Structural adaptations in a membrane enzyme that terminates endocannabinoid signaling

Optimization of alpha-ketooxazole inhibitors of fatty acid amide hydrolase

A series of alpha-ketooxazoles containing conformational constraints in the flexible C2 acyl side chain of 2 (OL-135) and representative oxazole C5 substituents were prepared and examined as inhibitors of fatty acid amide hydrolase (FAAH). Exceptionally potent and selective FAAH inhibitors emerged from the series (e.g., 6, Ki = 200 and 260 pM for rat and rhFAAH). With simple and small C5 oxazole substituents, each series bearing a biphenylethyl, phenoxyphenethyl, or (phenoxymethyl)phenethyl C2 side chain was found to follow a well-defined linear relationship between -log Ki and Hammett sigmap of a magnitude (rho = 2.7-3.0) that indicates that the substituent electronic effect dominates, confirming its fundamental importance to the series and further establishing its predictive value. Just as significantly, the nature of the C5 oxazole substituent substantially impacts the selectivity of the inhibitors whereas the effect of the C2 acyl chain was more subtle but still significant even in the small series examined. Combination of these independent features, which display generalized trends across a range of inhibitor series, simultaneously improves FAAH potency and selectivity and can provide exquisitely selective and potent FAAH inhibitors.

Influence of sulfur oxidation state and steric bulk upon trifluoromethyl ketone (TFK) binding kinetics to carboxylesterases and fatty acid amide hydrolase (FAAH)

Carboxylesterases metabolize numerous exogenous and endogenous ester-containing compounds including the chemotherapeutic agent CPT-11, anti-influenza viral agent oseltamivir, and many agrochemicals. Trifluoromethyl ketone (TFK)-containing compounds with a sulfur atom beta to the ketone moiety are some of the most potent carboxylesterase and amidase inhibitors identified to date. This study examined the effects of alkyl chain length (i.e., steric effects) and sulfur oxidation state upon TFK inhibitor potency (IC50) and binding kinetics (k(i)). The selective carboxylesterase inhibitor benzil was used as a non-TFK containing control. These effects were examined using two commercial esterases (porcine and rabbit liver esterase) and two human recombinant esterases (hCE-1 and hCE-2) as well as human recombinant fatty acid amide hydrolase (FAAH). In addition, the inhibition mechanism was examined using a combination of 1H NMR, X-ray crystallography, and ab initio calculations. Overall, the data show that while sulfur oxidation state profoundly affects both inhibitor potency and binding kinetics, the steric effects dominate and override the contributions of sulfur oxidation. In addition, the data suggest that inclusion of a sulfur atom beta to the ketone contributes an increase (approximately 5-fold) in inhibitor potency due to effects upon ketone hydration and/or intramolecular hydrogen bond formation. These results provide further information on the nature of the TFK binding interaction and will be useful in increasing our understanding of this basic biochemical process.

Novel ketooxazole based inhibitors of fatty acid amide hydrolase (FAAH)

Efforts to improve the properties of the well studied ketooxazole FAAH inhibitor OL-135 resulted in the discovery of a novel propylpiperidine series of FAAH inhibitors that has a modular design and superior properties to OL-135. The efficacy of one of these compounds was demonstrated in a rat spinal nerve ligation model of neuropathic pain in rats.

Fatty acid amide hydrolase: a gate-keeper of the endocannabinoid system

The family of endocannabinoids contains several polyunsaturated fatty acid amides such as anandamide (AEA), but also esters such as 2-arachidonoylglycerol (2-AG). These compounds are the main endogenous agonists of cannabinoid receptors, able to mimic several pharmacological effects of Delta9-tetrahydrocannabinol (Delta9-THC), the active principle of Cannabis sativa preparations like hashish and marijuana. The activity of AEA at its receptors is limited by cellular uptake, through a putative membrane transporter, followed by intracellular degradation by fatty acid amide hydrolase (FAAH). Growing evidence demonstrates that FAAH is the critical regulator of the endogenous levels of AEA, suggesting that it may serve as an attractive therapeutic target for the treatment of human disorders. In particular, FAAH inhibitors may be next generation therapeutics of potential value for the treatment of pathologies of the central nervous system, and of peripheral tissues. Investigations into the structure and function of FAAH, its biological and therapeutic implications, as well as a description of different families of FAAH inhibitors, are the topic of this chapter.

