Structure of rat monoamine oxidase A and its specific recognitions for substrates and inhibitors

Chemical Composition, Antioxidant Activities, Antidepressant Effect, and Lipid Peroxidation of Peruvian Blueberry: Molecular Docking Studies on Targets Involved in Oxidative Stress and Depression

Blueberries (Vaccinium corymbosum L.) are cultivated worldwide and are among the best dietary sources of bioactive compounds with beneficial health effects. This study aimed to investigate the components of Peruvian blueberry using high-performance liquid chromatography coupled to electrospray ionization and quadrupole time-of-flight mass spectrometry (HPLC-ESI-QTOF-MS/MS), identifying 11 compounds. Furthermore, we assessed in vitro the antioxidant activity and in vivo the antidepressant effect using a rat model and protective effect on lipid peroxidation (in the serum, brain, liver, and stomach). We also conducted molecular docking simulations with proteins involved in oxidative stress and depression for the identified compounds. Antioxidant activity was assessed by measuring total phenolic and flavonoid contents, as well as using 1,1-diphenyl-2-picrylhydrazin (DPPH), 2,2'-azino-bis-(3-ethylbenzothiazoline-6-sulfonic) acid (ABTS•+), and ferric-reducing antioxidant power (FRAP) assays. Peruvian blueberries demonstrated higher antioxidant activity than Vaccinium corymbosum fruits from Chile, Brazil, the United States, Turkey, Portugal, and China. The results showed that oral administration of Peruvian blueberries (10 and 20 mg/kg) for 28 days significantly (p < 0.001) increased swimming and reduced immobility in the forced swimming test (FST). Additionally, at doses of 40 and 80 mg/kg, oxidative stress was reduced in vivo (p < 0.001) by decreasing lipid peroxidation in brain, liver, stomach, and serum. Molecular docking and absorption, distribution, metabolism, excretion, and toxicity (ADMET) predictions were performed. In the molecular docking studies, quercitrin and 3,5-di-O-caffeoylquinic acid showed the best docking scores for nicotinamide adenine dinucleotide phosphate (NADPH) oxidase, superoxide dismutase, and xanthine oxidase; while 3,5-dicaffeoylquinic acid methyl ester and caffeoyl coumaroylquinic acid had the best docking scores for monoamine oxidase and serotonin receptor 5-HT2. In summary, our results suggest that the antidepressant and protective effects against lipid peroxidation might be related to the antioxidant activity of Peruvian Vaccinium corymbosum L.

Potent, selective and reversible hMAO-B inhibition by benzalphthalides: Synthesis, enzymatic and cellular evaluations and virtual docking and predictive studies

Monoaminooxidases (MAOs) are important targets for drugs used in the treatment of neurological and psychiatric disorders and particularly on Parkinson's Disease (PD). Compounds containing a trans-stilbenoid skeleton have demonstrated good selective and reversible MAO-B inhibition. Here, twenty-two (Z)-3-benzylidenephthalides (benzalphthalides, BPHs) displaying a trans-stilbenoid skeleton have been synthesised and evaluated as inhibitors of the MAO-A and MAO-B isoforms. Some BPHs have selectively inhibited MAO-B, with IC50 values ranging from sub-nM to μM. The most potent compound with IC50 = 0.6 nM was the 3',4'-dichloro-BPH 16, which showed highly selective and reversible MAO-B inhibitory activity. Furthermore, the most selective BPHs displayed a significant protection against the apoptosis, and mitochondrial toxic effects induced by 6-hydroxydopamine (6OHDA) on SH-SY5Y cells, used as a cellular model of PD. The results of virtual binding studies on the most potent compounds docked in MAO-B and MAO-A were in agreement with the potencies and selectivity indexes found experimentally. Additionally, related to toxicity risks, drug-likeness and ADME properties, the predictions found for the most relevant BPHs in this research were within those ranges established for drug candidates.

MAO inhibiting phytochemicals from the roots of Glycyrrhiza glabra L

Glycyrrhizin, a natural compound that is substantially present in Glycyrrhiza glabra L. (Gg) root. Monoamine oxidase B (MAOB) inhibitor is used for the treatment of several important neuropsychological diseases like Parkinson's disease. Gg is known to possess psychoactive properties which relates to its MAO inhibitory potential. This study sought to determine the MAO inhibition property of glycyrrhizin from Gg root extract. The Aqueous extract containing glycyrrhizin was isolated from the root of Gg and characterized using TLC, HPLC, and LC-MS techniques. In silico docking was conducted using Extra precision Glide 2018, Schrödinger docking suite. In addition, the pharmacokinetic properties of the compounds were predicted using SwissADME. The binding energies of the glycyrrhizin correlated well with their in vitro MAO inhibitory potential. Glycyrrhizin exhibited potent inhibitory activity towards MAOB whereas, an aqueous extract of Gg root inhibits both A and B forms of MAO enzyme. Further, molecular docking and molecular dynamics simulation showed that liquiritigenin and methoxyglabridin showed higher stability than other inhibitor compounds from the Gg root extract. These observations suggest that the phytochemicals from the Gg root extract have potent MAO inhibition properties, which can be exploited for the treatment of neurodegenerative disorders.Communicated by Ramaswamy H. Sarma.

The quest for new robust bacterial monoamine oxidases

Microbial enzymes are versatile, cost-effective, and sustainable tools, making them a preferred choice for enzymatic processes. Santema et al. harnessed AlphaFold, a cutting-edge structure prediction tool, to discover new thermophilic monoamine oxidases (MAO) that could be relevant for drug development and use in biotechnology fields. The new enzyme displays thermal robustness, offering a unique structure-to-function profile compared to known MAOs. This bacterial enzyme, paired with recent advancements in enzyme engineering, has the potential to meet the biotech sector's need for customized enzymes.

