Chloroperoxidase-Catalyzed Epoxidation of Cis-β-Methylstyrene: NH-S Hydrogen Bonds and Proximal Helix Dipole Change the Catalytic Mechanism and Significantly Lower the Reaction Barrier
Chloroperoxidase-Catalyzed Epoxidation of Cis-β-Methylstyrene: NH-S Hydrogen Bonds and Proximal Helix Dipole Change the Catalytic Mechanism and Significantly Lower the Reaction Barrier
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DOI：
10.1021/acs.jpcb.5b06731
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发表时间：
2015-11-12
影响因子：
3.3
通讯作者：
Chatfield, David C.
中科院分区：
文献类型：
--
作者：
Morozov, Alexander N.;Pardillo, Armando D.;Chatfield, David C.
轴向硫与骨架酰胺（NH-S）的近端氢键是血红素硫醇化酶如氯过氧化物酶（CPO）和细胞色素P450（P450）的保守特征。在CPO中，NH-S键的作用被近端螺旋的偶极矩放大。我们的气相DFT研究表明，近口袋效应显着提高CPO的反应性对烯烃底物的环氧化。有和没有近端口袋残基的模型的比较表明，有了它们，C-beta-O键形成的屏障降低了约4.6 kcal/mol，而C-alpha-O-C-beta环闭合变得无屏障。近端螺旋的偶极矩估计贡献了1/3的减少，而其余部分归因于NH-S键的影响。反应屏障的降低与向近端口袋的残基转移的电子密度增加相关。这种效应在二重态自旋表面上最为明显，并涉及电子转移机制的变化。一个完整的酶QMMM研究的二重态自旋表面给出了相同的障碍，气相DFT研究。自由能垒估计是与实验结果一致的CPO催化苯乙烯环氧化反应。
Proximal hydrogen bonding of the axial sulfur with the backbone amides (NH-S) is a conserved feature of heme-thiolate enzymes such as chloroperoxidase (CPO) and cytochrome P450 (P450). In CPO, the effect of NH-S bonds is amplified by the dipole moment of the proximal helix. Our gas-phase DFT studies show that the proximal pocket effect significantly enhances CPO's reactivity toward the epoxidation of olefmic substrates. Comparison of models with and without proximal pocket residues shows that with them, the barrier for C-beta-O bond formation is lowered by about similar to 4.6 kcal/mol, while C-alpha-O-C-beta ring closure becomes barrierless. The dipole moment of the proximal helix was estimated to contribute 1/3 of the decrease, while the rest is attributed to the effect of NH-S bonds. The decrease of the reaction barrier correlates with increased electron density transfer to residues of the proximal pocket. The effect is most pronounced on the doublet spin surface and involves a change in the electron-transfer mechanism. A full enzyme QMMM study on the doublet spin surface gives about the same barrier as the gas-phase DFT study. The free-energy barrier was estimated to be in agreement with the experimental results for the CPO-catalyzed epoxidation of styrene.