Endocannabinoids and related compounds: walking back and forth between plant natural products and animal physiology

Cannabis sativa has been known, used, and misused by mankind for centuries, and yet only over the last two decades has research stemming from the chemical constituents specific to this plant, the cannabinoids, started to provide fundamental insights into animal physiology and pathology, resulting in the development of new therapeutics. The discovery of the endocannabinoid system, and its targeting with two new pharmaceutical preparations now on the market in several countries, represent the most recent example of how studies on medicinal plants and on the mechanism of their biological effects can reveal, through a chain of breakthroughs, new systems of endogenous signals and physiological phenomena that can become the source of novel strategies for unmet therapeutic challenges.

The complications of promiscuity: endocannabinoid action and metabolism

In this review, we present our understanding of the action and metabolism of endocannabinoids and related endogenous molecules. It is clear that the interactions between the multiple endocannabinoid-like molecules (ECLs) are highly complex, both at the level of signal transduction and metabolism. Thus, ECLs are a group of ligands active at 7-transmembrane and nuclear receptors, as well as transmitter-gated and ion channels. ECLs and their metabolites can converge on common endpoints (either metabolic or signalling) through contradictory or reinforcing pathways. We highlight the complexity of the endocannabinoid system, based on the promiscuous nature of ECLs and their metabolites, as well as the synthetic modulators of the endocannabinoid system.

Therapeutic Potential of Endocannabinoid-Hydrolysing Enzyme Inhibitors

The specific protein target of delta9-tetrahydrocannabinol (delta9-THC), the main active ingredient of Cannabis sativa L., was characterized from rat brain nearly 20 years ago, and several endogenous compounds and proteins comprising the endocannabinoid (eCB) system have since been discovered. It has become evident that the eCB system consists of at least two cannabinoid receptors (i.e. the CB1 and CB2 receptors), in addition to their endogenous ligands (the eCBs) and several enzymes involved in the biosynthesis and catabolism of the eCBs. The two well-established eCBs, N-arachidonoylethanolamide (AEA) and 2-arachidonoylglycerol (2-AG), are produced by neurons on demand, act near their sites of synthesis and are effectively metabolized by fatty acid amide hydrolase (FAAH) and monoglyceride lipase (MGL), respectively. Inhibitors specifically targeting these enzymes could offer novel therapeutic approaches (e.g. for the treatment of pain and movement disorders). This MiniReview summarizes the literature concerning the potential therapeutic potential of FAAH and MGL inhibitors.

The contribution of cyclooxygenase-2 to endocannabinoid metabolism and action

The development of sensitive analytical methods for measurement of endocannabinoids, their metabolites, and related lipids, has underlined the complexity of the endocannabinoid system. A case can be made for an 'endocannabinoid soup' (akin to the inflammatory soup) whereby the net effect of a pathological state and/or a pharmacological intervention on this system is the result not only of changes in endocannabinoid levels but also of their metabolites and related compounds that affect their function. With respect to the metabolism of anandamide and 2-arachidonoylglycerol, the main hydrolytic enzymes involved are fatty acid amide hydrolase and monoacylglycerol lipase. However, other pathways can come into play when these are blocked. Cyclooxygenase-2 derived metabolites of anandamide and 2-arachidonoylglycerol have a number of properties, including effects upon cell viability, contraction of the cat iris sphincter (an effect mediated by a novel receptor), mobilization of calcium and modulation of synaptic transmission. Nonsteroidal anti-inflammatory agents, whose primary mode of action is the inhibition of cyclooxygenase, can also interact with the endocannabinoid system both in vitro and in vivo. Other enzymes, such as the lipoxygenase and cytochrome P450 oxidative enzymes, can also metabolize endocannabinoids and produce biologically active compounds. It is concluded that sensitive analytical methods, which allow for measurement of endocannabinoids and related lipids, should provide vital information as to the importance of these alternative metabolic pathways when the primary hydrolytic endocannabinoid metabolizing enzymes are inhibited.

