Loss of serotonin oxidation as a component of the altered substrate specificity in the Y444F mutant of recombinant human liver MAO A

Brunner syndrome caused by point mutation explained by multiscale simulation of enzyme reaction

Brunner syndrome is a disorder characterized by intellectual disability and impulsive, aggressive behavior associated with deficient function of the monoamine oxidase A (MAO-A) enzyme. These symptoms (along with particularly high serotonin levels) have been reported in patients with two missense variants in MAO-A (p.R45W and p.E446K). Herein, we report molecular simulations of the rate-limiting step of MAO-A-catalyzed serotonin degradation for these variants. We found that the R45W mutation causes a 6000-fold slowdown of enzymatic function, whereas the E446K mutation causes a 450-fold reduction of serotonin degradation rate, both of which are practically equivalent to a gene knockout. In addition, we thoroughly compared the influence of enzyme electrostatics on the catalytic function of both the wild type MAO-A and the p.R45W variant relative to the wild type enzyme, revealing that the mutation represents a significant electrostatic perturbation that contributes to the barrier increase. Understanding genetic disorders is closely linked to understanding the associated chemical mechanisms, and our research represents a novel attempt to bridge the gap between clinical genetics and the underlying chemical physics.

Monoamine oxidase inhibitors: A concise review with special emphasis on structure activity relationship studies

Monoamine oxidase enzyme is necessary for the management of brain functions. It oxidatively metabolizes monoamines and produces ammonia, aldehyde and hydrogen peroxide as by-products. Excessive production of by-products of monoamine metabolism generates free radicals which cause cellular apoptosis and several neurodegenerative disorders for example Alzheimer's disease, Parkinson's disease, depression and autism. The inhibition of MAOs is an attractive target for the treatment of neurological disorders. Clinically approved MAO inhibitors for example selegiline, rasagiline, clorgyline, pargyline etc. are irreversible in nature and cause some adverse effects while recently studied reversible MAO inhibitors are devoid of harmful effects of old monoamine oxidase inhibitors. In this review article we have listed various synthesized molecules containing different moieties like coumarin, chalcone, thiazole, thiourea, caffeine, pyrazole, chromone etc. along with their activity, mode of action, structure activity relationship and molecular docking studies.

High-Throughput Screening and Molecular Dynamics Simulation of Natural Product-like Compounds against Alzheimer's Disease through Multitarget Approach

Alzheimer’s disease (AD) is a progressive neurological disorder that affects 50 million people. Despite this, only two classes of medication have been approved by the FDA. Therefore, we have planned to develop therapeutics by multitarget approach. We have explored the library of 2029 natural product-like compounds for their multi-targeting potential against AD by inhibiting AChE, BChE (cholinergic pathway) MAO-A, and MOA-B (oxidative stress pathway) through in silico high-throughput screening and molecular dynamics simulation. Based on the binding energy of these target enzymes, approximately 189 compounds exhibited a score of less than −10 kcal/mol against all targets. However, none of the control inhibitors exhibited a binding affinity of less than −10 kcal/mol. Among these, the top 10 hits of compounds against all four targets were selected for ADME-T analysis. As a result, only F0850-4777 exhibited an acceptable range of physicochemical properties, drug-likeness, pharmacokinetics, and suitability for BBB permeation with high GI-A and non-toxic effects. The molecular dynamics study confirmed that F0850-4777 remained inside the binding cavity of targets in a stable conformation throughout the simulation and Prime-MM/GBSA study revealed that van der Waals’ energy (ΔG_vdW) and non-polar solvation or lipophilic energy (ΔG_{Sol_Lipo}) contribute favorably towards the formation of a stable protein–ligand complex. Thus, F0850-4777 could be a potential candidate against multiple targets of two pathophysiological pathways of AD and opens the doors for further confirmation through in vitro and in vivo systems.

