Reaction Mechanism of Monoamine Oxidase from QM/MM Calculations

Abstract

The flavoenzyme monoamine oxidase (MAO) is essential for the enzymatic decomposition of neurotransmitters. While it is commonly accepted that the rate limiting step of the reaction is the stereoselective abstraction of a hydrogen from the substrate, the precise mechanism is unknown. We modeled the reaction of human MAO-B with benzylamine by means of QM/MM calculations based on density functional theory. Oxidation of the unprotonated substrate was found to proceed with rates in good agreement with experimental values, while the protonated substrate does not react at room temperature. Our results support a concerted asynchronous polar nucleophilic mechanism. The lone pair of the amine-nitrogen interacts with a carbon atom of the flavin cofactor. During the reaction, this lone pair, as well as a proton, are transferred to the cofactor. Analysis of the electronic structure during the reaction rules out a radical mechanism.

Revealing Monoamine Oxidase B Catalytic Mechanisms by Means of the Quantum Chemical Cluster Approach

“…10 Thus, only NBA displays this interaction at too large a distance for a covalent bond but close enough to be considered as an attractive interaction. 11 Also, it is possible to observe that the interaction occurring in the Lys296-HOH-N5 motif is strengthened in the NBA transition state (Table S1). We believe that the conserved water molecule may partially compensate 19 the increased basicity of the flavin N5 as described for the PN mechanism in MAO (Scheme 1b).…”

“…The basic N5 of this adduct eventually abstracts the Cα-Hα from the substrate (Scheme 1a). 8 Abad et al, 11 using benzylamine as the MAO B substrate, described an asynchronous PN mechanism (lower pathway in Scheme 1a). This mechanism describes the amine moiety's lone pair (N am ) interaction with C4a of the isoalloxazine ring at a rather short distance (ca.…”

“…8 Despite the structural resemblance of both active sites, the electrostatic environment in MAO A and MAO B should not be identical, allowing for different degrees of charge transfer in the reactant complexes upon formation. In this context, Abad et al 11 described that MAO A performs the oxidative deamination of amines by a PN mechanism in contrast to the HT used by MAO B. The same authors 24 have recently claimed that the reaction rate is influenced by the N am -C4a interaction, and substituent effects on substrates only play a minor role, indicating that active site environmental effects bring the pathway closer to an asynchronous PN mechanism in MAO B.…”

“…10 The PN and HT mechanisms have been widely discussed in order to shed light on the nature of the experimental rate-determining step. For instance, Orru et al 8 addressed the dilemma regarding the same or different mechanisms for both MAO isoforms; Abad et al 11 used computational chemistry to favour the PN mechanism; Vianello et al 7 were the first to show computationally the prevailing feasibility of the HT over all other mechanisms, suggesting that this two-step hydride transfer mechanism might operate in MAO-B. However, as stated by the authors considering the structural resemblances of both isoforms, i.e.…”

Molecular aspects of monoamine oxidase B

“…Using different methods, two computational approaches support either the polar nucleophilic (Abad et al, 2013) or the hydride ion transfer (Vianello et al, 2012, Repic et al, 2014. The hydride ion transfer mechanism gives the lowest activation energy making it the most likely pathway, but better experimental evidence for the transition states for each calculated mechanism would be useful.…”