Fatty acid amide hydrolase: from characterization to therapeutics

Fatty acid amide hydrolase (FAAH) is an integral membrane enzyme within the amidase-signature family that terminates the action of several endogenous lipid messengers, including oleamide and the endocannabinoid anandamide. The hydrolysis of such messengers leads to molecules devoid of biological activity, and, therefore, modulates a number of neurobehavioral processes in mammals, including pain, sleep, feeding, and locomotor activity. Investigations into the structure and function of FAAH, its biological and therapeutic implications, as well as a description of different families of FAAH inhibitors are the topic of this review.

The N-acylethanolamine-hydrolyzing acid amidase (NAAA)

Bioactive N-acylethanolamines, including the endocannabinoid anandamide and anti-inflammatory N-palmitoylethanolamine, are hydrolyzed to fatty acids and ethanolamine in animal tissues by the catalysis of fatty acid amide hydrolase (FAAH). We recently cloned cDNA of N-acylethanolamine-hydrolyzing acid amidase (NAAA), another enzyme catalyzing the same reaction, from human, rat, and mouse. NAAA reveals no sequence homology with FAAH and belongs to the choloylglycine hydrolase family. The most striking catalytic property of NAAA is pH optimum at 4.5-5, which is consistent with its immunocytochemical localization in lysosomes. In rat, NAAA is highly expressed in lung, spleen, thymus, and intestine. Notably, the expression level of NAAA is exceptionally high in rat alveolar macrophages. The primary structure of NAAA exhibits 33-35% amino acid identity to that of acid ceramidase, a lysosomal enzyme hydrolyzing ceramide to fatty acid and sphingosine. NAAA actually showed a low, but detectable ceramide-hydrolyzing activity, while acid ceramidase hydrolyzed N-lauroylethanolamine. Thus, NAAA is a novel lysosomal hydrolase, which is structurally and functionally similar to acid ceramidase. These results suggest a unique role of NAAA in the degradation of N-acylethanolamines.

Acute intracerebroventricular administration of palmitoylethanolamide, an endogenous peroxisome proliferator-activated receptor-alpha agonist, modulates carrageenan-induced paw edema in mice

Peroxisome proliferator-activated receptor (PPAR)-alpha is a nuclear transcription factor. Although the presence of this receptor in different areas of central nervous system (CNS) has been reported, its role remains unclear. Palmitoylethanolamide (PEA), a member of the fatty-acid ethanolamide family, acts peripherally as an endogenous PPAR-alpha ligand, exerting analgesic and anti-inflammatory effects. High levels of PEA in the CNS have been found, but the specific function of this lipid remains to be clarified. Using carrageenan-induced paw edema in mice, we show that i.c.v. administration of PEA may control peripheral inflammation through central PPAR-alpha activation. A single i.c.v. administration of 0.01 to 1 microg of PEA, 30 min before carrageenan injection, reduced edema formation in the mouse carrageenan test. This effect was mimicked by 0.01 to 1 microg of GW7647 [2-[[4-[2-[[(cyclohexylamino)carbonyl](4-cyclohexylbutyl)amino]ethyl]phenyl]thio]-2-methylpropanoic acid], a synthetic PPAR-alpha agonist. Moreover, central PEA administration significantly reduced the expression of the proinflammatory enzymes cyclooxygenase-2 and inducible nitric-oxide synthase, and it significantly restored carrageenan-induced PPAR-alpha reduction in the spinal cord. To investigate the mechanism by which i.c.v. PEA attenuated the development of carrageenan-induced paw edema, we evaluated inhibitor kappaB-alpha (I kappa B-alpha) degradation and nuclear factor-kappaB (NF-kappaB) p65 activation in the cytosolic or nuclear extracts from spinal cord tissue. PEA prevented IkB-alpha degradation and NF-kappaB nuclear translocation, confirming the involvement of this transcriptional factor in the control of peripheral inflammation. The obligatory role of PPAR-alpha in mediating the effects of PEA was confirmed by the lack of the compounds anti-inflammatory effects in mutant mice lacking PPAR-alpha. In conclusion, our data show for the first time that PPAR-alpha activation in the CNS can control peripheral inflammation.