Monoamine Oxidase Inhibitors: From Classic to New Clinical Approaches

Monoamine oxidases (MAOs) are involved in the oxidative deamination of different amines and neurotransmitters. This pointed them as potential targets for several disorders and along the last 70 years a wide variety of MAO inhibitors have been developed as successful drugs for the treatment of complex diseases, being the first drugs approved for depression in the late 1950s. The discovery of two MAO isozymes (MAO-A and B) with different substrate selectivity and tissue expression patterns led to novel therapeutic approaches and to the development of new classes of inhibitors, such as selective irreversible and reversible MAO-B inhibitors and reversible MAO-A inhibitors. Significantly, MAO-B inhibitors constitute a widely studied group of compounds, some of them approved for the treatment of Parkinson's disease. Further applications are under development for the treatment of Alzheimer's disease, amyotrophic lateral sclerosis, and cardiovascular diseases, among others. This review summarizes the most important aspects regarding the development and clinical use of MAO inhibitors, going through mechanistic and structural details, new indications, and future perspectives. Monoamine oxidases (MAOs) catalyze the oxidative deamination of different amines and neurotransmitters. The two different isozymes, MAO-A and MAO-B, are located at the outer mitochondrial membrane in different tissues. The enzymatic reaction involves formation of the corresponding aldehyde and releasing hydrogen peroxide (H₂O₂) and ammonia or a substituted amine depending on the substrate. MAO's role in neurotransmitter metabolism made them targets for major depression and Parkinson's disease, among other neurodegenerative diseases. Currently, these compounds are being studied for other diseases such as cardiovascular ones.

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

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.

A comprehensive review of monoamine oxidase inhibitors as Anti-Alzheimer's disease agents: A review

Monoamine oxidases (MAO-A and MAO-B) are mammalian flavoenzyme, which catalyze the oxidative deamination of several neurotransmitters like norepinephrine, dopamine, tyramine, serotonin, and some other amines. The oxidative deamination produces several harmful side products like ammonia, peroxides, and aldehydes during the biochemical reaction. The concentration of biochemical neurotransmitter alteration in the brain by MAO is directly related with several neurological disorders like Alzheimer's disease and Parkinson's disease (PD). Activated MAO also contributes to the amyloid beta (Aβ) aggregation by two successive cleft β-secretase and γ-secretase of amyloid precursor protein (APP). Additionally, activated MAO is also involved in aggregation of neurofibrillary tangles and cognitive destruction through the cholinergic neuronal damage and disorder of the cholinergic system. MAO inhibition has general anti-Alzheimer's disease effect as a consequence of oxidative stress reduction prompted by MAO enzymes. In this review, we outlined and addressed recent understanding on MAO enzymes such as their structure, physiological function, catalytic mechanism, and possible therapeutic goals in AD. In addition, it also highlights the current development and discovery of potential MAO inhibitors (MAOIs) from various chemical scaffolds.

Bioinformatic Analysis of the Flavin-Dependent Amine Oxidase Superfamily: Adaptations for Substrate Specificity and Catalytic Diversity

The flavin-dependent amine oxidase (FAO) superfamily consists of over 9000 nonredundant sequences represented in all domains of life. Of the thousands of members identified, only 214 have been functionally annotated to date, and 40 unique structures are represented in the Protein Data Bank. The few functionally characterized members share a catalytic mechanism involving the oxidation of an amine substrate through transfer of a hydride to the FAD cofactor, with differences observed in substrate specificities. Previous studies have focused on comparing a subset of superfamily members. Here, we present a comprehensive analysis of the FAO superfamily based on reaction mechanism and substrate recognition. Using a dataset of 9192 sequences, a sequence similarity network, and subsequently, a genome neighborhood network were constructed, organizing the superfamily into eight subgroups that accord with substrate type. Likewise, through phylogenetic analysis, the evolutionary relationship of subgroups was determined, delineating the divergence between enzymes based on organism, substrate, and mechanism. In addition, using sequences and atomic coordinates of 22 structures from the Protein Data Bank to perform sequence and structural alignments, active-site elements were identified, showing divergence from the canonical aromatic-cage residues to accommodate large substrates. These specificity determinants are held in a structural framework comprising a core domain catalyzing the oxidation of amines with an auxiliary domain for substrate recognition. Overall, analysis of the FAO superfamily reveals a modular fold with cofactor and substrate-binding domains allowing for diversity of recognition via insertion/deletions. This flexibility allows facile evolution of new activities, as shown by reinvention of function between subfamilies.

