How Monoamine Oxidase A Decomposes Serotonin: An Empirical Valence Bond Simulation of the Reactive Step

Abstract

The enzyme-catalyzed degradation of the biogenic amine serotonin is an essential regulatory mechanism of its level in the human organism. In particular, monoamine oxidase A (MAO A) is an important flavoenzyme involved in the metabolism of monoamine neurotransmitters. Despite extensive research efforts, neither the catalytic nor the inhibition mechanisms of MAO enzymes are currently fully understood. In this article, we present the quantum mechanics/molecular mechanics simulation of the rate-limiting step for the serotonin decomposition, which consists of hydride transfer from the serotonin methylene group to the N5 atom of the flavin moiety. Free-energy profiles of the reaction were computed by the empirical valence bond method. Apart from the enzymatic environment, the reference reaction in the gas phase was also simulated, facilitating the estimation of the catalytic effect of the enzyme. The calculated barrier for the enzyme-catalyzed reaction of 14.82 ± 0.81 kcal mol^-1 is in good agreement with the experimental value of 16.0 kcal mol^-1, which provides strong evidence for the validity of the proposed hydride-transfer mechanism. Together with additional experimental and computational work, the results presented herein contribute to a deeper understanding of the catalytic mechanism of MAO A and flavoenzymes in general, and in the long run, they should pave the way toward applications in neuropsychiatry.

Figure 1

Mechanism of the rate-limiting step of MAO A-catalyzed serotonin decomposition, involving hydride transfer from the serotonin methylene group to the N5 atom of the FAD cofactor. Please note that the intermediate formally consists of two ionic species and the reaction in this respect is strongly dependent on the polar environment. It is worth to emphasize that in the following reactions, which do not represent the rate-limiting steps, the formed imine is deprotonated by FADH^– and hydrolyzed, while the reduced flavin is reoxidized back to FAD.

Figure 2

Structure of MAO A with serotonin in the active site. The flavin and serotonin moieties are represented using colored sticks, serotonin carbon atoms are depicted in green, and flavin carbon atoms are depicted in black.

Figure 3

Reactant (left), transition-state (middle), and product (right) structures of the MAO A active site with the reacting serotonin molecule. The serotonin moiety is denoted by SRO and the flavin moiety by FAD. Serotonin carbon atoms are depicted in green, and flavin carbon atoms are depicted in black. Please note that in the transition state, the transferring hydride ion is located about halfway between the reactive carbon Cα atom of serotonin and the flavin N5 atom. The averaged distances between the reactive carbon Cα atom and the flavin N5 atom are 3.01, 2.67, and 3.21 Å for the reactants, transition state, and products, respectively.

Figure 4

Reaction profiles for the decomposition of neutral serotonin. The gas-phase profiles are depicted in black, while the MAO A-catalyzed profiles are in red. The reaction coordinate is defined as the energy difference between EVB states 2 and 1 and is commonly used in displaying EVB free-energy profiles.