Structure-activity relationship of a series of inhibitors of monoacylglycerol hydrolysis--comparison with effects upon fatty acid amide hydrolase

A series of 32 heterocyclic analogues based on the structure of 2-arachidonoylglycerol (2-AG) were synthesized and tested for their ability to inhibit monoacylglycerol lipase and fatty acid amide hydrolase activities. The designed compounds feature a hydrophobic moiety and different heterocyclic subunits that mimic the glycerol fragment. This series has allowed us to carry out the first systematic structure-activity relationship study on inhibition of 2-AG hydrolysis. The most promising compounds were oxiran-2-ylmethyl (5Z,8Z,11Z,14Z)-icosa-5,8,11,14-tetraenoate (1) and tetrahydro-2H-pyran-2-ylmethyl (5Z,8Z,11Z,14Z)-icosa-5,8,11,14-tetraenoate (5). They inhibited cytosolic 2-oleoylglycerol (2-OG) hydrolysis completely (IC50 values of 4.5 and 5.6 muM, respectively). They also blocked, albeit less potently, 2-OG hydrolysis in membrane fractions (IC50 values of 19 and 26 muM, respectively) and anandamide hydrolysis (IC50 values of 12 and 51 muM, respectively). These compounds will be useful in delineating the importance of the cytosolic hydrolytic activity in the regulation of 2-AG levels and, hence, its potential as a target for drug development.

TheN-Acylethanolamine-Mediated Regulatory Pathway in Plants

While cannabinoids are secondary metabolites synthesized by just a few plant species, N-acylethanolamines (NAEs) are distributed widely in the plant kingdom, and are recovered in measurable, bioactive quantities in many plant-derived products. NAEs in higher plants are ethanolamides of fatty acids with acyl-chain lenghts of C12-C(18) and zero to three C=C bonds. Generally, the most-abundant NAEs found in plants and vertebrates are similar, including NAE 16 : 0, 18 : 1, 18 : 2, and 18 : 3. Like in animal systems, NAEs are formed in plants from N-acylphosphatidylethanolamines (NAPEs), and they are hydrolyzed by an amidase to yield ethanolamine and free fatty acids (FFA). Recently, a homologue of the mammalian fatty acid amide hydrolase (FAAH-1) was identified in Arabidopsis thaliana and several other plant species. Overexpression of Arabidopsis FAAH (AtFAAH) resulted in plants that grew faster, but were more sensitive to biotic and abiotic insults, suggesting that the metabolism of NAEs in plants resides at the balance between growth and responses to environmental stresses. Similar to animal systems, exogenously applied NAEs have potent and varied effects on plant cells. Recent pharmacological approaches combined with molecular-genetic experiments revealed that NAEs may act in certain plant tissues via specific membrane-associated proteins or by interacting with phospholipase D-alpha, although other, direct targets for NAE action in plants are likely to be discovered. Polyunsaturated NAEs can be oxidized via the lipoxygenase pathway in plants, producing an array of oxylipin products that have received little attention so far. Overall, the conservation of NAE occurrence and metabolic machinery in plants, coupled with the profound physiological effects of elevating NAE content or perturbing endogenous NAE metabolism, suggest that an NAE-mediated regulatory pathway, sharing similarities with the mammalian endocannabinoid pathway, indeed exists.

Structure-activity relationships of alpha-ketooxazole inhibitors of fatty acid amide hydrolase

A systematic study of the structure-activity relationships of 2b (OL-135), a potent inhibitor of fatty acid amide hydrolase (FAAH), is detailed targeting the C2 acyl side chain. A series of aryl replacements or substituents for the terminal phenyl group provided effective inhibitors (e.g., 5c, aryl = 1-napthyl, Ki = 2.6 nM), with 5hh (aryl = 3-ClPh, Ki = 900 pM) being 5-fold more potent than 2b. Conformationally restricted C2 side chains were examined, and many provided exceptionally potent inhibitors, of which 11j (ethylbiphenyl side chain) was established to be a 750 pM inhibitor. A systematic series of heteroatoms (O, NMe, S), electron-withdrawing groups (SO, SO2), and amides positioned within and hydroxyl substitutions on the linking side chain were investigated, which typically led to a loss in potency. The most tolerant positions provided effective inhibitors (12p, 6-position S, Ki = 3 nM, or 13d, 2-position OH, Ki = 8 nM) comparable in potency to 2b. Proteome-wide screening of selected inhibitors from the systematic series of >100 candidates prepared revealed that they are selective for FAAH over all other mammalian serine proteases.

