Large spectral change due to amide modes of a β-sheet upon the formation of an early photointermediate of middle rhodopsin
Large spectral change due to amide modes of a β-sheet upon the formation of an early photointermediate of middle rhodopsin
复制标题
在中间视紫红质的早期光中间体形成时，由于 β-折叠的酰胺模式而导致大的光谱变化
作者：
Furutani;Y.;Okitsu;T.;Reissig;L.;Mizuno;M.;Homma;M.;Wada;A.;Mizutani;Y.;Sudo;Y.
视紫红质在七个跨膜螺旋中以视网膜为发色团。最近，我们发现了一种独特的视紫红质(中视紫质，MR)，它在进化上位于细菌视紫红质和感觉性视紫红质II之间，并在其基态下容纳了三种视网膜异构体(全反式、13-顺式和独特的11-环体)。在这项研究中，我们用时间分辨傅立叶变换红外光谱研究了MR光循环过程中蛋白质部分和视网膜生色团的结构变化。通过全局指数拟合分析，确定了三个光中间体，其衰减时间常数分别为95μS、0.9ms和>；∼10ms。根据最近发表的结果，第一和第三中间体被归因于全光循环，而第二中间体很可能是在可见光区光谱上没有变化的中间体，它形成于第一和第三态之间，或者是由于13-环体的激活而产生的。通过比较视网膜和蛋白质部分的不同同位素标记的光诱导差异光谱，我们得出结论：在MR的全光循环过程中，亲水性部分的β-Sheet结构发生了显著变化，这可能与蛋白质的活性状态有关。这是微生物(类型1)视紫红质中MR的特征。
Rhodopsin contains retinal as the chromophore within seven transmembrane helices. Recently, we found a unique rhodopsin (middle rhodopsin, MR), which is evolutionarily located between the well-studied bacteriorhodopsin and sensory rhodopsin II, and which accommodates three retinal isomers in its ground state (the all-trans, the 13-cis, and, uniquely, the 11-cisisomers). In this study, we investigated structural changes of both the protein moiety and the retinal chromophore during photocycles of MR by time-resolved Fourier-transform infrared spectroscopy. Three photointermediates with decay time constants of 95 μs, 0.9 ms, and >∼10 ms were identified by the global exponential fitting analysis. The first and third intermediates were attributed to the all-transphotocycle, in accordance with recently published results, whereas the second intermediate was likely one that was spectroscopically silent in the visible region and that was formed between the first and third states or resulted from the activation of the 13-cisisomer. By comparing light-induced difference spectra with various isotope labels in either the retinal or the protein moiety, we concluded that a β-sheet structure in the hydrophilic part was significantly altered during the all-transphotocycle of MR, which may involve an active state of the protein. This feature is characteristic of MR among microbial (type-1) rhodopsins.