Oxidation reaction by xanthine oxidase. Theoretical study of reaction mechanism
Oxidation reaction by xanthine oxidase. Theoretical study of reaction mechanism
复制标题
DOI：
10.1021/ja068584d
复制
发表时间：
2007-07-04
影响因子：
15
通讯作者：
Sakaki, Shigeyoshi
中科院分区：
文献类型：
--
作者：
Amano, Tatsuo;Ochi, Noriaki;Sakaki, Shigeyoshi
利用代表反应中心的模型复合物和典型的基准底物甲酰胺，从理论上研究了含钼酶（黄嘌呤氧化酶）的氧化过程。对之前提出的反应机制进行了系统的比较。在先前理论上讨论的协同和逐步机制中，氧化反应在适度的活化势垒下发生。然而，产物的稳定性不如反应物复合物，这表明这些机制不太可能。而且，协同机制的产物与同位素实验结果并不一致。除了这些机制之外，这里还新研究了由活性位点去质子化引发的另一种机制。在该反应的过渡态中，甲酰胺的碳原子与Mo中心的氧代配体相互作用，并且氢原子从碳原子移动到硫代配体。该反应在适度的活化势垒和相当大的放热下发生。此外，该机理的产物与同位素实验结果一致。此外，我们的计算清楚地表明，在谷氨酸和底物存在的情况下，活性位点的去质子化会发生相当大的放热。在去质子化活性位点的情况下，逐步机制的中间体无法优化。从所有这些结果来看，应该得出结论，具有去质子化活性位点的一步机制是最合理的。
The oxidation process by molybdenum-containing enzyme, xanthine oxidase, is theoretically studied with a model complex representing the reaction center and a typical benchmark substrate, formamide. Comparisons were systematically made among reaction mechanisms proposed previously. In the concerted and stepwise mechanisms that were theoretically discussed previously, the oxidation reaction takes place with a moderate activation barrier. However, the product is less stable than the reactant complex, which indicates that these mechanisms are unlikely. Moreover, the product of the concerted mechanism is not consistent with the isotope experimental result. In addition to those mechanisms, another mechanism initiated by the deprotonation of the active site was newly investigated here. In the transition state of this reaction, the carbon atom of formamide interacts with the oxo ligand of the Mo center and the hydrogen atom is moving from the carbon atom to the thioxo ligand. This reaction takes place with a moderate activation barrier and considerably large exothermicity. Furthermore, the product by this mechanism is consistent with the isotope experimental result. Also, our computations clearly show that the deprotonation of the active site occurs with considerable exothermicity in the presence of glutamic acid and substrate. The intermediate of the stepwise mechanism could not be optimized in the case of the deprotonated active site. From all these results, it should be concluded that the one-step mechanism with the deprotonated active site is the most plausible.