Insights into the mechanism and inhibition of fatty acid amide hydrolase from quantum mechanics/molecular mechanics (QM/MM) modelling

Impact of the Recognition Part of Dipeptidyl Nitroalkene Compounds on the Inhibition Mechanism of Cysteine Proteases Cruzain and Cathepsin L

Cysteine proteases (CPs) are an important class of enzymes, many of which are responsible for several human diseases. For instance, cruzain of protozoan parasite Trypanosoma cruzi is responsible for the Chagas disease, while the role of human cathepsin L is associated with some cancers or is a potential target for the treatment of COVID-19. However, despite paramount work carried out during the past years, the compounds that have been proposed so far show limited inhibitory action against these enzymes. We present a study of proposed covalent inhibitors of these two CPs, cruzain and cathepsin L, based on the design, synthesis, kinetic measurements, and QM/MM computational simulations on dipeptidyl nitroalkene compounds. The experimentally determined inhibition data, together with the analysis and the predicted inhibition constants derived from the free energy landscape of the full inhibition process, allowed describing the impact of the recognition part of these compounds and, in particular, the modifications on the P2 site. The designed compounds and, in particular, the one with a bulky group (Trp) at the P2 site show promising in vitro inhibition activities against cruzain and cathepsin L for use as a starting lead compound in the development of drugs with medical applications for the treatment of human diseases and future designs.

Synthesis, Molecular Modeling and Biological Evaluation of Metabolically Stable Analogues of the Endogenous Fatty Acid Amide Palmitoylethanolamide

Palmitoylethanolamide (PEA) belongs to the class of N-acylethanolamine and is an endogenous lipid potentially useful in a wide range of therapeutic areas; products containing PEA are licensed for use in humans as a nutraceutical, a food supplement, or food for medical purposes for its analgesic and anti-inflammatory properties demonstrating efficacy and tolerability. However, the exogenously administered PEA is rapidly inactivated; in this process, fatty acid amide hydrolase (FAAH) plays a key role both in hepatic metabolism and in intracellular degradation. So, the aim of the present study was the design and synthesis of PEA analogues that are more resistant to FAAH-mediated hydrolysis. A small library of PEA analogues was designed and tested by molecular docking and density functional theory calculations to find the more stable analogue. The computational investigation identified RePEA as the best candidate in terms of both synthetic accessibility and metabolic stability to FAAH-mediated hydrolysis. The selected compound was synthesized and assayed ex vivo to monitor FAAH-mediated hydrolysis and to confirm its anti-inflammatory properties. ¹H-NMR spectroscopy performed on membrane samples containing FAAH in integral membrane protein demonstrated that RePEA is not processed by FAAH, in contrast with PEA. Moreover, RePEA retains PEA’s ability to inhibit LPS-induced cytokine release in both murine N9 microglial cells and human PMA-THP-1 cells.

Mechanism of inhibition of SARS-CoV-2 Mpro by peptidyl Michael acceptor explained by QM/MM simulations and design of new derivatives with tunable chemical reactivity

The SARS-CoV-2 main protease (M^pro) is essential for replication of the virus responsible for the COVID-19 pandemic, and one of the main targets for drug design. Here, we simulate the inhibition process of SARS-CoV-2 M^pro with a known Michael acceptor (peptidyl) inhibitor, N3. The free energy landscape for the mechanism of the formation of the covalent enzyme-inhibitor product is computed with QM/MM molecular dynamics methods. The simulations show a two-step mechanism, and give structures and calculated barriers in good agreement with experiment. Using these results and information from our previous investigation on the proteolysis reaction of SARS-CoV-2 M^pro, we design two new, synthetically accessible N3-analogues as potential inhibitors, in which the recognition and warhead motifs are modified. QM/MM modelling of the mechanism of inhibition of M^pro by these novel compounds indicates that both may be promising candidates as drug leads against COVID-19, one as an irreversible inhibitor and one as a potential reversible inhibitor.

