The Energetics and Molecular Dynamics of the Proton Pumping Photocycle in Bacteriorhodopsin

The energy stored in the primary photochemical event of light adapted bacteriorhodopsin (~16 kcal mol-1) is shown to be sufficient to pump two, but not three, protons per photocycle. A two channel model of the proton pumping photocycle is presented which accommodates the available experimental data. This model is energetically efficient because the primary event is used to destabilize both channels simultaneously. Two separate electrostatic gradients are generated, and the dark reactions that occur following the primary event restabilize both channels via proton translocation processes that ultimately pump two protons. The “primary” channel pumps the first proton in parallel. with the formation of the M412 intermediate. The “secondary” channel pumps the second proton during the O660→bR568 dark reaction provided the electrochemical gradient is below a given threshold. Ground state 13-cis→all-trans reisomerization is accomplished during the N520→O660 dark reaction due to formation of a protonated Schiff base chromophore with a single (secondary) counterion near the β-ionylidene ring. This latter counterion stabilizes a chromophore resonance structure which has a high degree of bond order reversal lowering significantly the barrier to ground state C13=C14 isomerization.

光适应细菌视紫红质的初级光化学过程中储存的能量（约16千卡/摩尔）足以在每个光循环中泵出两个质子，但不足以泵出三个质子。提出了一种质子泵光循环的双通道模型，该模型与现有的实验数据相符。这个模型在能量利用上是高效的，因为初级过程被用于同时使两个通道不稳定。产生了两个独立的静电梯度，初级过程之后发生的暗反应通过质子转运过程使两个通道重新稳定，最终泵出两个质子。“初级”通道在M412中间体形成的同时泵出第一个质子。如果电化学梯度低于给定阈值，“次级”通道在O660→bR568暗反应期间泵出第二个质子。由于在β-紫罗烯环附近形成了具有单个（次级）反离子的质子化席夫碱生色团，基态13 - 顺式→全反式异构化在N520→O660暗反应期间完成。后一种反离子稳定了一种生色团共振结构，该结构具有高度的键序反转，显著降低了基态C13 = C14异构化的能垒。