Near-Infrared Fluorescence Probe for Monoamine Oxidase A with a Large Stokes Shift for Intraoperative Navigation

Monoamine oxidase A (MAO-A) is a dimeric flavoprotein that is found in the mitochondrial membrane. Currently, there is a lack of near-infrared fluorescent probes (NIR-FPs) with good specificity and high sensitivity for detecting MAO-A, making it difficult to accurately recognize and image cells in vitro and in vivo. In this study, the NIR-FP DDM-NH2 was designed and synthesized in order to detect MAO-A specifically in live biological systems. The probe comprised two functional components: dicyanoisophosphone as an NIR dye precursor and alanine as a recognition moiety. After identifying MAO-A, the probe exhibited an NIR emission peak at 770 nm with a significant Stokes shift (180 nm), 11-fold response factor, low detection limit of 99.7 nM, and considerably higher affinity toward MAO-A than that toward MAO-B, indicating high sensitivity. In addition, DDM-NH2 was effective when applied to the image-based assessment of MAO-A activity in HeLa cells, zebrafish, and tumor-bearing mice, demonstrating great potential for visualization-based research and MAO-A application in vivo.

The carboxy terminus causes interfacial assembly of oleate hydratase on a membrane bilayer

The soluble flavoprotein oleate hydratase (OhyA) hydrates the 9-cis double bond of unsaturated fatty acids. OhyA substrates are embedded in membrane bilayers; OhyA must remove the fatty acid from the bilayer and enclose it in the active site. Here, we show that the positively charged helix-turn-helix motif in the carboxy terminus (CTD) is responsible for interacting with the negatively charged phosphatidylglycerol (PG) bilayer. Super-resolution microscopy of Staphylococcus aureus cells expressing green fluorescent protein fused to OhyA or the CTD sequence shows subcellular localization along the cellular boundary, indicating OhyA is membrane-associated and the CTD sequence is sufficient for membrane recruitment. Using cryo-electron microscopy, we solved the OhyA dimer structure and conducted 3D variability analysis of the reconstructions to assess CTD flexibility. Our surface plasmon resonance experiments corroborated that OhyA binds the PG bilayer with nanomolar affinity and we found the CTD sequence has intrinsic PG binding properties. We determined that the nuclear magnetic resonance structure of a peptide containing the CTD sequence resembles the OhyA crystal structure. We observed intermolecular NOE from PG liposome protons next to the phosphate group to the CTD peptide. The addition of paramagnetic MnCl2 indicated the CTD peptide binds the PG surface but does not insert into the bilayer. Molecular dynamics simulations, supported by site-directed mutagenesis experiments, identify key residues in the helix-turn-helix that drive membrane association. The data show that the OhyA CTD binds the phosphate layer of the PG surface to obtain bilayer-embedded unsaturated fatty acids.

Monoamine Oxidase-A (MAO-A) Inhibitors Screened from the Autodisplayed Fv-Antibody Library

Serotonin-like mimotopes were screened from the Fv-antibody library to be used as inhibitors against monoamine oxidase A (MAO-A). The Fv-antibody [corresponding to the VH region of immunoglobulin G (IgG)] consists of three complementarity-determining regions and four frame regions. The Fv-antibody library was prepared by site-directed mutagenesis of CDR3, which consists of 11 amino acid residues. Three target clones were screened from the Fv-antibody library, and the binding affinity of the screened clones to the monoclonal anti-serotonin antibody was analyzed using fluorescence-activated cell sorting. The screened Fv-antibodies were expressed as soluble proteins fused with green fluorescence protein. Additionally, the screened CDR3 regions (11 residues) of the selected Fv-antibodies were synthesized as peptides with linking amino acid residues. The binding constants (KD) of the three serotonin-like mimotopes (Fv-antibodies and peptides) were estimated using a surface plasmon resonance biosensor. The inhibitory activity (IC50) of the serotonin-like mimotopes (Fv-antibodies and peptides) was estimated separately for MAO-A and MAO-B enzymes and compared with that of conventional inhibitors. Finally, the screened serotonin-like mimotopes were used to treat a cell line (SH-SY5Y, ATCC code: CRL-2266) expressing serotonin receptors. This was done to confirm the following two aspects: (1) the binding of mimotopes to the serotonin receptors on the cell surface and (2) the inhibitory activity of mimotopes against MAO-A enzymes in the cell lysates.

Inhibiting Monoamine Oxidase in CNS and CVS would be a Promising Approach to Mitigating Cardiovascular Complications in Neurodegenerative Disorders

The flavoenzyme monoamine oxidases (MAOs) are present in the mitochondrial outer membrane and are responsible for the metabolism of biogenic amines. MAO deamination of biological amines produces toxic byproducts such as amines, aldehydes, and hydrogen peroxide, which are significant in the pathophysiology of multiple neurodegenerative illnesses. In the cardiovascular system (CVS), these by-products target the mitochondria of cardiac cells leading to their dysfunction and producing redox imbalance in the endothelium of the blood vessels. This brings up the biological relationship between the susceptibility of getting cardiovascular disorders in neural patients. In the current scenario, MAO inhibitors are highly recommended by physicians worldwide for the therapy and management of various neurodegenerative disorders. Many interventional studies reveal the benefit of MAO inhibitors in CVS. Drug candidates who can target both the central and peripheral MAO could be a better to compensate for the cardiovascular comorbidities observed in neurodegenerative patients.

Natural Products as Monoamine Oxidase Inhibitors: Potential Agents for Neurological Disorders

The role of medicinal plants has been advantageous due to their manifestation through various cellular and molecular mechanisms. Inhibition of the monoamine oxidase enzyme is suspected to be a highly effective treatment for various neurological illnesses like Alzheimer's disease, Parkinson's disease, depression, social phobia, and panic disorders. The study of phytochemicals and plant extracts used as a traditional source of medication revealed that they possess the vast potential for monoamine oxidase inhibition. Thus, the article focuses on the potential use of plant extracts and phytochemicals as sources of novel MAO inhibitors for treating neurological disorders. Exhaustive literature search revealed that a variety of phytochemicals from the categories such as flavonoids, alkaloids, glycosides, alkyl phenyl ketones, coumarin derivatives and essential oils have displayed potential MAO inhibition. This review highlights the progress made in the discovery and development of plant-based MAO inhibitors and aims to provide medicinal chemists with an overview of this information to aid in the development of clinically viable drugs.

