Evidence for alternative binding modes in the interaction of benzylamine analogues with bovine liver monoamine oxidase B

Alkaloids of the Cactaceae — The Classics

Alkaloids of the Cactaceae have been studied for the last 120 years. The first half of that period provided the “classic” compounds, after which a large number of usually very similar analogs were isolated or determined with modern methods. Although some unusual synthetic approaches have been developed, their preparation is generally quite straightforward. Their biosynthesis has been studied but, particularly in the case of the isoquinoline compounds, important aspects have not been addressed. Due to its striking effects, the pharmacology of mescaline has been studied more intensely than that of the other phenethylamines present in cacti, followed only by hordenine. The many 1,2,3,4-tetrahydroisoquinoline alkaloids have attracted much less interest and have often been considered practically inactive. Nevertheless, some recorded activities of this group of compounds suggests a need for additional studies, especially in connection with their co-administration with mescaline, as in dried cacti and in beverages prepared from them.

Why p-OMe- and p-Cl-β-Methylphenethylamines Display Distinct Activities upon MAO-B Binding

Despite their structural and chemical commonalities, p-chloro-β-methylphenethylamine and p-methoxy-β-methylphenethylamine display distinct inhibitory and substrate activities upon MAO-B binding. Density Functional Theory (DFT) quantum chemical calculations reveal that β-methylation and para-substitution underpin the observed activities sustained by calculated transition state energy barriers, attained conformations and key differences in their interactions in the enzyme’s substrate binding site. Although both compounds meet substrate requirements, it is clear that β-methylation along with the physicochemical features of the para-substituents on the aromatic ring determine the activity of these compounds upon binding to the MAO B-isoform. While data for a larger set of compounds might lend generality to our conclusions, our experimental and theoretical results strongly suggest that the contrasting activities displayed depend on the conformations adopted by these compounds when they bind to the enzyme.

Computational Study of the pKa Values of Potential Catalytic Residues in the Active Site of Monoamine Oxidase B

Monoamine oxidase (MAO), which exists in two isozymic forms, MAO A and MAO B, is an important flavoenzyme responsible for the metabolism of amine neurotransmitters such as dopamine, serotonin, and norepinephrine. Despite extensive research effort, neither the catalytic nor the inhibition mechanisms of MAO have been completely understood. There has also been dispute with regard to the protonation state of the substrate upon entering the active site, as well as the identity of residues that are important for the initial deprotonation of irreversible acetylenic inhibitors, in accordance with the recently proposed mechanism. Therefore, in order to investigate features essential for the modes of action of MAO, we have calculated pKa values of three relevant tyrosine residues in the MAO B active site, with and without dopamine bound as the substrate (as well as the pKa of the dopamine itself in the active site). The calculated pKa values for Tyr188, Tyr398, and Tyr435 in the complex are found to be shifted upward to 13.0, 13.7, and 14.7, respectively, relative to 10.1 in aqueous solution, ruling out the likelihood that they are viable proton acceptors. The altered tyrosine pKa values could be rationalized as an interplay of two opposing effects: insertion of positively charged bulky dopamine that lowers tyrosine pKa values, and subsequent removal of water molecules from the active site that elevates tyrosine pKa values, in which the latter prevails. Additionally, the pKa value of the bound dopamine (8.8) is practically unchanged compared to the corresponding value in aqueous solution (8.9), as would be expected from a charged amine placed in a hydrophobic active site consisting of aromatic moieties. We also observed potentially favorable cation-π interactions between the -NH3(+) group on dopamine and aromatic moieties, which provide a stabilizing effect to the charged fragment. Thus, we offer here theoretical evidence that the amine is most likely to be present in the active site in its protonated form, which is similar to the conclusion from experimental studies of MAO A (Jones et al. J. Neural Trans.2007, 114, 707-712). However, the free energy cost of transferring the proton from the substrate to the bulk solvent is only 1.9 kcal mol(-1), leaving open the possibility that the amine enters the chemical step in its neutral form. In conjunction with additional experimental and computational work, the data presented here should lead toward a deeper understanding of mechanisms of the catalytic activity and irreversible inhibition of MAO B, which can allow for the design of novel and improved MAO B inhibitors.

The pH dependence of kinetic isotope effects in monoamine oxidase A indicates stabilization of the neutral amine in the enzyme-substrate complex

A common feature of all the proposed mechanisms for monoamine oxidase is the initiation of catalysis with the deprotonated form of the amine substrate in the enzyme-substrate complex. However, recent steady-state kinetic studies on the pH dependence of monoamine oxidase led to the suggestion that it is the protonated form of the amine substrate that binds to the enzyme. To investigate this further, the pH dependence of monoamine oxidase A was characterized by both steady-state and stopped-flow techniques with protiated and deuterated substrates. For all substrates used, there is a macroscopic ionization in the enzyme-substrate complex attributed to a deprotonation event required for optimal catalysis with a pK(a) of 7.4-8.4. In stopped-flow assays, the pH dependence of the kinetic isotope effect decreases from approximately 13 to 8 with increasing pH, leading to assignment of this catalytically important deprotonation to that of the bound amine substrate. The acid limb of the bell-shaped pH profile for the rate of flavin reduction over the substrate binding constant (k(red)/K(s), reporting on ionizations in the free enzyme and/or free substrate) is due to deprotonation of the free substrate, and the alkaline limb is due to unfavourable deprotonation of an unknown group on the enzyme at high pH. The pK(a) of the free amine is above 9.3 for all substrates, and is greatly perturbed (DeltapK(a) approximately 2) on binding to the enzyme active site. This perturbation of the substrate amine pK(a) on binding to the enzyme has been observed with other amine oxidases, and likely identifies a common mechanism for increasing the effective concentration of the neutral form of the substrate in the enzyme-substrate complex, thus enabling efficient functioning of these enzymes at physiologically relevant pH.

