What are the signatures of tunnelling in enzyme-catalysed reactions?,Faraday Discussions

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What are the signatures of tunnelling in enzyme-catalysed reactions?
Faraday Discussions ( IF 3.3 ) Pub Date : 2019-12-16 , DOI: 10.1039/c9fd00044e
Linus O Johannissen ₁ , Andreea I Iorgu , Nigel S Scrutton , Sam Hay

Affiliation

While it is well established that thermally-activated quantum mechanical tunnelling of light particles (electrons and light atoms, typically hydrogen) plays a role in many enzyme-catalysed reactions, there are few definitive experimental signatures of atomic tunnelling and no clear methods of directly estimating the relative tunnelling contribution from typical experimental data. As most enzyme reactions involve the binding/capture of freely diffusing substrate(s), reactions are typically initiated by mixing and experimental conditions must then be compatible with liquid water (the solvent). This precludes the classic test of tunnelling: the observation of temperature-independent rate constants at cryogenic temperatures. Instead, H-tunnelling is usually inferred from kinetic isotope effects that are larger than the semiclassical limit. Often, the temperature dependence of the reaction is also measured over the experimentally accessible range (∼278-313 K for mesophilic enzymes), with resulting data analysed and interpreted using variations of Arrhenius, Eyring or Marcus theory. The apparent Arrhenius and Eyring activation parameters allow some quantitative comparison of different reactions, but do not directly provide any information about tunnelling, while the validity of parameters derived from non-adiabatic models such as Marcus theory are questionable due to the partially adiabatic nature of these reactions. Here, we use the correlation found between apparent activation enthalpy and entropy across several series of enzyme variants and tunnelling contributions determined using computational chemistry in an attempt to question and define new signatures of hydrogen tunnelling, which can be used to interpret typical experimental kinetic data measured for enzyme-catalysed reactions.

中文翻译：

酶催化反应中隧穿的特征是什么？

众所周知，轻质粒子（电子和轻原子，通常是氢）的热激活量子机械隧穿在许多酶催化的反应中起作用，但原子隧穿的明确实验特征很少，也没有明确的直接估算方法典型实验数据的相对隧穿贡献。由于大多数酶反应涉及自由扩散底物的结合/捕获，因此反应通常是通过混合引发的，然后实验条件必须与液态水（溶剂）相容。这排除了经典的隧穿测试：在低温下观察温度无关的速率常数。取而代之的是，H隧道效应通常是由大于半经典限值的动力学同位素效应推断出来的。经常，还可在实验上可达到的范围内（对嗜温性酶而言约为278-313 K）测量反应的温度依赖性，并使用Arrhenius，Eyring或Marcus理论的变化对所得数据进行分析和解释。表观的Arrhenius和Eyring激活参数允许对不同反应进行定量比较，但不直接提供有关隧穿的任何信息，而非绝热模型（例如Marcus理论）衍生的参数的有效性由于这些模型的部分绝热性质而令人怀疑。反应。在这里，我们使用跨酶系列变体的表观激活焓和熵与使用计算化学方法确定的隧穿贡献之间的相关性来试图质疑和定义氢隧穿的新特征，

更新日期：2019-12-17

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