Severe deficiency of the fatty acid amide hydrolase (FAAH) activity segregates with the Huntington's disease mutation in peripheral lymphocytes

The search for peripheral markers of neurodegenerative diseases aims at identifying molecules that could help in monitoring the effects of future therapeutics in easily accessible cells. Here we focused on the involvement of the endocannabinoid system in Huntington's disease (HD). We assayed peripheral lymphocytes from HD patients and healthy controls, and found that the activity of the fatty acid amide hydrolase (FAAH), the enzyme that degrades the endocannabinoid anandamide (AEA), was dramatically decreased (down to less than 10%) in HD compared to healthy subjects. Concomitantly, the endogenous levels of AEA were approximately 6-fold higher in HD versus healthy lymphocytes, while the other elements of the endocannabinoid system were not affected by HD. Low FAAH activity in HD lymphocytes was not due to down-regulation of protein expression, but rather to blockage of enzyme activity by a cytosolic and irreversible inhibitor. Finally, pre-HD patients showed defective FAAH activity, as did the brain of HD patients compared with healthy controls. Taken together, our data indicate that FAAH activity in lymphocytes mirrors some of the metabolic changes which take place in the brain, it is a measurable non-genetic peripheral marker that segregates with the HD mutation, and it might serve as a target to test chemicals active on the widespread toxic effects of the mutant protein.

The role of hydrophobic interactions in positioning of peripheral proteins in membranes

BACKGROUND

Three-dimensional (3D) structures of numerous peripheral membrane proteins have been determined. Biological activity, stability, and conformations of these proteins depend on their spatial positions with respect to the lipid bilayer. However, these positions are usually undetermined.

RESULTS

We report the first large-scale computational study of monotopic/peripheral proteins with known 3D structures. The optimal translational and rotational positions of 476 proteins are determined by minimizing energy of protein transfer from water to the lipid bilayer, which is approximated by a hydrocarbon slab with a decadiene-like polarity and interfacial regions characterized by water-permeation profiles. Predicted membrane-binding sites, protein tilt angles and membrane penetration depths are consistent with spin-labeling, chemical modification, fluorescence, NMR, mutagenesis, and other experimental studies of 53 peripheral proteins and peptides. Experimental membrane binding affinities of peripheral proteins were reproduced in cases that did not involve a helix-coil transition, specific binding of lipids, or a predominantly electrostatic association. Coordinates of all examined peripheral proteins and peptides with the calculated hydrophobic membrane boundaries, subcellular localization, topology, structural classification, and experimental references are available through the Orientations of Proteins in Membranes (OPM) database.

CONCLUSION

Positions of diverse peripheral proteins and peptides in the lipid bilayer can be accurately predicted using their 3D structures that represent a proper membrane-bound conformation and oligomeric state, and have membrane binding elements present. The success of the implicit solvation model suggests that hydrophobic interactions are usually sufficient to determine the spatial position of a protein in the membrane, even when electrostatic interactions or specific binding of lipids are substantial. Our results demonstrate that most peripheral proteins not only interact with the membrane surface, but penetrate through the interfacial region and reach the hydrocarbon interior, which is consistent with published experimental studies.

Arabidopsis amidase 1, a member of the amidase signature family

Amidase 1 (AMI1), a specific indole-3-acetamide amidohydrolase, is an Arabidopsis thaliana amidase signature enzyme that catalyzes the synthesis of indole-3-acetic acid from indole-3-acetamide. Amidase signature family members catalyze a diverse range of enzymatic reactions and are found widespread in nature, for instance in bacteria, mammals, and plants. At the protein level, the family members share a conserved stretch of approximately 50-130 amino acids, the name-giving amidase signature. Elucidation of the crystal structures of a mammalian fatty acid amide hydrolase and the bacterial malonamidase E2 revealed an unusual Ser-cisSer-Lys catalytic triad in proteins of this family. In addition, other members, such as the amidase from Rhodococcus rhodochrous strain J1 or Sulfolobus solfataricus, seem to use an accessory Cys-cisSer-Lys center. AMI1 possesses all conserved amino-acid residues of the Ser-cisSer-Lys triad, but lacks the CX(3)C motif and therefore the Cys-cisSer-Lys catalytic site. Using a set of point-mutated variants of AMI1 and chemical modifications, we analyzed the relative importance of single amino-acid residues of AMI1 with respect to substrate conversion. These experiments revealed that a specific serine residue, Ser137, is essential for AMI1 enzymatic activity. We also report structural and functional differences of AMI1 from other amidase signature enzymes.

