Lanyi J K
Department of Physiology and Biophysics, University of California, Irvine, California, 92697, USA.
J Struct Biol. 1998 Dec 15;124(2-3):164-78. doi: 10.1006/jsbi.1998.4044.
The atomic structure of bacteriorhodopsin and the outlines of its proton transport mechanism are now available. Photoisomerization of the retinal in the chromophore creates a steric and electrostatic conflict at the retinal binding site. The free energy gain sets off a sequence of reactions in which directed proton transfers take place between the protonated retinal Schiff base, Asp-85, and Asp-96. These internal steps, and other proton transfers at and near the two aqueous interfaces, add up to the translocation of a proton from the cytoplasmic to the extracellular side of the membrane. Bound water plays a crucial role in proton conduction in both extracellular and cytoplasmic regions, but the means by which the protons move from site to site differ. Proton release to the extracellular surface is through interaction of a hydrogen-bonded chain of identified aspartic acid, arginine, water, and glutamic acid residues with Asp-85, while proton uptake from the cytoplasmic surface utilizes a single aspartic acid, Asp-96, whose protonation state appears to be regulated by the protein conformation dependent hydration of this region. The directionality of the translocation is ensured by the accessibility of the Schiff base to the extracellular and cytoplasmic directions after the retinal is photoisomerized, as well as the changing proton affinities of the acceptor Asp-85 and donor Asp-96.
细菌视紫红质的原子结构及其质子转运机制的概况现已明晰。发色团中视黄醛的光异构化在视黄醛结合位点产生了空间和静电冲突。自由能的增加引发了一系列反应，在此过程中，质子在质子化的视黄醛席夫碱、天冬氨酸-85和天冬氨酸-96之间进行定向转移。这些内部步骤，以及在两个水相界面处及附近的其他质子转移，共同促成了质子从膜的细胞质一侧转运到细胞外一侧。结合水在细胞外和细胞质区域的质子传导中都起着关键作用，但质子在不同位点之间移动的方式有所不同。质子释放到细胞外表面是通过一条由特定的天冬氨酸、精氨酸、水和谷氨酸残基组成的氢键链与天冬氨酸-85相互作用实现的，而从细胞质表面摄取质子则利用单个天冬氨酸，即天冬氨酸-96，其质子化状态似乎受该区域蛋白质构象依赖性水合作用的调节。视黄醛光异构化后，席夫碱对细胞外和细胞质方向的可及性，以及受体天冬氨酸-85和供体天冬氨酸-96不断变化的质子亲和力，确保了转运的方向性。
