Crystallographic analysis of primary visual photochemistry
Crystallographic analysis of primary visual photochemistry
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DOI：
10.1002/anie.200600595
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发表时间：
2006-01-01
影响因子：
16.6
通讯作者：
Okada, Tetsuji
中科院分区：
文献类型：
--
作者：
Nakamichi, Hitoshi;Okada, Tetsuji
Photochemical cis–trans isomerization of the retinylidene chromophore is the mechanism of the primary visual process in the eye. 11-cis-Retinal is covalently linked not only to the dim-light photoreceptor protein rhodopsin but also to each of the three color pigments in humans. All of these proteins belong to the superfamily of Gprotein coupled receptors (GPCRs). To achieve extreme sensitivity for photon detection, the photoisomerization reaction to the alltrans form in these pigments has to be strictly designed for efficient formation of the active form of the protein moiety, which catalyzes guanosine diphosphate to triphosphate (GDP–GTP) exchange in the α-subunit of the Gprotein. To understand the molecular mechanism of this primary photochemistry, detailed structural analysis is required. Recent advances in X-ray crystallographic studies of rhodopsin [1–3] have opened a way to investigate the photoreaction with subatomic resolution. The first intermediate that can be trapped following light absorption by rhodopsin is bathorhodopsin (Figure 1 a). Formation of bathorhodopsin occurs on an ultrafast timescale of a few hundred femtoseconds at room temperature.[4] It exhibits a high quantum yield of 0.67 [5] and stores about two-thirds of the photon energy (36kcal molÀ1),[6] which is used to drive the formation of subsequent intermediates. Spectroscopic studies at low temperature or with time resolution yielded indirect structural data for this photochemical intermediate. Resonance Raman spectra of trapped bathorhodopsin indicated a strongly distorted chromophore in the region where the isomerization has taken place;[7] this distortion was recently substantiated and quantified to some extent.[8] Photoaffinity labeling studies of the bathorhodopsin intermediate indicated that the “loose” end of the chromophore, that is, the β-ionone ring, does not change its position relative to the dark state,[9] confirming the notion that the perturbation caused by the photoreaction is initially localized on a relatively small part of the chromophore. This view is supported by recent solid-state NMR spectroscopic observations of the methyl groups of the βionone ring which show that the ring is retained through strong selective interactions within the binding site into the activated state.[10] Also, magic-angle spinning solid-state NMR spectra of isotopically labeled bathorhodopsin revealed only minor differences in chemical shifts compared to rhodopsin, indicating that the protein environment of the chromophore, in particular the interaction with the counterion Glu113, does not change significantly.[11]