A MBP-FAAH fusion protein as a tool to produce human and rat fatty acid amide hydrolase: expression and pharmacological comparison

Fatty acid amide hydrolase (FAAH), a membrane-anchored enzyme responsible for the termination of endocannabinoid signalling, is an attractive target for treating conditions such as pain and anxiety. Inhibitors of the enzyme, optimized using rodent FAAH, are known but their pharmacology and medicinal chemistry properties on the human FAAH are missing. Therefore recombinant human enzyme would represent a powerful tool to evaluate new drug candidates. However, the production of high amounts of enzyme is hampered by the known refractiveness of FAAH to overexpression. Here, we report the successful overexpression of rat and human FAAH as a fusion to the E. coli maltose-binding protein, retaining catalytic properties of native FAAH. Several known FAAH inhibitors were tested and differences in their potencies toward the human and rat FAAH were found, underscoring the importance of using a human FAAH in the development of inhibitors.

Potent and selective alpha-ketoheterocycle-based inhibitors of the anandamide and oleamide catabolizing enzyme, fatty acid amide hydrolase

A study of the structure-activity relationships (SAR) of 2f (OL-135), a potent inhibitor of fatty acid amide hydrolase (FAAH), is detailed, targeting the 5-position of the oxazole. Examination of a series of substituted benzene derivatives (12-14) revealed that the optimal position for substitution was the meta-position with selected members approaching or exceeding the potency of 2f. Concurrent with these studies, the effect of substitution on the pyridine ring of 2f was also examined. A series of small, nonaromatic C5-substituents was also explored and revealed that the K(i) follows a well-defined correlation with the Hammett sigma(p) constant (rho = 3.01, R2 = 0.91) in which electron-withdrawing substituents enhance potency, leading to inhibitors with K(i)s as low as 400 pM (20n). Proteomic-wide screening of the inhibitors revealed that most are exquisitely selective for FAAH over all other mammalian proteases, reversing the 100-fold preference of 20a (C5 substituent = H) for the enzyme TGH.

Three hydrolases and a transferase: comparative analysis of active-site dynamics via the BioSimGrid database

Comparative molecular dynamics (MD) simulations enable us to explore the conformational dynamics of the active sites of distantly related enzymes. We have used the BioSimGrid (http://www.biosimgrid.org) database to facilitate such a comparison. Simulations of four enzymes were analyzed. These included three hydrolases and a transferase, namely acetylcholinesterase, outer-membrane phospholipase A, outer-membrane protease T, and PagP (an outer-membrane enzyme which transfers a palmitate chain from a phospholipid to lipid A). A set of 17 simulations were analyzed corresponding to a total of approximately 0.1 micros simulation time. A simple metric for active-site integrity was used to demonstrate the existence of clusters of dynamic conformational behaviour of the active sites. Small (i.e. within a cluster) fluctuations appear to be related to the function of an enzymatically active site. Larger fluctuations (i.e. between clusters) correlate with transitions between catalytically active and inactive states. Overall, these results demonstrate the potential of a comparative MD approach to analysis of enzyme function. This approach could be extended to a wider range of enzymes using current high throughput MD simulation and database methods.

Biochemistry and pharmacology of endovanilloids

Endovanilloids are defined as endogenous ligands and activators of transient receptor potential (TRP) vanilloid type 1 (TRPV1) channels. The first endovanilloid to be identified was anandamide (AEA), previously discovered as an endogenous agonist of cannabinoid receptors. In fact, there are several similarities, in terms of opposing actions on the same intracellular signals, role in the same pathological conditions, and shared ligands and tissue distribution, between TRPV1 and cannabinoid CB(1) receptors. After AEA and some of its congeners (the unsaturated long chain N-acylethanolamines), at least 2 other families of endogenous lipids have been suggested to act as endovanilloids: (i) unsaturated long chain N-acyldopamines and (ii) some lipoxygenase (LOX) metabolites of arachidonic acid (AA). Here we discuss the mechanisms for the regulation of the levels of the proposed endovanilloids, as well as their TRPV1-mediated pharmacological actions in vitro and in vivo. Furthermore, we outline the possible pathological conditions in which endovanilloids, acting at sometimes aberrantly expressed TRPV1 receptors, might play a role.

