Váró G
Institute of Biophysics, Biological Research Center of the Hungarian Academy of Sciences, H-6701, Szeged, Hungary.
Biochim Biophys Acta. 2000 Aug 30;1460(1):220-9. doi: 10.1016/s0005-2728(00)00141-9.
The light-activated proton-pumping bacteriorhodopsin and chloride ion-pumping halorhodopsin are compared. They belong to the family of retinal proteins, with 25% amino acid sequence homology. Both proteins have seven alpha helices across the membrane, surrounding the retinal binding pocket. Photoexcitation of all-trans retinal leads to ion transporting photocycles, which exhibit great similarities in the two proteins, despite the differences in the ion transported. The spectra of the K, L, N and O intermediates, calculated using time-resolved spectroscopic measurements, are very similar in both proteins. The absorption kinetic measurements reveal that the chloride ion transporting photocycle of halorhodopsin does not have intermediate M characteristic for deprotonated Schiff base, and intermediate L dominates the process. Energetically the photocycle of bacteriorhodopsin is driven mostly by the decrease of the entropic energy, while the photocycle of halorhodopsin is enthalpy-driven. The ion transporting steps were characterized by the electrogenicity of the intermediates, calculated from the photoinduced transient electric signal measurements. The function of both proteins could be described with the 'local access' model developed for bacteriorhodopsin. In the framework of this model it is easy to understand how bacteriorhodopsin can be converted into a chloride pump, and halorhodopsin into a proton pump, by changing the ion specificity with added ions or site-directed mutagenesis.
对光激活的质子泵视紫红质和氯离子泵嗜盐视紫红质进行了比较。它们属于视网膜蛋白家族，氨基酸序列同源性为25%。两种蛋白都有七个跨膜的α螺旋，围绕着视网膜结合口袋。全反式视网膜的光激发导致离子转运光循环，尽管所转运的离子不同，但这两种蛋白的光循环表现出很大的相似性。利用时间分辨光谱测量计算得到的K、L、N和O中间体的光谱在两种蛋白中非常相似。吸收动力学测量表明，嗜盐视紫红质的氯离子转用光循环没有去质子化席夫碱特有的中间体M，中间体L主导该过程。从能量角度来看，视紫红质的光循环主要由熵能的降低驱动，而嗜盐视紫红质的光循环是由焓驱动的。离子转运步骤通过根据光诱导瞬态电信号测量计算得到的中间体的电致性来表征。两种蛋白的功能都可以用为视紫红质开发的“局部通道”模型来描述。在这个模型的框架内，很容易理解通过添加离子或定点诱变改变离子特异性，视紫红质如何能转化为氯离子泵，嗜盐视紫红质如何能转化为质子泵。