Design and SAR Analysis of Covalent Inhibitors Driven by Hybrid QM/MM Simulations

Quantum mechanics/molecular mechanics (QM/MM) hybrid technique is emerging as a reliable computational method to investigate and characterize chemical reactions occurring in enzymes. From a drug discovery perspective, a thorough understanding of enzyme catalysis appears pivotal to assist the design of inhibitors able to covalently bind one of the residues belonging to the enzyme catalytic machinery. Thanks to the current advances in computer power, and the availability of more efficient algorithms for QM-based simulations, the use of QM/MM methodology is becoming a viable option in the field of covalent inhibitor design. In the present review, we summarized our experience in the field of QM/MM simulations applied to drug design problems which involved the optimization of agents working on two well-known drug targets, namely fatty acid amide hydrolase (FAAH) and epidermal growth factor receptor (EGFR). In this context, QM/MM simulations gave valuable information in terms of geometry (i.e., of transition states and metastable intermediates) and reaction energetics that allowed to correctly predict inhibitor binding orientation and substituent effect on enzyme inhibition. What is more, enzyme reaction modelling with QM/MM provided insights that were translated into the synthesis of new covalent inhibitor featured by a unique combination of intrinsic reactivity, on-target activity, and selectivity.

A perspective review on fatty acid amide hydrolase (FAAH) inhibitors as potential therapeutic agents

Fatty acid amide hydrolase (FAAH) is an important enzyme creditworthy of hydrolyzing endocannabinoids and related-amidated signalling lipids, discovery of which has pioneered novel arena of pharmacological canvasses to unwrap its curative potency in various diseased circumstances. It presents contemporary basis for understanding molecules regulating and mediating inflammatory reactions, pain, anxiety, depression, and neurodegeneration. FAAH inhibitors form vital approach for discovery of therapeutic agents that are concerned with local elevation of endocannabinoids under certain stimuli, debarring adverse/unwanted secondary effects from global activation of cannabinoid receptors by exogenous cannabimimetics. During past decades, several molecules with excellent potency developed through tailor-made approaches entered into clinical trials, but none could reach market. Hence, hunt for novel, non-toxic and selective FAAH inhibitors are on horizon. This review summarizes present perception on FAAH in conjunction with its structure, mechanism of catalysis and biological functions. It also foregrounds recent development of molecules belonging to diverse chemical classes as potential FAAH inhibitors bobbing up from in-depth chemical, mechanistic and computational studies published since 2015-November 2019, focusing on their potency. This review will assist readers to obtain rationale on FAAH as potential target for addressing various disease conditions, acquiring significant knowledge on recently established inhibitor scaffolds and their development potentials. New technologies including MD-MM simulations and 3D-QSAR studies allow mechanistic characterization of enzyme. Assessment of in-vitro and in-vivo efficacy of existing FAAH inhibitors will facilitate researchers to design novel ligands utilizing modern drug design methods. The discussions will also impose precaution in decision making process, quashing possibility of late stage failure.

Fatty acid amide hydrolase inhibitors: a patent review (2009-2014)

INTRODUCTION

Fatty acid amide hydrolase (FAAH) is a key enzyme responsible for the degradation of the endocannabinoid anandamide. FAAH inactivation is emerging as a strategy to treat several CNS and peripheral diseases, including inflammation and pain. The search for effective FAAH inhibitors has thus become a key focus in present drug discovery.

AREAS COVERED

Patents and patent applications published from 2009 to 2014 in which novel chemical classes are claimed to inhibit FAAH.

EXPERT OPINION

FAAH is a promising target for treating many disease conditions including pain, inflammation and mood disorders. In the last few years, remarkable efforts have been made to develop new FAAH inhibitors (either reversible and irreversible) characterized by excellent potency and selectivity, to complete the arsenal of tools for modulating FAAH activity. The failure of PF-04457845 in a Phase II study on osteoarthritis pain has not flattened the interest in FAAH inhibitors. New clinical trials on 'classical' FAAH inhibitors are now ongoing, and new strategies based on compounds with peculiar in vivo distribution (e.g., peripheral) or with multiple pharmacological activities (e.g., FAAH and COX) are under investigation and could boost the therapeutic potential of this class in the next future.

