The pKa of the protonated Schiff bases of gecko cone and octopus visual pigments.
The pKa of the protonated Schiff bases of gecko cone and octopus visual pigments.
复制标题
壁虎视锥细胞和章鱼视觉色素的质子化希夫碱的 pKa。
DOI：
10.1016/s0006-3495(94)80544-2
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发表时间：
1994
影响因子：
3.4
通讯作者：
Tsuda,M
中科院分区：
文献类型：
--
作者：
Liang,J;Steinberg,G;Livnah,N;Sheves,M;Ebrey,TG;Tsuda,M
视色素由通过质子化席夫碱键与其脱辅基蛋白结合的视黄醛组成。光使发色团异构化，并最终在漂白循环的Meta II阶段引起该席夫碱键的去质子化。视色素的Meta II中间体是色素的活性形式，其结合并激活G蛋白转导蛋白，启动视觉级联反应。Schiff碱的去质子化对于形成Meta II中间体是必需的。本文用光谱滴定法研究了章鱼视紫红质和壁虎视锥色素P521的席夫碱质子结合亲合力pKa。几种含氟的视黄醛类似物在Schiff碱区域附近具有强吸电子特性，并降低了模型化合物中Schiff碱的pKa。我们用这些氟化和其他视网膜类似物再生了章鱼和壁虎的视色素。对这些人造颜料的实验表明，在提高pH值时看到的光谱变化确实反映了席夫碱的pKa，而不是颜料的变性或颜料中其他基团的去质子化。章鱼视紫红质的席夫碱pKa为10.4，壁虎视锥色素的pKa为9.9。我们还表明，虽然Cl-离子的去除导致相当大的蓝移在壁虎锥色素P521，它影响的席夫碱pKa非常小，表明λ max的视觉色素和它的席夫碱pKa不紧密耦合。
A visual pigment is composed of retinal bound to its apoprotein by a protonated Schiff base linkage. Light isomerizes the chromophore and eventually causes the deprotonation of this Schiff base linkage at the meta II stage of the bleaching cycle. The meta II intermediate of the visual pigment is the active form of the pigment that binds to and activates the G protein transducin, starting the visual cascade. The deprotonation of the Schiff base is mandatory for the formation of meta II intermediate. We studied the proton binding affinity, pKa, of the Schiff base of both octopus rhodopsin and the gecko cone pigment P521 by spectral titration. Several fluorinated retinal analogs have strong electron withdrawing character around the Schiff base region and lower the Schiff base pKa in model compounds. We regenerated octopus and gecko visual pigments with these fluorinated and other retinal analogs. Experiments on these artificial pigments showed that the spectral changes seen upon raising the pH indeed reflected the pKa of the Schiff base and not the denaturation of the pigment or the deprotonation of some other group in the pigment. The Schiff base pKa is 10.4 for octopus rhodopsin and 9.9 for the gecko cone pigment. We also showed that although the removal of Cl- ions causes considerable blue-shift in the gecko cone pigment P521, it affects the Schiff base pKa very little, indicating that the lambda max of visual pigment and its Schiff base pKa are not tightly coupled.