Protective effects of niacin following high fat rich diet: an in-vivo and in-silico study

Niacin had long been understood as an antioxidant. There were reports that high fat diet (HFD) may cause psychological and physical impairments. The present study was aimed to experience the effect of Niacin on % growth rate, cumulative food intake, motor activity and anxiety profile, redox status, 5-HT metabolism and brain histopathology in rats. Rats were administered with Niacin at a dose of 50 mg/ml/kg body weight for 4 weeks following normal diet (ND) and HFD. Behavioral tests were performed after 4 weeks. Animals were sacrificed to collect brain samples. Biochemical, neurochemical and histopathological studies were performed. HFD increased food intake and body weight. The exploratory activity was reduced and anxiety like behavior was observed in HFD treated animals. Activity of antioxidant enzymes was decreased while oxidative stress marker and serotonin metabolism in the brain of rat were increased in HFD treated animals than ND fed rats. Morphology of the brain was also altered by HFD administration. Conversely, Niacin treated animals decreased food intake and % growth rate, increased exploratory activity, produced anxiolytic effects, decreased oxidative stress and increased antioxidant enzyme and 5-HT levels following HFD. Morphology of brain is also normalized by the treatment of Niacin following HFD. In-silico studies showed that Niacin has a potential binding affinity with degradative enzyme of 5-HT i.e. monoamine oxidase (MAO) A and B with an energy of ~ - 4.5 and - 5.0 kcal/mol respectively. In conclusion, the present study showed that Niacin enhanced motor activity, produced anxiolytic effect, and reduced oxidative stress, appetite, growth rate, increased antioxidant enzymes and normalized serotonin system and brain morphology following HFD intake. In-silico studies suggested that increase 5-HT was associated with the binding of MAO with Niacin subsequentially an inhibition of the degradation of monoamine. It is suggested that Niacin has a great antioxidant potential and could be a good therapy for the treatment of HFD induced obesity.

Clorgyline Analogs Synergize with Azoles against Drug Efflux in Candida auris

Concern about the global emergence of multidrug-resistant fungal pathogens led us to explore the use of combination therapy to combat azole resistance in Candida auris. Clorgyline had previously been shown to be a multi-target inhibitor of Cdr1 and Mdr1 efflux pumps of Candida albicans and Candida glabrata. A screen for antifungal sensitizers among synthetic analogs of Clorgyline detected interactions with the C. auris efflux pump azole substrates Posaconazole and Voriconazole. Of six Clorgyline analogs, M19 and M25 were identified as potential sensitizers of azole resistance. M19 and M25 were found to act synergistically with azoles against resistant C. auris clade I isolates and recombinant Saccharomyces cerevisiae strains overexpressing C. auris efflux pumps. Nile Red assays with the recombinant strains showed M19 and M25 inhibited the activity of Cdr1 and Mdr1 efflux pumps that are known to play key roles in azole resistance in C. auris clades I, III, and IV. While Clorgyline, M19 and M25 uncoupled the Oligomycin-sensitive ATPase activity of Cdr1 from C. albicans and C. auris, their mode of action is yet to be fully elucidated. The experimental combinations described herein provides a starting point to combat azole resistance dominated by overexpression of CauCdr1 in C. auris clades I and IV and CauMdr1 in C. auris clade III.

The monoaminergic system is a bilaterian innovation

Monoamines like serotonin, dopamine, and adrenaline/noradrenaline (epinephrine/norepinephrine) act as neuromodulators in the nervous system. They play a role in complex behaviours, cognitive functions such as learning and memory formation, as well as fundamental homeostatic processes such as sleep and feeding. However, the evolutionary origin of the genes required for monoaminergic modulation is uncertain. Using a phylogenomic approach, in this study, we show that most of the genes involved in monoamine production, modulation, and reception originated in the bilaterian stem group. This suggests that the monoaminergic system is a bilaterian novelty and that its evolution may have contributed to the Cambrian diversification.

Antidepressant activity of phytochemicals of Mangifera indica seeds assisted by integrated computational analysis

Mangifera indica L., also known as mango, is a tropical fruit that belongs to the Anacardiaceae family and is prized for its juiciness, unique flavour, and worldwide popularity. The current study aimed to probe into antidepressant power (ADP) of MIS in animals and confirmation of ADP with in silico induced-fit molecular docking. The depression model was prepared by exposing mice to various stressors from 9:00 am to 2:00 pm during 42 days study period. MIS extract and fluoxetine were given daily for 30 min before exposing animals to stressors. ADP was evaluated by various behavioural tests and biochemical analysis. Results showed increased physical activity in mice under behavioural tests, plasma nitrite and malondialdehyde (MDA) levels and monoamine oxidase A (MAO-A) activity decreased dose-dependently in MIS treated mice and superoxide dismutases (SOD) levels increased in treated groups as compared to disease control. With the peculiar behaviour and significant interactions of the functional residues of target proteins with selected ligands along with the best absorption, distribution, metabolism, excretion, and toxicity (ADMET) properties, it is concluded that catechin could be the best MAO-A inhibitor at a binding energy of -8.85 kcal/mol, and two hydrogen bonds were generated with Cys406 (A) and Gly443 (A) residues of the active binding site of MAO-A enzyme. While catechin at -6.86 kcal/mol generated three hydrogen bonds with Ala263 (A) and Gly434 (A) residues of the active site of monoamine oxidase B (MAO-B) enzyme and stabilized the best conformation. Therefore, it is highly recommended to test the selected lead-like compound catechin in the laboratory with biological system analysis to confirm its activity as MAO-A and MAO-B inhibitors so it can be declared as one of the novel therapeutic options with anti-depressant activity. Our findings concluded that M. indica seeds could be a significant and alternative anti-depressant therapy.