Structural insights into the mechanism of amine oxidation by monoamine oxidases A and B

Due to their pharmacological importance in the oxidation of amine neurotransmitters, the membrane-bound flavoenzymes monoamine oxidase A and monoamine oxidase B have attracted numerous investigations and, as a result, two different mechanisms; the single electron transfer and the polar nucleophilic mechanisms, have been proposed to describe their catalytic mechanisms. This review compiles the recently available structural data on both enzymes with available mechanistic data as well as current NMR data on flavin systems to provide an integration of the approaches. These conclusions support the proposal that a polar nucleophilic mechanism for amine oxidation is the most consistent mechanistic scheme as compared with the single electron transfer mechanism.

Human and rat monoamine oxidase-A are differentially inhibited by (S)-4-alkylthioamphetamine derivatives: insights from molecular modeling studies

Four enantiomerically pure (S)-4-alkylthioamphetamine derivatives were evaluated as monoamine oxidase (MAO) inhibitors using the human and rat isoforms of the enzyme. Molecular dockings were performed in order to gain insights regarding the binding mode of these inhibitors. All compounds were potent and selective MAO-A inhibitors although different rank orders of potencies were observed against the enzymes from different species. This behavior can be rationalized on the basis of different binding modes to each enzyme, as determined in silico. These findings further support the concept that MAO inhibitory activity of novel compounds, determined with enzymes from diverse mammalian species, should be considered with caution if human MAO is the final target to be addressed.

Docking of novel reversible monoamine oxidase-B inhibitors: efficient prediction of ligand binding sites and estimation of inhibitors thermodynamic properties

Monoamine oxidase (MAO, EC 1.4.3.4) is a flavoenzyme bound to the mitochondrial outer membranes of the cells, which is responsible for the oxidative deamination of neurotransmitter and dietary amines. It has two distinct isozymic forms, designated MAO-A and MAO-B, each displaying different substrate and inhibitor specificities. They are the well-known target for antidepressant, Parkinson's disease and neuroprotective drugs. Elucidation of the x-ray crystallographic structure of MAO-B has opened the way for molecular modeling studies. In this research 12 reversible and MAO-B selective inhibitors have been docked computationally to the active site of the MAO-B enzyme. AutoDock 3.0.5 was employed to perform the automated molecular docking. The result of docking studies generated thermodynamic properties, such as free energy of bindings (DeltaG(b)) and inhibition constants (K (i)) for the inhibitors. Moreover, 3D pictures of inhibitor-enzyme complexes afforded valuable data regarding the binding orientation of each inhibitor in the active site of MAO-B.

A computational study on the amine-oxidation mechanism of monoamine oxidase: Insight into the polar nucleophilic mechanism

The proposed polar nucleophilic mechanism of MAO was investigated using quantum chemical calculations employing the semi-empirical PM3 method. In order to mimic the reaction at the enzyme's active site, the reactions between the flavin and the p-substituted benzylamine substrate analogs were modeled. Activation energies and rate constants of all the reactions were calculated and compared with the published experimental data. The results showed that electron-withdrawing groups at the para position of benzylamine increase the reaction rate. A good correlation between the log of the calculated rate constants and the electronic parameter (sigma) of the substituent was obtained. These results agree with the previous kinetic experiments on the effect of p-substituents on the reduction of MAO-A by benzylamine analogs. In addition, the calculated rate constants showed a correlation with the rate of reduction of the flavin in MAO-A. In order to verify the results obtained from the PM3 method single-point B3LYP/6-31G*//PM3 calculations were performed. These results demonstrated a strong reduction in the activation energy for the reaction of benzylamine derivatives having electron-withdrawing substituents, which is in agreement with the PM3 calculations and the previous experimental QSAR study. PM3 and B3LYP/6-31G* energy surfaces were obtained for the overall reaction of benzylamine with flavin. Results suggest that PM3 is a reasonable method for studying this kind of reaction. These theoretical findings support the proposed polar nucleophilic mechanism for MAO-A.

Interactions of D-amphetamine with the active site of monoamine oxidase-A

Reversible monoamine oxidase A inhibitors (RIMA) are used as antidepressants but little is known about how they interact with the active site of the enzyme. Heterologous expression of human liver MAO-A in yeast provides sufficient protein for molecular studies and direct observation of the changes in the spectrum of the FAD co-factor when inhibitors bind. Using the reversible inhibitor, D-amphetamine, as a model compound, a concentration-dependent change in the spectrum with clean isosbestic points was observed. The decrease in absorbance between 400 and 500 nm gave a dissociation constant for binding similar to the K(i) value. Anaerobic reduction yielded the semiquinone spectrum only and the midpoint potential was the same as the free enzyme. Full reduction was not possible with dithionite as the reductant, suggesting that the semiquinone-reduced couple had a much lower midpoint potential than the free enzyme. In contrast, with substrate, which reduces the enzyme on an equimolar basis, the semiquinone is never seen. In anaerobic stopped-flow experiments, amphetamine inhibits completely the reoxidation of the reduced enzyme in contrast to a substrate such as 2-phenylethylamine (the desmethyl analogue of amphetamine) that accelerates the rate 12-fold. The spectral changes in MAO-A permit the examination of inhibitor interaction with the redox co-factor. Stacking of the inhibitor and flavin rings constitutes part of the interaction but, taking into account other evidence, steric factors may be the clue to the differences between substrate and inhibitor.