Keys to Lipid Selection in Fatty Acid Amide Hydrolase Catalysis: Structural Flexibility, Gating Residues and Multiple Binding Pockets

The fatty acid amide hydrolase (FAAH) regulates the endocannabinoid system cleaving primarily the lipid messenger anandamide. FAAH has been well characterized over the years and, importantly, it represents a promising drug target to treat several diseases, including inflammatory-related diseases and cancer. But its enzymatic mechanism for lipid selection to specifically hydrolyze anandamide, rather than similar bioactive lipids, remains elusive. Here, we clarify this mechanism in FAAH, examining the role of the dynamic paddle, which is formed by the gating residues Phe432 and Trp531 at the boundary between two cavities that form the FAAH catalytic site (the “membrane-access” and the “acyl chain-binding” pockets). We integrate microsecond-long MD simulations of wild type and double mutant model systems (Phe432Ala and Trp531Ala) of FAAH, embedded in a realistic membrane/water environment, with mutagenesis and kinetic experiments. We comparatively analyze three fatty acid substrates with different hydrolysis rates (anandamide > oleamide > palmitoylethanolamide). Our findings identify FAAH’s mechanism to selectively accommodate anandamide into a multi-pocket binding site, and to properly orient the substrate in pre-reactive conformations for efficient hydrolysis that is interceded by the dynamic paddle. Our findings therefore endorse a structural framework for a lipid selection mechanism mediated by structural flexibility and gating residues between multiple binding cavities, as found in FAAH. Based on the available structural data, this exquisite catalytic strategy for substrate specificity seems to be shared by other lipid-degrading enzymes with similar enzymatic architecture. The mechanistic insights for lipid selection might assist de-novo enzyme design or drug discovery efforts.

We describe a new structural enzymatic framework to regulate substrate specificity in lipid-degrading enzymes such as fatty acid amide hydrolase (FAAH), a key enzyme for the endocannabinoid lipid signaling that hydrolyzes a variety of lipids, however with different catalytic rates. The identified novel mechanism and key features for lipid selection in FAAH are then analysed in the context of other relevant lipid-degrading enzymes. Through the integration of microsecond-long molecular dynamics simulations with mutagenesis and kinetic experiments, our study suggests that structural flexibility, gating residues and multiple cavities in one catalytic site are keys to lipid selection in the endocannabinoid system. Our results suggest that the structural framework proposed here could likely be a general enzymatic strategy of other lipid-degrading enzymes to select the preferred lipid substrate within a broad spectrum of biologically active lipids. This new, and likely general, structural framework for lipid selection in FAAH could therefore now encourage additional experimental verifications of the role of ligand and structural flexibility, as regulated by key gating residues at the boundaries of multiple cavities forming a single catalytic site, as observed in several other lipid-degrading enzymes.

Physical Nature of Fatty Acid Amide Hydrolase Interactions with Its Inhibitors: Testing a Simple Nonempirical Scoring Model

Fatty acid amide hydrolase (FAAH) is an enzyme responsible for the deactivating hydrolysis of fatty acid ethanolamide neuromodulators. FAAH inhibitors have gained considerable interest due to their possible application in the treatment of anxiety, inflammation, and pain. In the context of inhibitor design, the availability of reliable computational tools for predicting binding affinity is still a challenging task, and it is now well understood that empirical scoring functions have several limitations that in principle could be overcome by quantum mechanics. Herein, systematic ab initio analyses of FAAH interactions with a series of inhibitors belonging to the class of the N-alkylcarbamic acid aryl esters have been performed. In contrast to our earlier studies of other classes of enzyme-inhibitor complexes, reasonable correlation with experimental results required us to consider correlation effects along with electrostatic term. Therefore, the simplest comprehensive nonempirical model allowing for qualitative predictions of binding affinities for FAAH ligands consists of electrostatic multipole and second-order dispersion terms. Such a model has been validated against the relative stabilities of the benchmark S66 set of biomolecular complexes. As it does not involve parameters fitted to experimentally derived data, this model offers a unique opportunity for generally applicable inhibitor design and virtual screening.