In Vivo Study of Monoamine Oxidases Using Multisite Intracerebral Microdialysis

The activity of monoamine oxidases (MAOs) in the brain is often associated with neurodegenerative diseases. The study of MAOs in vivo or ex vivo is generally performed using MAO inhibitors and rarely using substrates. We present a pharmacological approach using intracerebral microdialysis to study the activity of MAO in the striatum and the prefrontal cortex of rats. It consists of applying ascending concentrations of 3-methoxytyramine (3-MT) as a substrate via the probes and measuring the indirect product homovanillic acid generated by MAO activity. We present herein the methodologies comprising our in-house stereotaxic procedures in rats, the microdialysis perfusion system and the substrate application, and the neurochemical analysis of the samples.

MAO Visible Spectroscopy for Ligand Interactions, Redox Chemistry, and Kinetics of Irreversible Inhibition

The covalently bound FAD cofactor in monoamine oxidase (MAO) is reduced by the amine substrate and reoxidized by oxygen. Visible spectroscopy provides a convenient tool to study the interaction of ligands and the kinetics of the half-reactions for mechanistic investigations. Equilibrium redox titrations allow measurement of redox potentials, while rapid mixing experiments allow determination of the rate of reduction by different substrates and of covalent adduct formation by irreversible inactivators. Three techniques are described: (1) measuring ligand interactions by alterations in the spectrum, especially at 495 nm; (2) reducing MAO, including the essentials for anaerobic procedures; and (3) studying kinetics of reduction, reoxidation, or inactivation of MAO.

Crystallization of Human Monoamine Oxidase B

The interest in monoamine oxidases A and B (MAO A and B) is due to their central role in regulating the balance of neurotransmitters, both in the central nervous system and in peripheral organs. As validated drug targets for depression and Parkinson's disease, the elucidation of their crystal structures was an essential step to guide drug design investigations. The development of the heterologous expression system of MAO B in Pichia pastoris and the identification of the detergent, buffer, and precipitant conditions allowed to determine the first crystal structure of human MAO B in 2002. A detailed protocol to obtain reproducible MAO B crystals is described.

Affinity probes based on small-molecule inhibitors for tumor imaging

Methods for molecular imaging of target areas, including optical imaging, radionuclide imaging, magnetic resonance imaging and other imaging technologies, are helpful for the early diagnosis and precise treatment of cancers. In addition to cancer management, small-molecule inhibitors are also used for developing cancer target probes since they act as the tight-binding ligands of overexpressed proteins in cancer cells. This review aims to summarize the structural designs of affinity probes based on small-molecule inhibitors from the aspects of the inhibitor, linker, dye and radionuclide, and discusses the influence of the modification of these structures on affinity and pharmacokinetics. We also present examples of inhibitor affinity probes in clinical applications, and these summaries will provide insights for future research and clinical translations.

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.

Questions in the Chemical Enzymology of MAO

We have structure, a wealth of kinetic data, thousands of chemical ligands and clinical information for the effects of a range of drugs on monoamine oxidase activity in vivo. We have comparative information from various species and mutations on kinetics and effects of inhibition. Nevertheless, there are what seem like simple questions still to be answered. This article presents a brief summary of existing experimental evidence the background and poses questions that remain intriguing for chemists and biochemists researching the chemical enzymology of and drug design for monoamine oxidases (FAD-containing EC 4.1.3.4).

Insights into the Mechanism of the Therapeutic Potential of Herbal Monoamine Oxidase Inhibitors in Neurological Diseases

Monoamine oxidase (MAO) is an enzyme that catalyzes the deamination of monoamines and other proteins. MAO's hyperactivation results in the massive generation of reactive oxygen species, which leads to a variety of neurological diseases such as Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis, Huntington's disease, and depression-like disorders. Although synthetic MAO inhibitors are clinically available, they are associated with side effects such as hepatotoxicity, cheese reaction, hypertensive crisis, and so on, necessitating the investigation of alternative MAO inhibitors from a natural source with a safe profile. Herbal medications have a significant impact on the prevention of many diseases; additionally, they have fewer side effects and serve as a precursor for drug development. This review discusses the potential of herbal MAO inhibitors as well as their associated mechanism of action, with an aim to foster future research on herbal MAO inhibitors as potential treatment for neurological diseases.

Investigating the Disordered and Membrane-Active Peptide A-Cage-C Using Conformational Ensembles

The driving forces and conformational pathways leading to amphitropic protein-membrane binding and in some cases also to protein misfolding and aggregation is the subject of intensive research. In this study, a chimeric polypeptide, A-Cage-C, derived from α-Lactalbumin is investigated with the aim of elucidating conformational changes promoting interaction with bilayers. From previous studies, it is known that A-Cage-C causes membrane leakages associated with the sporadic formation of amorphous aggregates on solid-supported bilayers. Here we express and purify double-labelled A-Cage-C and prepare partially deuterated bicelles as a membrane mimicking system. We investigate A-Cage-C in the presence and absence of these bicelles at non-binding (pH 7.0) and binding (pH 4.5) conditions. Using in silico analyses, NMR, conformational clustering, and Molecular Dynamics, we provide tentative insights into the conformations of bound and unbound A-Cage-C. The conformation of each state is dynamic and samples a large amount of overlapping conformational space. We identify one of the clusters as likely representing the binding conformation and conclude tentatively that the unfolding around the central W23 segment and its reorientation may be necessary for full intercalation at binding conditions (pH 4.5). We also see evidence for an overall elongation of A-Cage-C in the presence of model bilayers.

Structural exploration of multifunctional monoamine oxidase B inhibitors as potential drug candidates against Alzheimer's disease

AD is one of the most typical neurodegenerative disorders that suffer many seniors worldwide. Recently, MAO inhibitors have received increasing attention not only for their roles involved in monoamine neurotransmitters metabolism and oxidative stress but also for their additional neuroprotective and neurorescue effects against AD. The curiosity in MAO inhibitors is reviving, and novel MAO-B inhibitors recently developed with ancillary activities (e.g., Aβ aggregation and AChE inhibition, anti-ROS and chelating activities) have been proposed as multitarget drugs foreshadowing a positive outlook for the treatment of AD. The current review describes the recent development of the design, synthesis, and screening of multifunctional ligands based on MAO-B inhibition for AD therapy. Structure-activity relationships and rational design strategies of the synthetic or natural product derivatives (chalcones, coumarins, chromones, and homoisoflavonoids) are discussed.