Monoamine oxidase-B inhibitors as potential neurotherapeutic agents: An overview and update

Monoamine oxidase (MAO) inhibitors have made significant contributions and remain an indispensable approach of molecular and mechanistic diversity for the discovery of antineurodegenerative drugs. However, their usage has been hampered by nonselective and/or irreversible action which resulted in drawbacks like liver toxicity, cheese effect, and so forth. Hence, the search for selective MAO inhibitors (MAOIs) has become a substantial focus in current drug discovery. This review summarizes our current understanding on MAO-A/MAO-B including their structure, catalytic mechanism, and biological functions with emphases on the role of MAO-B as a potential therapeutic target for the development of medications treating neurodegenerative disorders. It also highlights the recent developments in the discovery of potential MAO-B inhibitors (MAO-BIs) belonging to diverse chemical scaffolds, arising from intensive chemical-mechanistic and computational studies documented during past 3 years (2015-2018), with emphases on their potency and selectivity. Importantly, readers will gain knowledge of various newly established MAO-BI scaffolds and their development potentials. The comprehensive information provided herein will hopefully accelerate ideas for designing novel selective MAO-BIs with superior activity profiles and critical discussions will inflict more caution in the decision-making process in the MAOIs discovery.

Curcumin-based pyrazoline analogues as selective inhibitors of human monoamine oxidase A

A series of 2-methoxy-4-(5-phenyl-4,5-dihydro-1H-pyrazol-3-yl)phenol (pyrazoline) derivatives (2-6) have been synthesized and tested for human monoamine oxidase (hMAO) inhibitory activity. The most active derivative (2) behaved as a competitive hMAO-A inhibitor, with an inhibition constant value of 0.08 μM and a strong hMAO-A selectivity (K_i(hMAO-B)/K_i(hMAO-A) > 1751). In addition, 2 exhibited little to no cytotoxic effects up to a 25 μM concentration and provided the best blood-brain barrier permeability among the derivatives synthesized. Molecular dynamics simulations revealed that a chlorine substituent at the para-position of the phenyl ring in 2 enabled a π-π stacking interaction with Tyr407 and Tyr444 that resulted in the formation of an "aromatic sandwich" structure. Consequently, this tight-binding aromatic cage culminated in a dramatically reduced active site volume that is believed to be the origin of the observed selectivity between the hMAO-A and hMAO-B isozymes. Removal of the chlorine from 2 disrupted the favorable intermolecular interactions and resulted in a selectivity change towards hMAO-B.

ONIOM calculations on serotonin degradation by monoamine oxidase B: insight into the oxidation mechanism and covalent reversible inhibition

Monoamine oxidase (MAO) is an enzyme which catalyzes the oxidation of neurotransmitter amines and regulates their level. There are two forms of the enzyme with 70% similarity, known as MAO-A and MAO-B. MAO inhibitors are used in the treatment of neurological disorders such as depression, Parkinson's and Alzheimer's diseases. Therefore, understanding the chemical steps of MAO catalyzed amine oxidation is crucial for rational drug design. However, despite many experimental studies and recent computational efforts in the literature, the amine oxidation mechanism by MAO enzymes is still controversial. The polar nucleophilic mechanism and hydride transfer mechanisms are under debate in recent QM/MM studies. In this study, the serotonin oxidation mechanism by MAO was explored via the ONIOM (QM : QM) methodology at the M06-2X/6-31+G(d,p):PM6 level. A modified MAO mechanism involving a covalent reversible inhibition step via formation of flavin N5 ylide was proposed. This mechanism can be used to modulate the potency and reversibility of novel mechanism-based covalent inhibitors by intelligent modifications of the structure of the inhibitors. NBO donor-acceptor analysis confirms that the rate-determining αC-H cleavage step is a hybrid of hydride and proton transfer where hydride transfer dominates over the proton transfer. The functional role of covalent FAD was also investigated by calculating the activation energy of noncovalent FAD models where a 22 fold decrease in the rate of catalysis was predicted. Geometrical features imply that the function of the covalent bond in FAD might be to maintain the correct geometry and conformation for a more efficient catalysis.