Computational insights into function and inhibition of fatty acid amide hydrolase

The Fatty Acid Amide Hydrolase (FAAH) enzyme is a membrane-bound serine hydrolase responsible for the deactivating hydrolysis of a family of naturally occurring fatty acid amides. FAAH is a critical enzyme of the endocannabinoid system, being mainly responsible for regulating the level of its main cannabinoid substrate anandamide. For this reason, pharmacological inhibition of FAAH, which increases the level of endogenous anandamide, is a promising strategy to cure a variety of diseases including pain, inflammation, and cancer. Much structural, mutagenesis, and kinetic data on FAAH has been generated over the last couple of decades. This has prompted several informative computational investigations to elucidate, at the atomic-level, mechanistic details on catalysis and inhibition of this pharmaceutically relevant enzyme. Here, we review how these computational studies - based on classical molecular dynamics, full quantum mechanics, and hybrid QM/MM methods - have clarified the binding and reactivity of some relevant substrates and inhibitors of FAAH. We also discuss the experimental implications of these computational insights, which have provided a thoughtful elucidation of the complex physical and chemical steps of the enzymatic mechanism of FAAH. Finally, we discuss how computations have been helpful for building structure-activity relationships of potent FAAH inhibitors.

Optimization of an elastic network augmented coarse grained model to study CCMV capsid deformation

The major protective coat of most viruses is a highly symmetric protein capsid that forms spontaneously from many copies of identical proteins. Structural and mechanical properties of such capsids, as well as their self-assembly process, have been studied experimentally and theoretically, including modeling efforts by computer simulations on various scales. Atomistic models include specific details of local protein binding but are limited in system size and accessible time, while coarse grained (CG) models do get access to longer time and length scales but often lack the specific local interactions. Multi-scale models aim at bridging this gap by systematically connecting different levels of resolution. Here, a CG model for CCMV (Cowpea Chlorotic Mottle Virus), a virus with an icosahedral shell of 180 identical protein monomers, is developed, where parameters are derived from atomistic simulations of capsid protein dimers in aqueous solution. In particular, a new method is introduced to combine the MARTINI CG model with a supportive elastic network based on structural fluctuations of individual monomers. In the parametrization process, both network connectivity and strength are optimized. This elastic-network optimized CG model, which solely relies on atomistic data of small units (dimers), is able to correctly predict inter-protein conformational flexibility and properties of larger capsid fragments of 20 and more subunits. Furthermore, it is shown that this CG model reproduces experimental (Atomic Force Microscopy) indentation measurements of the entire viral capsid. Thus it is shown that one obvious goal for hierarchical modeling, namely predicting mechanical properties of larger protein complexes from models that are carefully parametrized on elastic properties of smaller units, is achievable.

Quantum mechanics/molecular mechanics modeling of fatty acid amide hydrolase reactivation distinguishes substrate from irreversible covalent inhibitors

Carbamate and urea derivatives are important classes of fatty acid amide hydrolase (FAAH) inhibitors that carbamoylate the active-site nucleophile Ser241. In the present work, the reactivation mechanism of carbamoylated FAAH is investigated by means of a quantum mechanics/molecular mechanics (QM/MM) approach. The potential energy surfaces for decarbamoylation of FAAH covalent adducts, derived from the O-aryl carbamate URB597 and from the N-piperazinylurea JNJ1661610, were calculated and compared to that for deacylation of FAAH acylated by the substrate oleamide. Calculations show that a carbamic group bound to Ser241 prevents efficient stabilization of transition states of hydrolysis, leading to large increments in the activation barrier. Moreover, the energy barrier for the piperazine carboxylate was significantly lower than that for the cyclohexyl carbamate derived from URB597. This is consistent with experimental data showing slowly reversible FAAH inhibition for the N-piperazinylurea inhibitor and irreversible inhibition for URB597.