Investigation into MAO B-Mediated Formation of CC112273, a Major Circulating Metabolite of Ozanimod, in Humans and Preclinical Species: Stereospecific Oxidative Deamination of (S)-Enantiomer of Indaneamine (RP101075) by MAO B

Ozanimod, recently approved for treating relapsing multiple sclerosis, produced a disproportionate, active, MAO B-catalyzed metabolite (CC112273) that showed remarkable interspecies differences and led to challenges in safety testing. This study explored the kinetics of CC112273 formation from its precursor RP101075. Incubations with human liver mitochondrial fractions revealed K _Mapp, V _max, and intrinsic clearance (Cl_int) for CC112273 formation to be 4.8 μM, 50.3 pmol/min/mg protein, and 12 μl/min/mg, respectively, whereas Michaelis-Menten constant (K _M) with human recombinant MAO B was 1.1 μM. Studies with liver mitochondrial fractions from preclinical species led to K _Mapp, V _max, and Cl_int estimates of 3.0, 35, and 33 μM, 80.6, 114, 37.3 pmol/min/mg, and 27.2, 3.25, and 1.14 μl/min/mg in monkey, rat, and mouse, respectively, and revealed marked differences between rodents and primates, primarily attributable to differences in the K _M Comparison of Cl_int estimates revealed monkey to be ∼2-fold more efficient and the mouse and rat to be 11- and 4-fold less efficient than humans in CC112273 formation. The influence of stereochemistry on MAO B-mediated oxidation was also investigated using the R-isomer of RP101075 (RP101074). This showed marked selectivity toward catalysis of the S-isomer (RP101075) only. Docking into MAO B crystal structure suggested that although both the isomers occupied its active site, only the orientation of RP101075 presented the C-H on the α-carbon that was ideal for the C-H bond cleavage, which is a requisite for oxidative deamination. These studies explain the basis for the observed interspecies differences in the metabolism of ozanimod as well as the substrate stereospecificity for formation of CC112273. SIGNIFICANCE STATEMENT: This study evaluates the enzymology and the species differences of the major circulating metabolite of ozanimod, CC112273. Additionally, the study also explores the influence of stereochemistry on MAO B-catalyzed reactions. The study is of significance to the DMD readers given that this oxidation is catalyzed by a non-cytochrome P450 enzyme, and that marked species difference and notable stereospecificity was observed in MAO B-catalyzed biotransformation when the indaneamine enantiomers were used as substrates.

Coumarin-Chalcone Hybrids as Inhibitors of MAO-B: Biological Activity and In Silico Studies

Fourteen coumarin-derived compounds modified at the C3 carbon of coumarin with an α,β-unsaturated ketone were synthesized. These compounds may be designated as chalcocoumarins (3-cinnamoyl-2H-chromen-2-ones). Both chalcones and coumarins are recognized scaffolds in medicinal chemistry, showing diverse biological and pharmacological properties among which neuroprotective activities and multiple enzyme inhibition, including mitochondrial enzyme systems, stand out. The evaluation of monoamine oxidase B (MAO-B) inhibitors has aroused considerable interest as therapeutic agents for neurodegenerative diseases such as Parkinson's. Of the fourteen chalcocumarins evaluated here against MAO-B, ChC4 showed the strongest activity in vitro, with IC₅₀ = 0.76 ± 0.08 µM. Computational docking, molecular dynamics and MM/GBSA studies, confirm that ChC4 binds very stably to the active rMAO-B site, explaining the experimental inhibition data.

Evaluation of the inhibition of monoamine oxidase A by bioactive coffee compounds protecting serotonin degradation

Monoamine oxidase A (MAO-A) is a major enzyme responsible for the deamination of neurotransmitters such as serotonin (5-HT) in the central nervous system. The decrease in 5-HT levels is accompanied by disorders at the affective and somatic levels, leading to depression and disorders of the satiety center. The aim of this study was to evaluate the degree of MAO-A inhibition by chlorogenic acids, as well as green, light-, and dark-roasted coffee extracts and bioactive compounds from beans of the species Coffea canephora and Coffea arabica. Data for analysis was obtained using isothermal titration calorimetry and molecular docking. The results showed that caffeine and ferulic acid, as well as green Robusta coffee, demonstrated the greatest inhibition of MAO-A activity, which may increase the bioavailability of serotonin. We believe that green coffee shows potential antidepressant activity by inhibiting MAO-A, and may be used for treating depression and potentially, type 2 diabetes.

Effects of CATECHIN on reserpine-induced vacuous chewing movements: behavioral and biochemical analysis

This study evaluated the effect of (+)-catechin, a polyphenolic compound, on orofacial dyskinesia (OD) induced by reserpine in mice. The potential modulation of monoaminoxidase (MAO) activity, tyrosine hydroxylase (TH) and glutamic acid decarboxylase (GAD67) immunoreactivity by catechin were used as biochemical endpoints. The interaction of catechin with MAO-A and MAO-B was determined in vitro and in silico. The effects of catechin on OD induced by reserpine (1 mg/kg for 4 days, subcutaneously) in male Swiss mice were examined. After, catechin (10, 50 or 100 mg/kg, intraperitoneally) or its vehicle were given for another 20 days. On the 6th, 8th, 15th and 26th day, vacuous chewing movements (VCMs) and locomotor activity were quantified. Biochemical markers (MAO activity, TH and GAD67 immunoreactivity) were evaluated in brain structures. In vitro, catechin inhibited both MAO isoforms at concentrations of 0.34 and 1.03 mM being completely reversible for MAO-A and partially reversible for MAO-B. Molecular docking indicated that the catechin bound in the active site of MAO-A, while in the MAO-B it interacted with the surface of the enzyme in an allosteric site. In vivo, reserpine increased the VCMs and decreased the locomotor activity. Catechin (10 mg/kg), decreased the number of VCMs in the 8th day in mice pre-treated with reserpine without altering other behavioral response. Ex vivo, the MAO activity and TH and GAD67 immunoreactivity were not altered by the treatments. Catechin demonstrated a modest and transitory protective effect in a model of OD in mice.

