FTIR studies of the photoactivation processes in squid retinochrome
FTIR studies of the photoactivation processes in squid retinochrome
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DOI：
10.1021/bi050219w
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发表时间：
2005-06-07
期刊：
影响因子：
2.9
通讯作者：
Kandori, H
中科院分区：
文献类型：
--
作者：
Furutani, Y;Terakita, A;Kandori, H
视黄素是无脊椎动物视觉系统的一种光异构酶，它将全反式视网膜转化为11-顺式构型，并将其提供给视觉视紫红质。本文利用低温紫外可见光谱和傅里叶变换红外光谱研究了光诱导鱿鱼视黄素的结构变化。在PC脂质体中，鲁米维黄色素在77~230K的较宽温度范围内稳定，其初级中间体在视黄素中的热稳定性与视紫红质相反。FTIR光谱分析表明，鲁米维甲酸的发色团为松弛的平面11顺构体，与其高的热稳定性相一致。视黄素的发色团结合口袋似乎同时容纳全反式和11-顺式两种形式，没有大的扭曲，并且全反式和11-顺式发色团之间有限的蛋白质结构变化可能适合视黄素作为光异构酶的功能。D2O中N-D和O-D伸缩振动的分析表明，视黄色素中席夫碱的氢键比牛视紫红质和细菌视紫红质弱，而在强氢键条件下(O-D伸展在2334 cm(-1))，视黄素有水分子。在鲁米维黄色素中，水的氢键进一步加强。偏视黄色素的形成伴随着希夫碱的去质子化，以及α-螺旋的肽骨架的改变，以及可能的β-折叠的形成。研究发现，在pH 7.5的PC脂质体中，希夫碱质子不转移到其对离子Glu181，而直接释放到水相。这表明希夫碱的环境暴露在偏视黄色素中，这可能有利于希夫碱在11-顺式视黄醇向其穿梭蛋白的运输过程中的水解反应。
Retinochrome is a photoisomerase of the invertebrate visual system, which converts all-trans-retinal to the 11-cis configuration and supplies it to visual rhodopsin. In this paper, we studied lightinduced structural changes in squid retinochrome by means of low-temperature UV-visible and Fourier transform infrared (FTIR) spectroscopy. In PC liposomes, lumi-retinochrome was stable in the wide temperature range between 77 and 230 K. High thermal stability of the primary intermediate in retinochrome is in contrast to the case in rhodopsins. FTIR spectroscopy suggested that the chromophore of lumi-retinochrome is in a relaxed planar 11-cis form, being consistent with its high thermal stability. The chromophore binding pocket of retinochrome appears to accommodate both all-trans and 11-cis forms without a large distortion, and limited protein structural changes between all-trans and 11-cis chromophores may be suitable for the function of retinochrome as a photoisomerase. The analysis of N-D and O-D stretching vibrations in D2O revealed that the hydrogen bond of the Schiff base is weaker in retinochrome than in bovine rhodopsin and bacteriorhodopsin, while retinochrome has a water molecule under strongly hydrogen-bonded conditions (O-D stretch at 2334 cm(-1)). The hydrogen bond of the water is further strengthened in lumi-retinochrome. The formation of meta-retinochrome accompanies deprotonation of the Schiff base, together with the peptide backbone alterations of a-helices, and possible formation of beta-sheets. It was found that the Schiff base proton is not transferred to its counterion, Glu181, but directly released to the aqueous phase in PC liposomes (pH 7.5). This suggests that the Schiff base environment is exposed to solvent in meta-retinochrome, which may be advantageous for the hydrolysis reaction of the Schiff base in the transport of 11-cis-retinal to its shuttle protein.