Wagging the Tail: Essential Role of Substrate Flexibility in FAAH Catalysis

The serine hydrolase, fatty acid amide hydrolase (FAAH), is responsible for the intracellular degradation of anandamide and other bioactive fatty acid ethanolamides involved in the regulation of pain, inflammation, and other pathophysiological processes. The catalytic site of FAAH is composed of multiple cavities with mixed hydrophobic and hydrophilic properties, the role of which remains incompletely understood. Anandamide is thought to enter the active site through a "membrane-access" (MA) channel and position its flexible fatty acyl chain in a highly hydrophobic "acyl chain-binding" (AB) cavity to allow for hydrolysis to occur. Using microsecond molecular dynamics (MD) simulations of FAAH embedded in a realistic membrane/water environment, we show now that anandamide may not lock itself into the AB cavity but may rather assume catalytically significant conformations required for hydrolysis by moving its flexible arachidonoyl tail between the MA and AB cavities. This process is regulated by a phenylalanine residue (Phe432) located at the boundary between the two cavities, which may act as a "dynamic paddle." The results identify structural flexibility as a key determinant by which FAAH recognizes its primary lipid substrate.

Molecular dynamics simulations in drug design

This minireview focuses on recent developments in the application of molecular dynamics to drug design. Recent applications of endpoint free-energy computational methods such as molecular mechanics Poisson-Boltzmann surface area (MM-PBSA) and generalized Born surface area (MM-GBSA) and linear response methods are described. Recent progress in steered molecular dynamics applied to drug design is reviewed.

The increasing role of QM/MM in drug discovery

Since its first appearance in 1976, the quantum mechanics/molecular mechanics (QM/MM) approach has mostly been used to study the chemical reactions of enzymes, which are frequently the target of drug discovery programs. In principle, a detailed understanding of the enzymatic mechanism should help researchers to design a potent enzyme inhibitor or new drug. However, QM/MM has not yet had a widespread impact on structure-based drug design. This is mostly due to its high computational cost. We expect this to change with the recent and extraordinary increases in computational power, and with the availability of more efficient algorithms for QM/MM calculations. Here, we report on some representative examples of QM/MM studies, including our own research, of pharmaceutically relevant enzymes, such as ribonuclease H and fatty acid amide hydrolase (FAAH). We aim to show how QM/MM has traditionally been used to study enzymatic catalysis. In this regard, we discuss its potential to become a routinely used drug design tool. To support this, we also discuss selected computational studies where QM/MM insights have been helpful in improving the potency of covalent inhibitors of FAAH.

Covalent inhibitors of fatty acid amide hydrolase: a rationale for the activity of piperidine and piperazine aryl ureas

Recently, covalent drugs have attracted great interest in the drug discovery community, with successful examples that have demonstrated their therapeutic effects. Here, we focus on the covalent inhibition of the fatty acid amide hydrolase (FAAH), which is a promising strategy in the treatment of pain and inflammation. Among the most recent and potent FAAH inhibitors (FAAHi), there are the cyclic piperidine and piperazine aryl ureas. FAAH hydrolyzes efficiently the amide bond of these compounds, forming a covalent enzyme-inhibitor adduct. To rationalize this experimental evidence, we performed an extensive computational analysis centered on piperidine-based PF750 (1) and piperazine-based JNJ1661010 (2), two potent lead compounds used to generate covalent inhibitors as clinical candidates. We found that FAAH induces a distortion of the amide bond of the piperidine and piperazine aryl ureas. Quantum mechanics/molecular mechanics ΔE(LUMO-HOMO) energies indicate that the observed enzyme-induced distortion of the amide bond favors the formation of a covalent FAAH-inhibitor adduct. These findings could help in the rational structure-based design of novel covalent FAAHi.

Application of a SCC-DFTB QM/MM approach to the investigation of the catalytic mechanism of fatty acid amide hydrolase

Self-consistent charge density functional tight binding (SCC-DFTB) is a promising method for hybrid quantum mechanics/molecular mechanics (QM/MM) simulations of enzyme-catalyzed reactions. The acylation reaction of fatty acid amide hydrolase (FAAH), a promising drug target, was investigated by applying a SCC-DFTB/CHARMM27 scheme. Calculated potential energy barriers resulted in reasonable agreement with experiments for oleamide (OA) and oleoylmethyl ester (OME) substrates, outperforming previous calculations performed at the PM3/CHARMM22 level. Furthermore, the experimental preference of FAAH in hydrolyzing OA faster than OME was adequately reproduced by calculations. All these findings indicate that the SCC-DFTB/CHARMM27 approach can be successfully applied to mechanistic investigations of FAAH-catalyzed reactions.