Molecular Deceleration Regulates Toxicant Release to Prevent Cell Damage in Pseudomonas putida S16 (DSM 28022)

The underlying molecular mechanisms of flavin-dependent amine oxidases remain relatively poorly understood, even though many of these enzymes have been reported. The nicotine oxidoreductase NicA2 is a crucial enzyme for the first step of nicotine degradation in Pseudomonas putida S16 (DSM 28022). Here, we present the crystal structure of a ternary complex comprising NicA2 residues 21 to 482, flavin adenine dinucleotide (FAD), and nicotine at 2.25 Å resolution. Unlike other, related structures, NicA2 does not have an associated diacyl glycerophospholipid, wraps its substrate more tightly, and has an intriguing exit passage in which nine bulky amino acid residues occlude the release of its toxic product, pseudooxynicotine (PN). The replacement of these bulky residues by amino acids with small side chains effectively increases the catalytic turnover rate of NicA2. Our results indicate that the passage in wild-type NicA2 effectively controls the rate of PN release and thus prevents its rapid intracellular accumulation. It gives ample time for PN to be converted to less-harmful substances by downstream enzymes such as pseudooxynicotine amine oxidase (Pnao) before its accumulation causes cell damage or even death. The temporal metabolic regulation mode revealed in this study may shed light on the production of cytotoxic compounds.IMPORTANCE Flavin-dependent amine oxidases have received extensive attention because of their importance in drug metabolism, Parkinson's disease, and neurotransmitter catabolism. However, the underlying molecular mechanisms remain relatively poorly understood. Here, combining the crystal structure of NicA2 (an enzyme in the first step of the bacterial nicotine degradation pathway in Pseudomonas putida S16 (DSM 28022)), biochemical analysis, and mutant construction, we found an intriguing exit passage in which bulky amino acid residues occlude the release of the toxic product of NicA2, in contrast to other, related structures. The selective product exportation register for NicA2 has proven to be beneficial to cell growth. Those seeking to produce cytotoxic compounds could greatly benefit from the use of such an export register mechanism.

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.

Amphetamine Derivatives as Monoamine Oxidase Inhibitors

Amphetamine and its derivatives exhibit a wide range of pharmacological activities, including psychostimulant, hallucinogenic, entactogenic, anorectic, or antidepressant effects. The mechanisms of action underlying these effects are usually related to the ability of the different amphetamines to interact with diverse monoamine transporters or receptors. Moreover, many of these compounds are also potent and selective monoamine oxidase inhibitors. In the present work, we review how structural modifications on the aromatic ring, the amino group and/or the aliphatic side chain of the parent scaffold, modulate the enzyme inhibitory properties of hundreds of amphetamine derivatives. Furthermore, we discuss how monoamine oxidase inhibition might influence the pharmacology of these compounds.

Structure-Based Specific Detection and Inhibition of Monoamine Oxidases and Their Applications in Central Nervous System Diseases

Monoamine oxidases (MAOs) are the enzymes that catalyze the oxidation of monoamines, such as dopamine, norepinephrine, and serotonin, which serve as key neurotransmitters in the central nervous system (CNS). MAOs play important roles in maintaining the homeostasis of monoamines, and the aberrant expression or activation of MAOs underlies the pathogenesis of monoamine neurotransmitter disorders, including neuropsychiatric and neurodegenerative diseases. Clearly, detecting and inhibiting the activities of MAOs is of great value for the diagnosis and therapeutics of these diseases. Accordingly, many specific detection probes and inhibitors have been developed and substantially contributed to basic and clinical studies of these diseases. In this review, progress in the detecting and inhibiting of MAOs and their applications in mechanism exploration and treatment of neurotransmitter-related disorders is summarized. Notably, how the detection probes and inhibitors of MAOs were developed has been specifically addressed. It is hoped that this review will benefit the design of more effective and sensitive probes and inhibitors for MAOs, and eventually the treatment of monoamine neurotransmitter disorders.

An amine oxidase gene from mud crab, Scylla paramamosain, regulates the neurotransmitters serotonin and dopamine in vitro

Amine oxidase, which participates in the metabolic processing of biogenic amines, is widely found in organisms, including higher organisms and various microorganisms. In this study, the full-length cDNA of a novel amine oxidase gene was cloned from the mud crab, Scylla paramamosain, and termed SpAMO. The cDNA sequence was 2,599 bp in length, including an open reading frame of 1,521 bp encoding 506 amino acids. Two amino acid sequence motifs, a flavin adenine dinucleotide-binding domain and a flavin-containing amine oxidoreductase, were highly conserved in SpAMO. A quantitative real-time polymerase chain reaction analysis showed that the expression level of SpAMO after quercetin treatment was time- and concentration-dependent. The expression of SpAMO tended to decrease and then increase in the brain and haemolymph after treatment with 5 mg/kg/d quercetin; after treatment with 50 mg/kg/d quercetin, the expression of SpAMO declined rapidly and remained low in the brain and haemolymph. These results indicated that quercetin could inhibit the transcription of SpAMO, and the high dose (50 mg/kg/d) had a relatively significant inhibitory effect. SpAMO showed the highest catalytic activity on serotonin, followed by dopamine, β-phenylethylamine, and spermine, suggesting that the specific substrates of SpAMO are serotonin and dopamine. A bioinformatics analysis of SpAMO showed that it has molecular characteristics of spermine oxidase, but a quercetin test and enzyme activity study indicated that it also functions like monoamine oxidase. It is speculated that SpAMO might be a novel amine oxidase in S. paramamosain that has the functions of both spermine oxidase and monoamine oxidase.

