Is a "proton wire" concerted or stepwise? A model study of proton transfer in carbonic anhydrase
Is a "proton wire" concerted or stepwise? A model study of proton transfer in carbonic anhydrase
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DOI：
10.1021/jp021931v
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发表时间：
2003-01-30
影响因子：
3.3
通讯作者：
Karplus, M
中科院分区：
文献类型：
--
作者：
Cui, Q;Karplus, M
碳酸酐酶 (CA) 中质子转移反应的能量学已通过活性位点模型进行了研究。具体来说，研究了由多个水分子介导的从锌结合水分子到组氨酸残基的质子转移。当有两个或三个桥接水分子时，质子转移完全或几乎完全一致，并且仅存在一个鞍点。当额外的水分子形成环桥时，会形成一种中间体，其中一个水分子以水合氢离子的形式存在。与以前采用低水平理论或假设逐步机制的计算相反，从当前工作中获得的能量与实验估计大致一致。在所有情况下，过渡态都涉及多个质子的运动，这与实验观察结果一致，即 H2O/D2O 混合物中的反应速率对溶剂中 D2O 的分数呈指数依赖性。对于三个（W3）或四个水（W4），质子转移到“His 64”模型几乎不涉及过渡态，这表明质子受体的方向不如仅两个水（W2）重要。因此，W3和W4结果与实验观察一致，即CA的H64A突变体在良好缓冲的咪唑溶液中的许多动力学特性与野生型相似。随着桥接水分子数量的增加，势垒高度增加，势垒频率（以及因此隧道效应的贡献）减少。总的来说，这些研究表明CA中的质子转移反应对水桥的性质和结构敏感，这会受到蛋白质活性位点中水分子和氨基酸动力学的影响。
The energetics of proton transfer reactions in carbonic anhydrase (CA) have been studied with an active site model. Specifically, proton transfer from a zinc-bound water molecule to a histidine residue mediated by a numbers of water molecules was investigated. With two or three bridging water molecules, the proton transfers are fully or nearly fully concerted and only one saddle point exists. With an additional water molecule that forms a ring bridge, an intermediate is formed in which one of the water molecules exists as a hydronium ion. In contrast to previous calculations in which either a low-level of theory was employed or a stepwise mechanism was assumed, the energetics obtained from the current work are approximately consistent with the experimental estimates. In all of the scenarios, the motion of more than one proton is involved in the transition state, which is in agreement with the experimental observation that the reaction rates in H2O/D2O mixture have an exponential dependence on the fraction of D2O in the solvent. For three (W3) or four waters (W4), the proton transfer to the "His 64" model is hardly involved in the transition state, suggesting that the orientation of the proton acceptor is less important than for only two waters (W2). Thus, the W3 and W4 results are consistent with the experimental observation that many kinetic properties of the H64A mutant of CA in well-buffered imidazole solution are similar to the wild type. The barrier height increases, and the barrier frequency (and therefore, the contribution of tunneling) decreases as the number of bridging water molecules increases. Overall, these investigations demonstrate that the proton transfer reaction in CA is sensitive to the nature and structure of the water bridge, which would be influenced by the dynamics of the water molecules and amino acids in the active site of the protein.