Structural Fluctuations in Enzyme-Catalyzed Reactions: Determinants of Reactivity in Fatty Acid Amide Hydrolase from Multivariate Statistical Analysis of Quantum Mechanics/Molecular Mechanics Paths

The effects of structural fluctuations, due to protein dynamics, on enzyme activity are at the heart of current debates on enzyme catalysis. There is evidence that fatty acid amide hydrolase (FAAH) is an enzyme for which reaction proceeds via a high-energy, reactive conformation, distinct from the predominant enzyme-substrate complex (Lodola et al. Biophys. J. 2007, 92, L20-22). Identifying the structural causes of differences in reactivity between conformations in such complex systems is not trivial. Here, we show that multivariate analysis of key structural parameters can identify structural determinants of barrier height by analysis of multiple reaction paths. We apply a well-tested quantum mechanics/molecular mechanics (QM/MM) method to the first step of the acylation reaction between FAAH and oleamide substrate for 36 different starting structures. Geometrical parameters (consisting of the key bond distances that change during the reaction) were collected and used for principal component analysis (PCA), partial least-squares (PLS) regression analysis, and multiple linear regression (MLR) analysis. PCA indicates that different "families" of enzyme-substrate conformations arise from QM/MM molecular dynamics simulation and that rarely sampled, catalytically significant conformational states can be identified. PLS and MLR analyses allowed the construction of linear regression models, correlating the calculated activation barriers with simple geometrical descriptors. These analyses reveal the presence of two fully independent geometrical effects, explaining 78% of the variation in the activation barrier, which are directly correlated with transition-state stabilization (playing a major role in catalysis) and substrate binding. These results highlight the power of statistical approaches of this type in identifying crucial structural features that contribute to enzyme reactivity.

3-Heterocycle-phenyl N-alkylcarbamates as FAAH inhibitors: design, synthesis and 3D-QSAR studies

Carbamates are a well-established class of fatty acid amide hydrolase (FAAH) inhibitors. Here we describe the synthesis of meta-substituted phenolic N-alkyl/aryl carbamates and their in vitro FAAH inhibitory activities. The most potent compound, 3-(oxazol-2yl)phenyl cyclohexylcarbamate (2 a), inhibited FAAH with a sub-nanomolar IC(50) value (IC(50)=0.74 nM). Additionally, we developed and validated three-dimensional quantitative structure-activity relationships (QSAR) models of FAAH inhibition combining the newly disclosed carbamates with our previously published inhibitors to give a total set of 99 compounds. Prior to 3D-QSAR modeling, the degree of correlation between FAAH inhibition and in silico reactivity was also established. Both 3D-QSAR methods used, CoMSIA and GRID/GOLPE, produced statistically significant models with coefficient of correlation for external prediction (R(2) (PRED)) values of 0.732 and 0.760, respectively. These models could be of high value in further FAAH inhibitor design.

Enzyme mechanisms: fast reaction and computational approaches

Now, more than ever, enzymology and its development can be considered of vital importance to the progression of the biological sciences. With an increase in the numbers of enzymes being identified from genomic studies, enzymology is key to defining the structural and functional properties of these enzymes in order to establish their mechanisms of action and how they fit into metabolic networks. Along with the efforts of the bioinformaticians and systems biologists, such studies will ultimately lead to detailed descriptions of intricate biochemical pathways and allow identification of the most appropriate target enzymes for intervention in disease therapy. Thus the timing for the recent Biochemical Society Focused Meeting entitled 'Enzyme Mechanisms: Fast Reaction and Computational Approaches' was highly appropriate. The present paper represents an overview of this meeting, which was held at the Manchester Interdisciplinary Biocentre on 9-10 October 2008.