Department of Ophthalmology, Massachusetts Eye and Ear Infirmary, Boston, MA, USA.
Biophys J. 2010 Oct 6;99(7):2366-73. doi: 10.1016/j.bpj.2010.08.003.
Visual opsins bind 11-cis retinal at an orthosteric site to form rhodopsins but increasing evidence suggests that at least some are capable of binding an additional retinoid(s) at a separate, allosteric site(s). Microspectrophotometric measurements on isolated, dark-adapted, salamander photoreceptors indicated that the truncated retinal analog, β-ionone, partitioned into the membranes of green-sensitive rods; however, in blue-sensitive rod outer segments, there was an enhanced uptake of four or more β-ionones per rhodopsin. X-ray crystallography revealed binding of one β-ionone to bovine green-sensitive rod rhodopsin. Cocrystallization only succeeded with extremely high concentrations of β-ionone and binding did not alter the structure of rhodopsin from the inactive state. Salamander green-sensitive rod rhodopsin is also expected to bind β-ionone at sufficiently high concentrations because the binding site is present on its surface. Therefore, both blue- and green-sensitive rod rhodopsins have at least one allosteric binding site for retinoid, but β-ionone binds to the latter type of rhodopsin with low affinity and low efficacy.
视蛋白在正位点结合 11-顺式视黄醛形成视紫红质，但越来越多的证据表明，至少有一些视蛋白能够在另一个别构位点结合额外的类视黄醇。对分离的、暗适应的蝾螈光感受器进行的微量分光光度测量表明，截断的视黄醛类似物β-紫罗兰酮分配到绿色敏感棒状细胞的膜中；然而，在蓝色敏感的棒状细胞外段，每个视紫红质的β-紫罗兰酮的摄取增加了四个或更多。X 射线晶体学揭示了一个β-紫罗兰酮与牛绿色敏感棒状细胞视紫红质的结合。只有在β-紫罗兰酮的极高浓度下才能成功共晶化，并且结合不会改变视紫红质从非活性状态的结构。由于其表面存在结合位点，因此，蝾螈绿色敏感棒状细胞视紫红质也预计会在足够高的浓度下结合β-紫罗兰酮。因此，蓝色和绿色敏感棒状细胞视紫红质都至少有一个类视黄醇的别构结合位点，但β-紫罗兰酮与后者类型的视紫红质的亲和力和效力较低。