The structure of monoamine oxidases: past, present, and future

The first crystal structure of mammalian monoamine oxidases (MAOs) was solved in 2002; almost 65 years after, these FAD-dependent enzymes were discovered and classified as responsible for the oxidation of aromatic neurotransmitters. Both MAO A and MAO B feature a two-domain topology characterized by the Rossmann fold, interacting with dinucleotide cofactors, which is intimately associated to a substrate-binding domain. This globular body is endowed with a C-terminal α-helix that anchors the protein to the outer mitochondrial phospholipid bilayer. As monotopic membrane proteins, the structural elucidation of MAOs was a challenging task that required the screening of different detergent conditions for their purification and crystallization. MAO A and MAO B structures differ both in their oligomerization architecture and in details of their active sites. Purified human MAO B and rat MAO A are dimeric, whereas human MAO A was found to be monomeric, which is believed to result from the detergent treatments used to extract the protein from the membrane. The active site of MAOs consists of a hydrophobic cavity located in front of the flavin cofactor and extending to the protein surface. Some structural features are highly conserved in the two isozymes, such as a Tyr-Tyr aromatic sandwich in front of the flavin ring and a Lys residue hydrogen-bonded to the cofactor N5 atom, whereas a pair of gating residues (Phe208/Ile335 in MAO A; Ile199/Tyr326 in MAO B) specifically determines the different substrate and inhibitor properties of the two enzymes.

Monoamine Oxidases

Monoamine oxidases A and B (MAO A and B) are mammalian flavoenzymes bound to the outer mitochondrial membrane. They were discovered almost a century ago and they have been the subject of many biochemical, structural and pharmacological investigations due to their central role in neurotransmitter metabolism. Currently, the treatment of Parkinson's disease involves the use of selective MAO B inhibitors such as rasagiline and safinamide. MAO inhibition was shown to exert a general neuroprotective effect as a result of the reduction of oxidative stress produced by these enzymes, which seems to be relevant also in non-neuronal contexts. MAOs were successfully expressed as recombinant proteins in Pichia pastoris, which allowed a thorough biochemical and structural characterization. These enzymes are characterized by a globular water-soluble main body that is anchored to the mitochondrial membrane through a C-terminal α-helix, similar to other bitopic membrane proteins. In both MAO A and MAO B the enzyme active site consists of a hydrophobic cavity lined by residues that are conserved in the two isozymes, except for few details that determine substrate and inhibitor specificity. In particular, human MAO B features a dual-cavity active site whose conformation depends on the size of the bound ligand. This article provides a comprehensive and historical review of MAOs and the state-of-the-art of these enzymes as membrane drug targets.

3D similarities between the binding sites of monoaminergic target proteins

The study of binding site similarities can be relevant to understand the interaction of different drugs at several molecular targets. The increasing availability of protein crystal structures and the development of novel algorithms designed to evaluate three-dimensional similarities, represent a great opportunity to explore the existence of electronic and shape features shared by clinically relevant proteins, which could assist drug design and discovery. Proteins involved in the recognition of monoaminergic neurotransmitters, such as monoamine transporters or monoamine oxidases (MAO) have been related to several psychiatric and neurological disorders such as depression or Parkinson’s disease. In this work, we evaluated the possible existence of similarities among the binding sites of the serotonin transporter (SERT), the dopamine transporter (DAT), MAO-A and MAO-B. This study was carried out using molecular simulation methodologies linked to the statistical algorithm PocketMatch, which was modified in order to obtain similarities profiles. Our results show that DAT and SERT exhibit a high degree of 3-D similarities all along the pathway that is presumably involved in the substrate transport process. Distinct differences, on the other hand, were found both at the extracellular and the intracellular ends of the transporters, which might be involved in the selective initial recognition of the corresponding substrate. Similarities were also found between the active (catalytic) site of MAO-A and the extracellular vestibule of SERT (the S2 binding site). These results suggest some degree of structural convergence for these proteins, which have different functions, tissue distribution and genetic origin, but which share the same endogenous ligand (serotonin). Beyond the functional implications, these findings are valuable for the design of both selective and non-selective ligands.

Tuning Vector Stability and Integration Frequency Elevates Functional GPCR Production and Homogeneity in Saccharomyces cerevisiae

Membrane proteins play a valuable role in biotechnology, yet the difficulty of producing high yields of functional membrane protein limits their use in synthetic biology. The practical application of G protein-coupled receptors in whole cell biosensors, for example, is restricted to those that are functionally produced at the cell surface in the chosen host, limiting the range of detectable molecules. Here, we present a facile approach to significantly improve the yield and homogeneity of functional membrane proteins in Saccharomyces cerevisiae by altering only the choice of expression vector. Expression of a model GPCR, the human adenosine A₂a receptor, from commonly used centromeric and episomal vectors leads to low yields and cellular heterogeneity due to plasmid loss in 20-90% of the cell population. In contrast, homogeneous production of GPCR is attained using a multisite integrating vector or a novel, modified high copy vector that does not require genomic integration or addition of any selection agents. Finally, we introduce a FACS-based screen, which enables rapid isolation of cells with 4- to 15-fold increases in gene dosage and up to a 9-fold increase in functional protein yield without loss of homogeneity compared to a strain isolated through conventional, low-throughput methods. These results can be extended to improve the cellular homogeneity and yield of other membrane proteins, expanding the repertoire of useful receptors for synthetic biology applications.

Synthesis and Biological Evaluation of New Thiosemicarbazone Derivative Schiff Bases as Monoamine Oxidase Inhibitory Agents

Twenty-six novel thiosemicarbazone derivative B1–B26 were synthesized via condensation reactions between the corresponding thiosemicarbazides and aldehydes. The chemical characterization of the compounds was carried out by infrared (IR), mass (MS), proton and carbon nuclear magnetic resonance (¹H- and ¹³C-NMR) spectroscopic analyses. The compounds were investigated for their monoamine oxidase A (MAO-A) and monoamine oxidase B (MAO-B) inhibitory activity and most of them were more potent against MAO-A enzyme when compared with MAO-B enzyme. N-Cyclohexyl-2-[4-[(4-chlorophenyl)thio]benzylidene]hydrazine-1-carbothioamide (B24) was the most active compound against MAO-A. The enzyme kinetics study revealed that compound B24 has a reversible and competitive mode of binding. Interaction modes between compound B24 and MAO-A were clarified by docking studies. In addition, the favourable absorption, distribution, metabolism, and excretion (ADME) properties and non-toxic nature of compound B24 make this compound a promising MAO-A inhibitor.