New insights into Brunner syndrome and potential for targeted therapy

We report two families with Brunner syndrome living in one state of Australia. The first family had a predicted protein-truncating variant of monoamine oxidase A (MAOA) (p.S251KfsX2). Affected males had mild intellectual disability (ID), obsessive behaviour, limited friendships and were introverted and placid during clinical interview. The family disclosed episodic explosive aggression after a diagnosis was made. The second family had a missense variant in MAOA (p.R45W). Affected males had borderline-mild ID, attention deficit disorder and limited friendships. One had a history of explosive aggression in childhood and episodic symptoms of flushing, headaches and diarrhoea. Their carrier mother had normal intelligence but similar episodic symptoms. Characteristic biochemical abnormalities included high serum serotonin and urinary metanephrines and low urinary 5-hydroxyindoleacetic acid (5-HIAA) and vanillylmandelic acid (VMA). Symptomatic individuals in the second family had particularly high serotonin levels, and treatment with a serotonin reuptake inhibitor and dietary modification resulted in reversal of biochemical abnormalities, reduction of 'serotonergic' symptoms and behavioural improvement. Brunner syndrome should be considered as a cause of mild ID with paroxysmal behavioural symptoms. It can be screened for with serum/urine metanephrine and serotonin measurement. Cautious treatment with a serotonin reuptake inhibitor, dietary modifications and avoidance of medications contraindicated in patients on monoamine oxidase inhibitors can improve symptoms.

Molecular insights into human monoamine oxidase (MAO) inhibition by 1,4-naphthoquinone: evidences for menadione (vitamin K3) acting as a competitive and reversible inhibitor of MAO

Monoamine oxidase (MAO) catalyzes the oxidative deamination of biogenic and exogenous amines and its inhibitors have therapeutic value for several conditions including affective disorders, stroke, neurodegenerative diseases and aging. The discovery of 2,3,6-trimethyl-1,4-naphthoquinone (TMN) as a nonselective and reversible inhibitor of MAO, has suggested 1,4-naphthoquinone (1,4-NQ) as a potential scaffold for designing new MAO inhibitors. Combining molecular modeling tools and biochemical assays we evaluate the kinetic and molecular details of the inhibition of human MAO by 1,4-NQ, comparing it with TMN and menadione. Menadione (2-methyl-1,4-naphthoquinone) is a multitarget drug that acts as a precursor of vitamin K and an inducer of mitochondrial permeability transition. Herein we show that MAO-B was inhibited competitively by 1,4-NQ (K(i)=1.4 μM) whereas MAO-A was inhibited by non-competitive mechanism (K(i)=7.7 μM). Contrasting with TMN and 1,4-NQ, menadione exhibited a 60-fold selectivity for MAO-B (K(i)=0.4 μM) in comparison with MAO-A (K(i)=26 μM), which makes it as selective as rasagiline. Fluorescence and molecular modeling data indicated that these inhibitors interact with the flavin moiety at the active site of the enzyme. Additionally, docking studies suggest the phenyl side groups of Tyr407 and Tyr444 (for MAO-A) or Tyr398 and Tyr435 (for MAO-B) play an important role in the interaction of the enzyme with 1,4-NQ scaffold through forces of dispersion as verified for menadione, TMN and 1,4-NQ. Taken together, our findings reveal the molecular details of MAO inhibition by 1,4-NQ scaffold and show for the first time that menadione acts as a competitive and reversible inhibitor of human MAO.

