Åqvist Johan
Department of Cell and Molecular Biology, Biomedical Center, Uppsala University , Box 596, SE-751 24 Uppsala, Sweden.
Biochemistry. 2017 Aug 15;56(32):4169-4176. doi: 10.1021/acs.biochem.7b00523. Epub 2017 Aug 2.
The main problem for enzymes from psychrophilic species, which need to work near the freezing point of liquid water, is the exponential decay of reaction rates as the temperature is decreased. Cold-adapted enzymes have solved this problem by shifting the activation enthalpy-entropy balance for the catalyzed reaction compared to those of their mesophilic orthologs. To understand the structural basis of this universal feature, it is necessary to examine pairs of such orthologous enzymes, with known three-dimensional structures, at the microscopic level. Here, we use molecular dynamics free energy calculations in combination with the empirical valence bond method to evaluate the temperature dependence of the activation free energy for differently adapted triosephosphate isomerases. The results show that the enzyme from the psychrophilic bacterium Vibrio marinus indeed displays the characteristic shift in enthalpy-entropy balance, compared to that of the yeast ortholog. The origin of this effect is found to be located in a few surface-exposed protein loops that show differential mobilities in the two enzymes. Key mutations render these loops more mobile in the cold-adapted triosephosphate isomerase, which explains both the reduced activation enthalpy contribution from the protein surface and the lower thermostability.
对于来自嗜冷物种、需要在液态水的冰点附近发挥作用的酶来说，主要问题是随着温度降低反应速率呈指数衰减。与中温直系同源酶相比，冷适应酶通过改变催化反应的活化焓 - 熵平衡解决了这个问题。为了理解这一普遍特征的结构基础，有必要在微观层面研究具有已知三维结构的此类直系同源酶对。在此，我们结合分子动力学自由能计算和经验价键方法，评估不同适应性的磷酸丙糖异构酶活化自由能的温度依赖性。结果表明，与酵母直系同源酶相比，嗜冷细菌海弧菌的酶确实表现出焓 - 熵平衡的特征性转变。发现这种效应的起源位于一些表面暴露的蛋白质环中，这两个酶在这些环中表现出不同的流动性。关键突变使这些环在冷适应的磷酸丙糖异构酶中更具流动性，这既解释了蛋白质表面活化焓贡献的降低，也解释了较低的热稳定性。