Closing the Gate to the Active Site

Fatty acid amide hydrolase (FAAH) is a dimeric, membranebound enzyme that degrades neuromodulatory fatty acid amides and esters and is expressed in mammalian brain and peripheral tissues. The cleavage of approximately 30 amino acids from each subunit creates an FAAH variant that is soluble and homogeneous in detergent-containing buffers, opening the avenue to the in vitro mechanistic and structural studies. Here we have studied the stability of FAAH as a function of guanidinium hydrochloride concentration and of hydrostatic pressure. The unfolding transition was observed to be complex and required a fitting procedure based on a three-state process with a monomeric intermediate. The first transition was characterized by dimer dissociation, with a free energy change of approximately 11 kcal/mol that accounted for approximately 80% of the total stabilization energy. This process was also paralleled by a large change in the solvent-accessible surface area, because of the hydration occurring both at the dimeric interface and within the monomers. As a consequence, the isolated subunits were found to be much less stable (DeltaG approximately 3 kcal/mol). The addition of methoxyarachidonyl fluorophosphonate, an irreversible inhibitor of FAAH activity, enhanced the stability of the dimer by approximately 2 kcal/mol, toward denaturant- and pressure-induced unfolding. FAAH inhibition by methoxyarachidonyl fluorophosphonate also reduced the ability of the protein to bind to the membranes. These findings suggest that local conformational changes at the level of the active site might induce a tighter interaction between the subunits of FAAH, affecting the enzymatic activity and the interaction with membranes.

Fatty acid amide hydrolase inhibitors display broad selectivity and inhibit multiple carboxylesterases as off-targets

Fatty acid amide hydrolase (FAAH) is the primary regulator of several bioactive lipid amides including anandamide. Inhibitors of FAAH are potentially useful for the treatment of pain, anxiety, depression, and other nervous system disorders. However, FAAH inhibitors must display selectivity for this enzyme relative to the numerous other serine hydrolases present in the human proteome in order to be therapeutically acceptable. Here we employed activity-based protein profiling (ABPP) to assess the selectivity of FAAH inhibitors in multiple rat and human tissues. We discovered that some inhibitors, including carbamate compounds SA-47 and SA-72, and AM404 are exceptionally selective while others, like URB597, BMS-1, OL-135, and LY2077855 are less selective, displaying multiple off-targets. Since proteins around 60kDa constitute the major off-targets for URB597 and several other FAAH inhibitors with different chemical structures, we employed the multi-dimensional protein identification technology (MudPIT) approach to analyze their identities. We identified multiple carboxylesterase isozymes as bona fide off-targets of FAAH inhibitors. Consistently, enzymatic assay confirmed inhibition of carboxylesterase activities in rat liver by FAAH inhibitors. Since carboxylesterases hydrolyze a variety of ester-containing drugs and prodrugs, we speculate that certain FAAH inhibitors, by inhibiting carboxylesterases, might have drug-drug interactions with other medicines if developed as therapeutic agents.

Three-dimensional crystallization of membrane proteins

Although the examination of the protein data bank reveals an important backlog in the number of three-dimensional structures of membrane proteins, several recent successes are serving as preludes to what will become a very prosperous decade in this field. Systematic investigations of various factors affecting the stability of membrane proteins, as well as their potential to crystallize three dimensionally, have paved the way for such achievements. The importance of the role of detergents both at the level of purification and crystallization is now well established. In addition, the recognition of the protein-detergent complex as the entity to crystallize, as well as the understanding of its physical-chemical properties and discovery of factors affecting these, have permitted the design of better crystallization strategies. As a consequence of the various efforts in the field, new crystallization methods for membrane proteins are being implemented. These have already been successful and are expected to contribute significantly to the future successes. This chapter will review some basic principles in membrane protein crystallization and give an overview of the current state of the art in the field. Some practical guidelines to help the novice approach the problem of membrane protein crystallization from the initial step of protein purification to crystallogenesis will also be given.