Structures and Mechanism of the Monoamine Oxidase Family

Members of the monoamine oxidase family of flavoproteins catalyze the oxidation of primary and secondary amines, polyamines, amino acids, and methylated lysine side chains in proteins. The enzymes have similar overall structures, with conserved FAD-binding domains and varied substrate-binding sites. Multiple mechanisms have been proposed for the catalytic reactions of these enzymes. The present review compares the structures of different members of the family and the various mechanistic proposals.

Computational investigation on the structure-activity relationship of the biradical mechanism for monoamine oxidase

Although a considerable amount of mechanistic data has accumulated in literature, the detailed mechanism for amine oxidation by monoamine oxidase is still controversial. The single electron transfer mechanism (SET) has been widely discussed, but not completely understood yet. In the present study, the modified SET mechanism, proposed by Silverman et al., was explored by quantum chemical calculations. The ONIOM method was applied with UDFT/B3LYP/6-31 + G(d,p) for the higher layer and with UHF/6-31G(d) for the lower layer. Isoalloxazin heterocyclic ring and benzylamine were employed in the calculations to represent flavin and the substrate, respectively. The substituents CH(3), OH, OCH(3), H, F, Cl, Br, CF(3) and NO(2) were incorporated at the para position of benzylamine to explore structure-activity relationships. The structures of the reactant complex, transition state and product complex were fully optimized. Activation energies and rate constants of all the reactions were calculated. The results obtained from the linear regression analysis showed that electron-donating groups at the para position of benzylamine increase the reaction rate. A linear but inverse correlation between the log of the calculated rate constants (log k) and the electronic parameter of the substituent was observed (R = 0.93). In accordance with this result, a relatively weak inverse correlation between the calculated log k and the experimental log k was obtained (R = 0.78). The results are contrary to the previous kinetic experiments and the computational study on the effect of p-substituents in the flavin reduction of MAO A by p-substituted benzylamine analogs. Therefore, they present negative evidence for the modeled biradical mechanism.

High-level expression and purification of rat monoamine oxidase A (MAO A) in Pichia pastoris: comparison with human MAO A

The high-level heterologous expression in Pichia pastoris, purification and characterization of recombinant membrane-bound rat liver monoamine oxidase A (MAO A) are described. A 1-L culture of cells produces approximately 700 U of rat MAO A activity. The rat MAO A activity is found in outer mitochondrial membrane of the cell. Using a modification of the human MAO A purification procedure, approximately 200mg of recombinant rat MAO A is purified in a 43% yield and exhibits a molecular weight of approximately 60,000 kDa on SDS-PAGE. The purified enzyme contains a covalently bound FAD and forms a N(5) flavocyanine adduct on inhibition by clorgyline. Edman sequencing shows that the amino terminus of rat MAO A is blocked at an N-terminal threonyl residue. The purified rat enzyme exhibits a higher thermal stability than does purified human MAO A. Compared with human MAO A, rat MAO A oxidizes serotonin or kynuramine with twofold higher k(cat)/K(m) values, oxidizes phenethylamine with a 6.7-fold higher catalytic efficiency and benzylamine with a approximately 40-fold higher catalytic efficiency. Although approximately 90% identical in sequence to human MAO A, rat MAO A is a more efficient catalyst for amine neurotransmitter oxidation.

