Birge R R, Barlow R B
Department of Chemistry, Syracuse University, NY 13244, USA.
Biophys Chem. 1995 Jun-Jul;55(1-2):115-26. doi: 10.1016/0301-4622(94)00145-a.
Retinal photoreceptors generate discrete electrical events in the dark indistinguishable from those evoked by light and the resulting dark signals limit visual sensitivity at low levels of illumination. The random spontaneous events are strongly temperature dependent and in both vertebrate and invertebrate photoreceptors require activation energies usually in the range of 23 to 28 kcal mol-1. Recent molecular orbital studies and pH experiments on horseshoe crabs (Limulus) suggest that the thermal isomerization of a relatively unstable form of rhodopsin, one in which the Schiff-base linkage between the chromophore and protein is unprotonated, is responsible for thermal noise. This mechanism is examined in detail and compared to other literature models for photoreceptor noise. We conclude that this two-step process is likely to be the principal source of noise in all vertebrate and invertebrate photoreceptors. This model predicts that the rate of photoreceptor noise will scale in proportion to 10- xi, where xi is the pKa of the Schiff base proton on the retinyl chromophore. Nature minimizes photoreceptor noise by selecting a binding site geometry which shifts the pKa of the Schiff base proton to > 16, a value significantly larger than the pKa of the chromophore in bacteriorhodopsin (pKa approximately 13) or model protonated Schiff bases in solution (pKa approximately 7).
视网膜光感受器在黑暗中会产生离散的电活动，这些电活动与光诱发的电活动难以区分，并且由此产生的暗信号会在低光照水平下限制视觉敏感度。这些随机的自发活动强烈依赖于温度，在脊椎动物和无脊椎动物的光感受器中，其激活能通常在23至28千卡/摩尔的范围内。最近对鲎（鲎属）进行的分子轨道研究和pH实验表明，一种相对不稳定形式的视紫红质的热异构化是热噪声的原因，在这种形式中，发色团与蛋白质之间的席夫碱连接未被质子化。本文详细研究了这一机制，并与其他文献中关于光感受器噪声的模型进行了比较。我们得出结论，这一两步过程可能是所有脊椎动物和无脊椎动物光感受器中噪声的主要来源。该模型预测，光感受器噪声的速率将与10的负xi次方成比例，其中xi是视黄醛发色团上席夫碱质子的pKa值。自然界通过选择一种结合位点几何结构来最小化光感受器噪声，这种结构会将席夫碱质子的pKa值转移到大于16，这个值明显大于细菌视紫红质中发色团的pKa值（pKa约为13）或溶液中模型质子化席夫碱的pKa值（pKa约为7）。
