Substrate-dependent H/D kinetic isotope effects and the role of the di(μ-oxo)diiron(IV) core in soluble methane monooxygenase: a theoretical study

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.