Mutagenic probes of the role of Ser209 on the cavity shaping loop of human monoamine oxidase A

The available literature implicating human monoamine oxidase A (MAO A) in apoptotic processes reports levels of MAO A protein that do not correlate with activity, suggesting that unknown mechanisms may be involved in the regulation of catalytic function. Bioinformatic analysis suggests Ser209 as a possible phosphorylation site that may be relevant to catalytic function because it is adjacent to a six-residue loop termed the 'cavity shaping loop' from structural data. To probe the functional role of this site, MAO A Ser209Ala and Ser209Glu mutants were created and investigated. In its membrane-bound form, the MAO A Ser209Glu phosphorylation mimic exhibits catalytic and inhibitor binding properties similar to those of wild-type MAO A. Solubilization in detergent solution and purification of the Ser209Glu mutant results in considerable decreases in these functional parameters. By contrast, the MAO A Ser209Ala mutant exhibits similar catalytic properties to those of wild-type enzyme when purified. Compared to purified wild-type and Ser209Ala MAO A proteins, the Ser209Glu MAO A mutant shows significant differences in covalent flavin fluorescence yield, CD spectra and thermal stability. These structural differences in the purified MAO A Ser209Glu mutant are not exhibited in quantitative structure-activity relationship patterns using a series of para-substituted benzylamine analogs similar to the wild-type enzyme. These data suggest that Ser209 in MAO A does not appear to be the putative phosphorylation site for regulation of MAO A activity and demonstrate that the membrane environment plays a significant role in stabilizing the structure of MAO A and its mutant forms.

Insights into the mechanism of flavoprotein-catalyzed amine oxidation from nitrogen isotope effects on the reaction of N-methyltryptophan oxidase

The mechanism of N-methyltryptophan oxidase, a flavin-dependent amine oxidase from Escherichia coli, was studied using a combination of kinetic isotope effects and theoretical calculations. The 15(kcat/Km) kinetic isotope effect for sarcosine oxidation is pH-dependent with a limiting value of 0.994-0.995 at high pH. Density functional theory calculations on model systems were used to interpret these isotope effects. The isotope effects are inconsistent with proposed mechanisms involving covalent amine-flavin adducts but cannot by themselves conclusively distinguish between some discrete electron-transfer mechanisms and a direct hydride-transfer mechanism, although the latter mechanism is more consistent with the energetics of the reaction.

The therapeutic potential of monoamine oxidase inhibitors

Monoamine oxidase inhibitors were among the first antidepressants to be discovered and have long been used as such. It now seems that many of these agents might have therapeutic value in several common neurodegenerative conditions, independently of their inhibition of monoamine oxidase activity. However, many claims and some counter-claims have been made about the physiological importance of these enzymes and the potential of their inhibitors. We evaluate these arguments in the light of what we know, and still have to learn, of the structure, function and genetics of the monoamine oxidases and the disparate actions of their inhibitors.

Flavoenzyme catalysed oxidation of amines: roles for flavin and protein-based radicals

Amines are a carbon source for the growth of a number of bacterial species and they also play key roles in neurotransmission, cell growth and differentiation, and neoplastic cell proliferation. Enzymes have evolved to catalyse these reactions and these oxidoreductases can be grouped into the flavoprotein and quinoprotein families. The mechanism of amine oxidation catalysed by the quinoprotein amine oxidases is understood reasonably well and occurs through the formation of enzyme–substrate covalent adducts with TPQ (topaquinone), TTQ (tryptophan tryptophylquinone), CTQ (cysteine tryptophylquinone) and LTQ (lysine tyrosyl quinone) redox centres. Oxidation of amines by flavoenzymes is less well understood. The role of protein-based radicals and flavin semiquinone radicals in the oxidation of amines is discussed.

