The enzymatic hydration of CO2 into HCO3- by carbonic anhydrase (CA) is highly efficient and environment-friendly measure for CO2 sequestration. Here extensive MM MD and QM/MM MD simulations were used to explore the whole enzymatic process, and a full picture of the enzymatic hydration of CO2 by CA was achieved. Prior to CO2 hydration, the proton transfer from the water molecule (WT1) to His64 is the rate-limiting step with the free energy barrier of 10.4 kcal/mol, which leads to the ready state with the Zn-bound OH-. The nucleophilic attack of OH- on CO2 produces HCO3- with the free energy barrier of 4.4 kcal/mol and the free energy release of about 8.0 kcal/mol. Gln92 as the key residue manipulates both CO2 transportation to the active site and release of HCO3-. The unprotonated His64 in CA prefers in an inward orientation, while the outward conformation is favorable energetically for its protonated counterpart. The conformational transition of His64 between inward and outward correlates with its protonation state, which is mediated by the proton transfer and the product release. The whole enzymatic cycle has the free energy span of 10.4 kcal/mol for the initial proton transfer step and the free energy change of -6.5 kcal/mol. The mechanistic details provide a comprehensive understanding of the entire reversible conversion of CO2 into bicarbonate and roles of key residues in chemical and nonchemical steps for the enzymatic hydration of CO2.

中文翻译：

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His64 的构象变化和底物运输：深入​​了解碳酸酐酶对 CO2 酶促水合的全貌

通过碳酸酐酶 (CA) 将 CO2 酶促水合为 HCO3- 是一种高效且环保的 CO2 封存措施。在这里，广泛的 MM MD 和 QM/MM MD 模拟用于探索整个酶促过程，并获得了 CA 对 CO2 酶促水合的全貌。在 CO2 水合之前，质子从水分子 (WT1) 转移到 His64 是限速步骤，自由能垒为 10.4 kcal/mol，这导致与 Zn 结合的 OH- 处于就绪状态。OH- 对 CO2 的亲核攻击产生 HCO3-，其自由能垒为 4.4 kcal/mol，释放的自由能约为 8.0 kcal/mol。Gln92 作为关键残留物控制 CO2 向活性位点的运输和 HCO3- 的释放。CA 中未质子化的 His64 更倾向于向内，而外部构象对其质子化对应物在能量上是有利的。His64 在向内和向外之间的构象转变与其质子化状态相关，这是由质子转移和产物释放介导的。整个酶促循环的初始质子转移步骤的自由能跨度为 10.4 kcal/mol，自由能变化为 -6.5 kcal/mol。机理细节提供了对 CO2 向碳酸氢盐的整个可逆转化以及 CO2 酶促水合的化学和非化学步骤中关键残留物的作用的全面理解。这是由质子转移和产物释放介导的。整个酶促循环的初始质子转移步骤的自由能跨度为 10.4 kcal/mol，自由能变化为 -6.5 kcal/mol。机理细节提供了对 CO2 向碳酸氢盐的整个可逆转化以及 CO2 酶促水合的化学和非化学步骤中关键残留物的作用的全面理解。这是由质子转移和产物释放介导的。整个酶促循环的初始质子转移步骤的自由能跨度为 10.4 kcal/mol，自由能变化为 -6.5 kcal/mol。机理细节提供了对 CO2 向碳酸氢盐的整个可逆转化以及 CO2 酶促水合的化学和非化学步骤中关键残留物的作用的全面理解。