Petrovskaya L E, Balashov S P, Lukashev E P, Imasheva E S, Gushchin I Yu, Dioumaev A K, Rubin A B, Dolgikh D A, Gordeliy V I, Lanyi J K, Kirpichnikov M P
Shemyakin-Ovchinnikov Institute of Bioorganic Chemistry, Russian Academy of Sciences, Moscow, 117997, Russia.
Biochemistry (Mosc). 2015 Jun;80(6):688-700. doi: 10.1134/S000629791506005X.
This review covers the properties of a retinal protein (ESR) from the psychrotrophic bacterium Exiguobacterium sibiricum that functions as a light-driven proton pump. The presence of a lysine residue at the position corresponding to intramolecular proton donor for the Schiff base represents a unique structural feature of ESR. We have shown that Lys96 successfully facilitates delivery of protons from the cytoplasmic surface to the Schiff base, thus acting as a proton donor in ESR. Since proton uptake during the photocycle precedes Schiff base reprotonation, we conclude that this residue is initially in the uncharged state and acquires a proton for a short time after Schiff base deprotonation and M intermediate formation. Involvement of Lys as a proton donor distinguishes ESR from the related retinal proteins - bacteriorhodopsin (BR), proteorhodopsin (PR), and xanthorhodopsin (XR), in which the donor function is performed by residues with a carboxyl side chain. Like other eubacterial proton pumps (PR and XR), ESR contains a histidine residue interacting with the proton acceptor Asp85. In contrast to PR, this interaction leads to shift of the acceptor's pKa to more acidic pH, thus providing its ability to function over a wide pH range. The presence of a strong H-bond between Asp85 and His57, the structure of the proton-conducting pathways from cytoplasmic surface to the Schiff base and to extracellular surface, and other properties of ESR were demonstrated by solving its three-dimensional structure, which revealed several differences from known structures of BR and XR. The structure of ESR, its photocycle, and proton transfer reactions are discussed in comparison with homologous retinal proteins.
本综述涵盖了来自嗜冷细菌西伯利亚微小杆菌的一种视网膜蛋白（ESR）的特性，该蛋白作为光驱动质子泵发挥作用。在对应于席夫碱分子内质子供体的位置存在赖氨酸残基是ESR的一个独特结构特征。我们已经表明，赖氨酸96成功促进质子从细胞质表面传递到席夫碱，从而在ESR中充当质子供体。由于光循环过程中的质子摄取先于席夫碱再质子化，我们得出结论，该残基最初处于不带电状态，在席夫碱去质子化和M中间体形成后短时间内获得一个质子。赖氨酸作为质子供体的参与使ESR与相关的视网膜蛋白——细菌视紫红质（BR）、视紫质（PR）和黄视紫红质（XR）有所不同，在这些蛋白中，供体功能由具有羧基侧链的残基执行。与其他真细菌质子泵（PR和XR）一样，ESR含有一个与质子受体天冬氨酸85相互作用的组氨酸残基。与PR不同的是，这种相互作用导致受体的pKa向更酸性的pH值移动，从而使其能够在较宽的pH范围内发挥作用。通过解析其三维结构，证明了天冬氨酸85和组氨酸57之间存在强氢键、从细胞质表面到席夫碱以及到细胞外表面的质子传导途径的结构，以及ESR的其他特性，该结构揭示了与BR和XR已知结构的若干差异。与同源视网膜蛋白相比，讨论了ESR的结构、其光循环和质子转移反应。
