Structural analysis of photo-intermediates of squid rhodopsin

无脊椎动物的视紫红质与脊椎动物的视紫红质光化学性质有很大不同。最近对鱿鱼和牛视紫红质的晶体学研究表明，这些光感受器的结构存在着深刻的差异。在这里，我们进行了鱿鱼深紫质和人工9-顺式异构体异紫质的x射线晶体学研究。对在这些波长下照射的冷冻晶体的衍射数据进行分析，提供了各同分异构体状态的结构信息。结果表明，深紫红质的形成伴随着视网膜多烯链的中心部分向细胞质侧的大移动，s-离子环和席夫碱的小移动；即，视网膜采取右旋全反式配置。相反，异紫质中的视网膜呈平面9-顺式结构；即，与未光解状态下的11-顺式视网膜相比，在C9=C10键周围观察到有限的构型变化。

Photochemical properties of invertebrate rhodopsin are different largely from vertebrate rhodopsin. Recent crystallographic studies of squid and bovine rhodopsins suggest profound differences in the structure of these photoreceptors. Here, we perform X-ray crystallographic studies of squid bathorhodopsin and an artificial 9-cis isomer isorhodopsin. Analyses of diffraction data of frozen crystals that were illuminated at these wavelengths provided the structural information of each isomeric state. The result showed that the formation of bathorhodopsin is accompanied by a large movement of the central moiety of the retinal polyene chain towards the cytoplasmic side with small movements of the s-ionone ring and the Schiff base ; i. e., the retinal takes on a right-handed screwed all-trans configuration. Conversely, the retinal in isorhodopsin takes on a planar 9-cis configuration ; i. e., as compared to the 11-cis retinal in the unphotolysed state, a limited configurational change is observed around the C9=C10 bond.