酶促反应的溶剂粘度依赖性
Solvent viscosity dependence for enzymatic reactions
摘要：
提出了酶作用下溶剂粘度与反应速率常数关系的机理。它是基于酶活性位点中电场的波动产生的热平衡摇摆（曲柄轴运动）的刚性平面（其中的偶极矩$\approx 3.6 D$所在）的一个有利的位置和取向的肽基团（或可能是其中的几个）。因此，平面的摇摆导致偶极矩电场的波动。这些波动可以与反应坐标相互作用，因为后者反过来具有跃迁偶极矩，由于在移动的反应系统沿它的电荷分离。摇摆的肽基团的平面是敏感的微粘度的环境中的蛋白质内部和后者是溶剂粘度的函数。因此，我们得到了一个额外的因素，这些特性与反应速率常数的相互关系。我们认为，由于曲轴运动的特性，电场波动的频谱在某些频率处具有尖锐的共振峰，并且相应的傅立叶模式可以近似为振荡。我们采用了一个已知的结果，从理论上的热激活逃逸周期性驱动，以获得反应速率常数，并认为它产生可靠的描述preexponent溶剂粘度的依赖体现本身。所建议的机制，以掌握从实验中已知的这种依赖性的主要特征，并令人满意地产生的分数指数的上限在它。
英文摘要：
A mechanism for relationship of solvent viscosity with reaction rate constant at enzyme action is suggested. It is based on fluctuations of electric field in enzyme active site produced by thermally equilibrium rocking (cranckshaft motion) of the rigid plane (in which the dipole moment $\approx 3.6 D$ lies) of a favourably located and oriented peptide group (or may be a few of them). Thus the rocking of the plane leads to fluctuations of the electric field of the dipole moment. These fluctuations can interact with the reaction coordinate because the latter in its turn has transition dipole moment due to separation of charges at movement of the reacting system along it. The rocking of the plane of the peptide group is sensitive to the microviscosity of its environment in protein interior and the latter is a function of the solvent viscosity. Thus we obtain an additional factor of interrelationship for these characteristics with the reaction rate constant. We argue that due to the properties of the cranckshaft motion the frequency spectrum of the electric field fluctuations has a sharp resonance peak at some frequency and the corresponding Fourier mode can be approximated as oscillations. We employ a known result from the theory of thermally activated escape with periodic driving to obtain the reaction rate constant and argue that it yields reliable description of the preexponent where the dependence on solvent viscosity manifests itself. The suggested mechanism is shown to grasp the main feature of this dependence known from the experiment and satisfactorily yields the upper limit of the fractional index of a power in it.
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