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低温下细菌视紫红质的光循环揭示了构象亚态的分布屏障。

Bacteriorhodopsin photocycle at cryogenic temperatures reveals distributed barriers of conformational substates.

作者信息

Dioumaev Andrei K, Lanyi Janos K

机构信息

Department of Physiology and Biophysics, University of California, Irvine, CA 92697, USA.

出版信息

Proc Natl Acad Sci U S A. 2007 Jun 5;104(23):9621-6. doi: 10.1073/pnas.0703859104. Epub 2007 May 29.

Abstract

The time course of thermal reactions after illumination of 100% humidified bacteriorhodopsin films was followed with FTIR spectroscopy between 125 and 195 K. We monitored the conversion of the initial photoproduct, K, to the next, L intermediate, and a shunt reaction of the L state directly back to the initial BR state. Both reactions can be described by either multiexponential kinetics, which would lead to apparent end-state mixtures that contain increasing amounts of the product, i.e., L or BR, with increasing temperature, or distributed kinetics. Conventional kinetic schemes that could account for the partial conversion require reversible reactions, branching, or parallel cycles. These possibilities were tested by producing K or L and monitoring their interconversion at a single temperature and by shifting the temperature upward or downward after an initial incubation and after their redistribution. The results are inconsistent with any conventional scheme. Instead, we attribute the partial conversions to the other alternative, distributed kinetics, observed previously in myoglobin, which arise from an ensemble of frozen conformational substates at the cryogenic temperatures. In this case, the time course of the reactions reflects the progressive depletion of distinct microscopic substates in the order of their increasing activation barriers, with a distribution width for K to L reaction of approximately 7 kJ/mol.

摘要

在125至195K之间,用傅里叶变换红外光谱法跟踪了100%湿度的细菌视紫红质薄膜光照后热反应的时间进程。我们监测了初始光产物K向下一个L中间体的转化,以及L态直接回到初始BR态的分流反应。这两个反应都可以用多指数动力学来描述,这将导致明显的终态混合物,随着温度升高,产物(即L或BR)的含量增加,或者用分布动力学来描述。能够解释部分转化的传统动力学方案需要可逆反应、分支或平行循环。通过产生K或L并在单一温度下监测它们的相互转化,以及在初始孵育后和它们重新分布后向上或向下改变温度来测试这些可能性。结果与任何传统方案都不一致。相反,我们将部分转化归因于另一种选择,即分布动力学,这在肌红蛋白中也曾观察到,它源于低温下一系列冻结的构象亚态。在这种情况下,反应的时间进程反映了不同微观亚态按其活化能垒增加的顺序逐渐耗尽,K到L反应的分布宽度约为7kJ/mol。

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