Luecke H, Richter H T, Lanyi J K
Department of Molecular Biology and Biochemistry, University of California, Irvine, CA 92697, USA.
Science. 1998 Jun 19;280(5371):1934-7. doi: 10.1126/science.280.5371.1934.
Photoisomerization of the retinal of bacteriorhodopsin initiates a cyclic reaction in which a proton is translocated across the membrane. Studies of this protein promise a better understanding of how ion pumps function. Together with a large amount of spectroscopic and mutational data, the atomic structure of bacteriorhodopsin, determined in the last decade at increasing resolutions, has suggested plausible but often contradictory mechanisms. X-ray diffraction of bacteriorhodopsin crystals grown in cubic lipid phase revealed unexpected two-fold symmetries that indicate merohedral twinning along the crystallographic c axis. The structure, refined to 2.3 angstroms taking this twinning into account, is different from earlier models, including that most recently reported. One of the carboxyl oxygen atoms of the proton acceptor Asp85 is connected to the proton donor, the retinal Schiff base, through a hydrogen-bonded water and forms a second hydrogen bond with another water. The other carboxyl oxygen atom of Asp85 accepts a hydrogen bond from Thr89. This structure forms the active site. The nearby Arg82 is the center of a network of numerous hydrogen-bonded residues and an ordered water molecule. This network defines the pathway of the proton from the buried Schiff base to the extracellular surface.
细菌视紫红质中视黄醛的光异构化引发了一个循环反应，在此过程中质子跨膜转运。对这种蛋白质的研究有望让人们更好地理解离子泵的工作原理。结合大量光谱学和突变数据，过去十年间以不断提高的分辨率测定的细菌视紫红质的原子结构，提出了看似合理但往往相互矛盾的机制。在立方脂质相中生长的细菌视紫红质晶体的X射线衍射显示出意外的二次对称性，这表明沿晶体学c轴存在半面体孪晶。考虑到这种孪晶现象而精修至2.3埃的结构与早期模型不同，包括最近报道的模型。质子受体天冬氨酸85的一个羧基氧原子通过一个氢键连接的水分子与质子供体视黄醛席夫碱相连，并与另一个水分子形成第二个氢键。天冬氨酸85的另一个羧基氧原子接受苏氨酸89的氢键。这种结构构成了活性位点。附近的精氨酸82是众多氢键连接的残基和一个有序水分子网络的中心。这个网络定义了质子从埋藏的席夫碱到细胞外表面的路径。
