Acetylcholinesterase (AChE) is the crucial enzyme in the central nervous system. It is the target of various organophosphorus nerve agents and pesticides, and the inhibition of AChE is a therapeutic strategy for the treatment of various neurological-related diseases. The Glu202 is a key residue adjacent to the catalytic His447 and plays important role in catalysis. Although the Glu202 has long been considered as negatively charged in many studies, more and more evidences support a protonated Glu202. However, Glu202 is freely accessible by solvent, and thus it seems more reasonable for Glu202 to majorly take the deprotonated state. In the present work, we carried out a series of molecular dynamics simulations with the Glu202 adopting different protonation states. Our results show that the protonated Glu202 is important in maintaining the key hydrogen bond network that supports the catalytic triad, whereas the deprotonated Glu202 results in the collapse of the key hydrogen bond network which consequently destabilizes the catalytic His447. We also notice that different protonation states of Glu202 merely alters the binding mode of ACh. However, since the catalytic His447 is disrupted if Glu202 is deprotonated, His447 cannot facilitate the nucleophilic attack performed by Ser203. Therefore, the catalytic efficiency of ACh hydrolysis should be remarkably decreased if Glu202 is deprotonated. Our findings suggest that, when designing and developing highly active AChE inhibitors or proposing mechanistic hypotheses for AChE-catalyzed reactions, the protonated state of Glu202 should be considered.

中文翻译：

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Glu202 在乙酰胆碱酯酶中的质子化状态

乙酰胆碱酯酶 (AChE) 是中枢神经系统中的关键酶。它是各种有机磷神经毒剂和农药的靶点，抑制AChE是治疗各种神经相关疾病的治疗策略。Glu202 是与催化 His447 相邻的关键残基，在催化中起重要作用。尽管 Glu202 在许多研究中一直被认为是带负电荷的，但越来越多的证据支持质子化的 Glu202。然而，Glu202 可以通过溶剂自由接触，因此 Glu202 主要采取去质子化状态似乎更合理。在目前的工作中，我们对采用不同质子化状态的 Glu202 进行了一系列分子动力学模拟。我们的研究结果表明，质子化的 Glu2O2 对于维持支持催化三联体的关键氢键网络很重要，而去质子化的 Glu2O2 会导致关键氢键网络的崩溃，从而使催化 His447 不稳定。我们还注意到 Glu202 的不同质子化状态仅仅改变了 ACh 的结合模式。然而，如果 Glu202 被去质子化，则由于催化 His447 被破坏，因此 His447 不能促进 Ser203 进行的亲核攻击。因此，如果 Glu2O2 去质子化，ACh 水解的催化效率会显着降低。我们的研究结果表明，在设计和开发高活性 AChE 抑制剂或提出 AChE 催化反应的机制假设时，应考虑 Glu202 的质子化状态。