Photoisomerization efficiency in UV-absorbing visual pigments: Protein-directed isomerization of an unprotonated retinal schiff base
Photoisomerization efficiency in UV-absorbing visual pigments: Protein-directed isomerization of an unprotonated retinal schiff base
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DOI：
10.1021/bi7003763
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发表时间：
2007-05-29
期刊：
影响因子：
2.9
通讯作者：
Shichida, Yoshinori
中科院分区：
文献类型：
--
作者：
Tsutsui, Kei;Imai, Hiroo;Shichida, Yoshinori
视色素由视蛋白和发色团11-顺式-视黄醛组成，所述发色团11-顺式-视黄醛通过席夫碱键与视蛋白的特定赖氨酸残基结合。席夫碱发色团在吸收可见光的颜料中质子化，而在吸收紫外线的视觉颜料（UV颜料）中未质子化。为了研究未质子化的席夫碱是否可以在视蛋白环境中与质子化的席夫碱一样有效地进行光异构化，我们测量了牛视紫红质E113 Q突变体的量子产率，其中席夫碱在碱性pH下未质子化，以及小鼠UV色素（小鼠UV）。紫外线颜料的光敏性通过照射颜料，然后通过发色团提取和HPLC分析来测量。通过比较原始色素及其酸变性状态的最大吸收率来估计消光系数。牛视紫红质E113 Q突变体在pH 8.2时的量子产率，其中席夫碱是未质子化的，是显着低于野生型视紫红质，而突变体给出了一个量子产率几乎相同的野生型在pH 5.5时，其中席夫碱是质子化的。这些结果表明，席夫碱质子化在增加量子产率中起作用。小鼠紫外线，它有一个未质子化的希夫碱发色团的量子产率，显着高于未质子化形式的视紫红质E113 Q突变体，虽然它仍然低于可见光吸收色素。这些结果表明，小鼠UV色素的非质子化席夫碱生色团的有效光异构化的一个特定的机制。
A visual pigment consists of an opsin protein and a chromophore, 11-cis-retinal, which binds to a specific lysine residue of opsin via a Schiff base linkage. The Schiff base chromophore is protonated in pigments that absorb visible light, whereas it is unprotonated in ultraviolet-absorbing visual pigments (UV pigments). To investigate whether an unprotonated Schiff base can undergo photoisomerization as efficiently as a protonated Schiff base in the opsin environment, we measured the quantum yields of the bovine rhodopsin E113Q mutant, in which the Schiff base is unprotonated at alkaline pH, and the mouse UV pigment (mouse UV). Photosensitivities of UV pigments were measured by irradiation of the pigments followed by chromophore extraction and HPLC analysis. Extinction coefficients were estimated by comparing the maximum absorbances of the original pigments and their acid-denatured states. The quantum yield of the bovine rhodopsin E113Q mutant at pH 8.2, where the Schiff base is unprotonated, was significantly lower than that of wild-type rhodopsin, whereas the mutant gave a quantum yield almost identical to that of the wild type at pH 5.5, where the Schiff base is protonated. These results suggest that Schiff base protonation plays a role in increasing quantum yield. The quantum yield of mouse UV, which has an unprotonated Schiff base chromophore, was significantly higher than that of the unprotonated form of the rhodopsin E113Q mutant, although it was still lower than the visible-absorbing pigments. These results suggest that the mouse UV pigment has a specific mechanism for the efficient photoisomerization of its unprotonated Schiff base chromophore.