Deciphering the Two-Step Hydride Mechanism of Monoamine Oxidase Flavoenzymes

The complete two-step hydride transfer mechanism of amine oxidation involved in the metabolism of monoamine neurotransmitters was scrutinized by DFT calculations. In living organisms, this process is catalyzed by monoamine oxidase enzymes. Herein, we focus on some intriguing aspects of the reaction that may have been previously noticed but have not been clarified to date. The first step of the reaction includes the C–H bond cleavage on the methylene group vicinal to the amino group of the monoamine substrate and the subsequent transfer of hydrogen to the N5 atom of the flavin prosthetic group of the enzyme. We confirmed the nature of this step to be hydride transfer by evaluation of the pertinent HOMO–LUMO gap together with analysis of orbital contours alongside the intrinsic reaction coordinate profile. Next, we investigated the rather peculiar intermediate adduct that may form between the amine substrate and the flavin molecule, featuring an unusually long C–N bond of ∼1.62 Å. Although this bond is quite stable in the gas phase, the presence of just a few explicit water molecules facilitates its dissociation almost without energy input so that the amine-flavin intermediate can form an ionic pair instead. We attribute the existence of the unusual C–N bond to a fragile balance between opposing electronic structure effects, as evaluated by the natural bond orbital analysis. In line with this, the intermediate in the solution or in the enzyme active site can exist in two energetically almost equivalent forms, namely, as a covalently bound complex or as an ion pair, as suggested by previous studies. Finally, we characterized the transformation of the intermediate to the fully reduced flavin and imine products via proton transfer from the amino group to the flavin N1 atom, completing the reductive part of the catalytic cycle. Although we found that explicit solvation substantially boosts the kinetics of this step, the corresponding barrier is significantly lower than that in the hydride transfer step, confirming hydrogen abstraction as the rate-limiting step of amine oxidation and validating the two-step hydride transfer mechanism of monoamine oxidases.