Mai Binh Khanh, Kim Yongho
Department of Applied Chemistry, Kyung Hee University, 1 Seochun-Dong, Giheung-Gu, Yongin-Si, Gyeonggi-Do, 446-701 (Korea), Fax: (+82) 31-203-5773.
Chemistry. 2014 May 19;20(21):6532-41. doi: 10.1002/chem.201303466. Epub 2014 Apr 8.
Soluble methane monooxygenase (sMMO) is an enzyme that converts alkanes to alcohols using a di(μ-oxo)diiron(IV) intermediate Q at the active site. Very large kinetic isotope effects (KIEs) indicative of significant tunneling are observed for the hydrogen transfer (H-transfer) of CH4 and CH3 CN; however, a relatively small KIE is observed for CH3NO2. The detailed mechanism of the enzymatic H-transfer responsible for the diverse range of KIEs is not yet fully understood. In this study, variational transition-state theory including the multidimensional tunneling approximation is used to calculate rate constants to predict KIEs based on the quantum-mechanically generated intrinsic reaction coordinates of the H-transfer by the di(μ-oxo)diiron(IV) complex. The results of our study reveal that the role of the di(μ-oxo)diiron(IV) core and the H-transfer mechanism are dependent on the substrate. For CH4 , substrate binding induces an electron transfer from the oxygen to one Fe(IV) center, which in turn makes the μ-O ligand more electrophilic and assists the H-transfer by abstracting an electron from the C-H σ orbital. For CH3CN, the reduction of Fe(IV) to Fe(III) occurs gradually with substrate binding and H-transfer. The charge density and electrophilicity of the μ-O ligand hardly change upon substrate binding; however, for CH3NO2, there seems to be no electron movement from μ-O to Fe(IV) during the H-transfer. Thus, the μ-O ligand appears to abstract a proton without an electron from the C-H σ orbital. The calculated KIEs for CH4, CH3CN, and CH3NO2 are 24.4, 49.0, and 8.27, respectively, at 293 K, in remarkably good agreement with the experimental values. This study reveals that diverse KIE values originate mainly from tunneling to the same di(μ-oxo)diiron(IV) core for all substrates, and demonstrate that the reaction dynamics are essential for reproducing experimental results and understanding the role of the diiron core for methane oxidation in sMMO.
可溶性甲烷单加氧酶（sMMO）是一种利用活性位点的双（μ-氧代）双铁（IV）中间体Q将烷烃转化为醇的酶。对于CH4和CH3CN的氢转移（H-转移），观察到非常大的动力学同位素效应（KIEs），表明存在显著的隧穿现象；然而，对于CH3NO2，观察到的KIE相对较小。导致不同KIE范围的酶促H-转移的详细机制尚未完全理解。在本研究中，基于双（μ-氧代）双铁（IV）配合物的H-转移的量子力学生成的本征反应坐标，使用包括多维隧穿近似的变分过渡态理论来计算速率常数以预测KIEs。我们的研究结果表明，双（μ-氧代）双铁（IV）核心的作用和H-转移机制取决于底物。对于CH4，底物结合诱导电子从氧转移到一个Fe（IV）中心，这反过来使μ-O配体更具亲电性，并通过从C-H σ轨道夺取一个电子来协助H-转移。对于CH3CN，随着底物结合和H-转移，Fe（IV）逐渐还原为Fe（III）。底物结合后，μ-O配体的电荷密度和亲电性几乎没有变化；然而，对于CH3NO2，在H-转移过程中似乎没有电子从μ-O转移到Fe（IV）。因此，μ-O配体似乎从C-H σ轨道夺取一个没有电子的质子。在293 K时，CH4、CH3CN和CH3NO2的计算KIEs分别为24.4、49. 和8.27，与实验值非常吻合。本研究表明，不同的KIE值主要源于所有底物对同一个双（μ-氧代）双铁（IV）核心的隧穿，并证明反应动力学对于重现实验结果和理解双铁核心在sMMO中甲烷氧化的作用至关重要。
