La Jolla, CA 92093, USA.
Structure. 2004 Apr;12(4):703-15. doi: 10.1016/j.str.2004.03.001.
In the photosynthetic reaction center (RC) from Rhodobacter sphaeroides, the reduction of a bound quinone molecule Q(B) is coupled with proton uptake. When Asp-L213 is replaced by Asn, proton transfer is inhibited. Proton transfer was restored by two second-site revertant mutations, Arg-M233-->Cys and Arg-H177-->His. Kinetic effects of Cd(2+) on proton transfer showed that the entry point in revertant RCs to be the same as in the native RC. The structures of the parental and two revertant RCs were determined at resolutions of 2.10, 1.80, and 2.75 A. From the structures, we were able to delineate alternate proton transfer pathways in the revertants. The main changes occur near Glu-H173, which allow it to substitute for the missing Asp-L213. The electrostatic changes near Glu-H173 cause it to be a good proton donor and acceptor, and the structural changes create a cavity which accommodates water molecules that connect Glu-H173 to other proton transfer components.
在球形红细菌的光合反应中心（RC）中，结合醌分子Q(B)的还原与质子摄取相偶联。当天冬氨酸-L213被天冬酰胺取代时，质子转移受到抑制。通过两个第二位点回复突变，精氨酸-M233→半胱氨酸和精氨酸-H177→组氨酸，质子转移得以恢复。镉离子（Cd(2+)）对质子转移的动力学效应表明，回复型反应中心的进入点与天然反应中心相同。亲本反应中心和两个回复型反应中心的结构分别在2.10、1.80和2.75埃的分辨率下测定。从这些结构中，我们能够描绘出回复型反应中心中交替的质子转移途径。主要变化发生在谷氨酸-H173附近，使其能够替代缺失的天冬氨酸-L213。谷氨酸-H173附近的静电变化使其成为良好的质子供体和受体，结构变化形成了一个容纳水分子的腔，这些水分子将谷氨酸-H173与其他质子转移成分连接起来。