Mechanistic studies of mouse polyamine oxidase with N1,N12-bisethylspermine as a substrate

In mammalian cells, the flavoprotein polyamine oxidase catalyzes a key step in the catabolism of polyamines, the oxidation of N1-acetylspermine and N1-acetylspermidine to spermidine and putrescine, respectively. The mechanism of the mouse enzyme has been studied with N1,N12-bisethylspermine (BESPM) as a substrate. At pH 10, the pH optimum, the limiting rate of reduction of the flavin in the absence of oxygen is comparable to the k(cat) value for turnover, establishing reduction as rate-limiting. Oxidation of the reduced enzyme is a simple second-order reaction. No intermediates are seen in the reductive or oxidative half-reactions. The k(cat) value decreases below a pK(a) of 9.0. The k(cat)/K(m) value for BESPM exhibits a bell-shaped pH profile, with pK(a) values of 9.8 and 10.8. These pK(a) values are assigned to the substrate nitrogens. The rate constant for the reaction of the reduced enzyme with oxygen is not affected by a pH between 7.5 and 10. Active site residue Tyr430 is conserved in the homologous protein monoamine oxidase. Mutation of this residue to phenylalanine results in a 6-fold decrease in the k(cat) value and the k(cat)/K(m) value for oxygen due to a comparable decrease in the rate constant for flavin reduction. This moderate change is not consistent with this residue forming a tyrosyl radical during catalysis.

Inactivation of purified human recombinant monoamine oxidases A and B by rasagiline and its analogues

The inactivation of purified human recombinant monoamine oxidases (MAO) A and B by rasagiline [N-propargyl-1(R)-aminoindan] and four of its analogues [N-propargyl-1(S)-aminoindan (S-PAI), 6-hydroxy-N-propargyl-1(R)-aminoindan (R-HPAI), N-methyl-N-propargyl-1(R)-aminoindan (R-MPAI), and 6-(N-methyl-N-ethyl carbamoyloxy)-N-propargyl-1(R)-aminoindan (R-CPAI)] has been investigated. All compounds tested, with the exception of R-CPAI, form stoichiometric N(5) flavocyanine adducts with the FAD moiety of either enzyme. No H(2)O(2) is produced during either MAO A or MAO B inactivation, which demonstrates that covalent addition occurs in a single turnover. Rasagiline has the highest specificity for MAO B, as demonstrated by a 100-fold higher inhibition potency (k(inact)/K(i)) compared to MAO A, with the remaining compounds exhibiting lower isozyme specificities. MAO B and MAO A are more selective for the R-enantiomer (rasagiline) compared to the S-enantiomer (S-PAI) by 2500-fold and 17-fold, respectively. Differences in UV/vis and CD spectral data of the complexes of the studied compounds with both MAO A and MAO B are interpreted in light of crystallographic data of complexes of MAO B with rasagiline and its analogues (Binda, C.; et al. J. Med. Chem. 2004, 47, 1767-1774.

Analysis of conserved active site residues in monoamine oxidase A and B and their three-dimensional molecular modeling

Monoamine oxidase (MAO) is a key enzyme responsible for the degradation of serotonin, norepinephrine, dopamine, and phenylethylamine. It is an outer membrane mitochondrial enzyme existing in two isoforms, A and B. We have recently generated 14 site-directed mutants of human MAO A and B, and we found that four key amino acids, Lys-305, Trp-397, Tyr-407, and Tyr-444, in MAO A and their corresponding amino acids in MAO B, Lys-296, Trp-388, Tyr-398, and Tyr-435, play important roles in MAO catalytic activity. Based on the polyamine oxidase three-dimensional crystal structure, it is suggested that Lys-305, Trp-397, and Tyr-407 in MAO A and Lys-296, Trp-388, and Tyr-398 in MAO B may be involved in the non-covalent binding to FAD. Tyr-407 and Tyr-444 in MAO A (Tyr-398 and Tyr-435 in MAO B) may form an aromatic sandwich that stabilizes the substrate binding. Asp-132 in MAO A (Asp-123 in MAO B) located at the entrance of the U-shaped substrate-binding site has no effect on MAO A nor MAO B catalytic activity. The similar impact of analogous mutants in MAO A and MAO B suggests that these amino acids have the same function in both isoenzymes. Three-dimensional modeling of MAO A and B using polyamine oxidase as template suggests that the overall tertiary structure and the active sites of MAO A and B may be similar.