Synthesis, monoamine oxidase inhibition activity and molecular docking studies of novel 4-hydroxy-N'-[benzylidene or 1-phenylethylidene]-2-H/methyl/benzyl-1,2-benzothiazine-3-carbohydrazide 1,1-dioxides

Three series of 4-hydroxy-N^'-[benzylidene/1-phenylethylidene]-2-H/methyl/benzyl-1,2-benzothiazine-3-carbohydrazide 1,1-dioxides (9-11)a-l were synthesized and unraveled to be highly potent dual inhibitors of monoamine oxidases (MAO-A and MAO-B). All the examined compounds demonstrated IC₅₀ values in lower micro-molar range for both MAO-A as well as MAO-B. The most active MAO-A inhibitor was 4-hydroxy-N'-(1-phenylethylidene)-2H-benzo[e][1,2]thiazine-3-carbohydrazide 1,1-dioxide (9i) with an IC₅₀ value of 0.11 ± 0.005 μM, whereas, methyl 4-hydroxy-2H-benzo[e][1,2]thiazine-3-carboxylate 1,1-dioxide (3) was the most active MAO-B inhibitor with an IC₅₀ value of 0.21 ± 0.01 μM. Enzyme kinetics studies revealed that the most potent compounds inhibited both MAO enzymes (A & B) in a competitive fashion. Molecular docking studies were also performed to obtain an intuitive picture of inhibition potential for potent inhibitors. The high potency of these compounds is optimally combined with highly favorable ADME profile with predicted good oral bioavailability.

Synthesis of New Hydrazone Derivatives for MAO Enzymes Inhibitory Activity

In the present work, 14 new 1-substituted-2-phenylhydrazone derivatives were synthesized to evaluate their inhibitory activity against hMAO enzymes. The structures of the newly synthesized hydrazones 2a–2n were characterized by IR, ¹H-NMR, ¹³C-NMR, HR-MS spectroscopic methods. The inhibitory activity of compounds 2a–2n against hMAO-A and hMAO-B enzymes was elucidated by using an in-vitro Amplex Red^® reagent assay based on fluorometric methods. According to the activity studies, 2a and 2b were found to be the most active compounds against hMAO-A enzyme, with IC₅₀ values of 0.342 µM and 0.028 µM, respectively. The most active compounds 2a–2b were evaluated by means of enzyme kinetics and docking studies. Moreover, these compounds were subjected to cytotoxicity and genotoxicity tests to establish their preliminary toxicological profiles and were found to be non-cytotoxic and non-genotoxic. Consequently, the findings of this study display the biological importance of compounds 2a, 2b as selective, irreversible and competitive inhibitors of hMAO-A. Docking studies revealed that there is a strong interaction between hMAO-A and the most active compound 2b.

Assessment of Enzyme Inhibition: A Review with Examples from the Development of Monoamine Oxidase and Cholinesterase Inhibitory Drugs

The actions of many drugs involve enzyme inhibition. This is exemplified by the inhibitors of monoamine oxidases (MAO) and the cholinsterases (ChE) that have been used for several pharmacological purposes. This review describes key principles and approaches for the reliable determination of enzyme activities and inhibition as well as some of the methods that are in current use for such studies with these two enzymes. Their applicability and potential pitfalls arising from their inappropriate use are discussed. Since inhibitor potency is frequently assessed in terms of the quantity necessary to give 50% inhibition (the IC50 value), the relationships between this and the mode of inhibition is also considered, in terms of the misleading information that it may provide. Incorporation of more than one functionality into the same molecule to give a multi-target-directed ligands (MTDLs) requires careful assessment to ensure that the specific target effects are not significantly altered and that the kinetic behavior remains as favourable with the MTDL as it does with the individual components. Such factors will be considered in terms of recently developed MTDLs that combine MAO and ChE inhibitory functions.

Synthesis and evaluation of small molecules bearing a benzyloxy substituent as novel and potent monoamine oxidase inhibitors

A new series of small molecules bearing a benzyloxy substituent have been designed, synthesized and evaluated for hMAO inhibitory activity in vitro. Most of the compounds were potent and selective MAO-B inhibitors, and were weak inhibitors of MAO-A. In particular, compounds 9e (IC₅₀ = 0.35 μM) and 10e (IC₅₀ = 0.19 μM) were the most potent MAO-B inhibitors, and exhibited the highest selectivity for MAO-B (9e, SI > 285.7-fold and 10e, SI = 146.8-fold). In addition, the structure-activity relationships for MAO-B inhibition indicated that electron-withdrawing groups in the open small molecules were more suitable for MAO-B inhibition, and substitutions at the benzyloxy of the open small molecules, particularly with the halogen substituted benzyloxy, were more favorable for MAO-B inhibition. Molecular docking studies have been done to explain the potent MAO-B inhibition of the open small molecules. Furthermore, the representative compounds 9e and 10e showed low neurotoxicity in SH-SY5Y cells in vitro. So the small molecules bearing the benzyloxy substituent could be used to develop promising drug candidates for the therapy of neurodegenerative diseases.

A Green and Convenient Route for the Regioselective Synthesis of New Substituted 3-Aryl-5H-indeno[1,2-c]pyridazines as Potential Monoamine Oxidase Type A Inhibitors

Several indeno[1,2-c]pyridazines were efficiently synthesised using the one-pot, three-component reaction of substituted indanones, arylglyoxalmonohydrates, and hydrazine in the presence of 1,5-diazabicyclo[4,3,0]non-5-ene (DBN) in water at room temperature. These substituted 3-aryl indeno[1,2-c]pyridazines can be considered as potential monoamine oxidase type A (MAOA) inhibitors. The advantages of this new strategy are the novelty of the indenopyridazine derivatives, high regioselectivity, use of water as the solvent, no requirement for toxic metal catalysts, and good to excellent yields.