50(P-7) 視物質ロドプシン中における発色団レチナールのコンホメーションに関する研究(ポスター発表の部)

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The visual pigment rhodopsin (Rh) is comprised of an apoprotein opsin and 11-cis-retinylidene chromophore. Due to the steric interactions in Rh, the chromophore is twisted around the C6/C7 and C12/C13 single bonds. In order to determine the absolute sense of the twists, we have synthesized enantiomeric pairs of structure-locked retinal analogues, 2a/2b for the C6/C7 twist, and 3a/3b (13-trans) and 3c/3d (13-cis) for the C12/C13 twist. The twist around the 6-s-bond: The artificial retinal 2a and 2b were submitted to the binding study with opsin. It was found that 2a bound to opsin, generating the rhodopsin analog, Rh-(2a), whereas no binding was observed at 2b. Molecular modeling yielded the lowest energy conformation of 2a with a-35°twist around the C6/C7 bond. The CD of Rh-(2a) closely resembled that of native rhodopsin. In addition, a GTP binding assay showed Rh-(2a) to be 80% as active as the native pigment. The present results establish the absolute sense of helicity around 6-s-cis bond as being negative. The twist around the 12-s-bond: The binding study of 3a/3b and 3c/3d were performed. The β-locked analogs 3a and 3c bound to opsin, but not 3b and 3d. NMR measurements and molecular modeling of 3a showed that the lowest energy conformation has twist chair form of the seven-membered ring, and the C12/C13 twist was positive. However, the CD of Rh-(3a) exhibited only a weak positive CE and no obvious exciton couplet. It is possible that the CE represents the positive wing of the compressed chromophore as the CD of 3a, but the results implied that an opsin compression effect forces the chromophore in Rh-(3a) to be flattened. Efforts are ongoing to determine the absolute sense of twist around the chromophoric 12-s-bond, which is not clear in the recently determined x-ray structure of rhodopsin (2.8 A resolution) and controversial according to several other experimental and theoretical studies.
2001-09-01

50(P-7) 視物質ロドプシン中における発色団レチナールのコンホメーションに関する研究(ポスター発表の部)

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