Cannabinoids and pain

Convincing evidence from preclinical studies demonstrates that cannabinoids can reduce pain responses in a range of inflammatory and neuropathic pain models. The anatomical and functional data reveal cannabinoid receptor-mediated analgesic actions operating at sites concerned with the transmission and processing of nociceptive signals in brain, spinal cord and the periphery. The precise signalling mechanisms by which cannabinoids produce analgesic effects at these sites remain unclear; however, significant clues point to cannabinoid modulation of the functions of neurone and immune cells that mediate nociceptive and inflammatory responses. Intracellular signalling mechanisms engaged by cannabinoid receptors-like the inhibition of calcium transients and adenylate cyclase, and pre-synaptic modulation of transmitter release-have been demonstrated in some of these cell types and are predicted to play a role in the analgesic effects of cannabinoids. In contrast, the clinical effectiveness of cannabinoids as analgesics is less clear. Progress in this area requires the development of cannabinoids with a more favourable therapeutic index than those currently available for human use, and the testing of their efficacy and side-effects in high-quality clinical trials.

Elucidation of Hydrolysis Mechanisms for Fatty Acid Amide Hydrolase and Its Lys142Ala Variant via QM/MM Simulations

Fatty acid amide hydrolase (FAAH) is a serine hydrolase that degrades anandamide, an endocannabinoid, and oleamide, a sleep-inducing lipid, and has potential applications as a therapeutic target for neurological disorders. Remarkably, FAAH hydrolyzes amides and esters with similar rates; however, the normal preference for esters reemerges when Lys142 is mutated to alanine. To elucidate the hydrolysis mechanisms and the causes behind this variation of selectivity, mixed quantum and molecular mechanics (QM/MM) calculations were carried out to obtain free-energy profiles for alternative mechanisms for the enzymatic hydrolyses. The methodology features free-energy perturbation calculations in Monte Carlo simulations with PDDG/PM3 as the QM method. For wild-type FAAH, the results support a mechanism, which features proton transfer from Ser217 to Lys142, simultaneous proton transfer from Ser241 to Ser217, and attack of Ser241 on the substrate's carbonyl carbon to yield a tetrahedral intermediate, which subsequently undergoes elimination with simultaneous protonation of the leaving group by a Lys142-Ser217 proton shuttle. For the Lys142Ala mutant, a striking multistep sequence is proposed with simultaneous proton transfer from Ser241 to Ser217, attack of Ser241 on the carbonyl carbon of the substrate, and elimination of the leaving group and its protonation by Ser217. Support comes from the free-energy results, which well reproduce the observation that the Lys142Ala mutation in FAAH decreases the rate of hydrolysis for oleamide significantly more than for methyl oleate.

Arg-158 Is Critical in Both Binding the Substrate and Stabilizing the Transition-state Oxyanion for the Enzymatic Reaction of Malonamidase E2

Malonamidase E2 (MAE2) from Bradyrhizobium japonicum is an enzyme that hydrolyzes malonamate to malonate and has a Ser-cis-Ser-Lys catalytic triad at the active site. The crystal structures of wild type and mutant MAE2 exhibited that the guanido group of Arg-158 could be involved in the binding of malonamate in which the negative charge of the carboxyl group could destabilize a negatively charged transition-state oxyanion in the enzymatic reaction. In an attempt to elucidate the specific roles of Arg-158, site-directed mutants, R158Q, R158E, and R158K, were prepared (see Table 1). The crystal structure of R158Q determined at 2.2 Angstrom resolution showed that the guanido group of Arg-158 was important for the substrate binding with the marginal structural change upon the mutation. The k(cat) value of R158Q significantly decreased by over 1500-fold and the catalytic activity of R158E could not be detected. The k(cat) value of R158K was similar to that of the wild type with the K(m) value drastically increased by 100-fold, suggesting that Lys-158 of R158K can stabilize the negative charge of the carboxylate in the substrate to some extent and contribute to the stabilization of the transition-state oxyanion, but a single amine group of Lys-158 in R158K could not precisely anchor the carboxyl group of malonamate compared with the guanido group of Arg-158. Our kinetic and structural evidences demonstrate that Arg-158 in MAE2 should be critical to both binding the substrate and stabilizing the transition-state oxyanion for the catalytic reaction of MAE2.

Polyunsaturated fatty acids, membrane organization, T cells, and antigen presentation

Dietary supplementation with polyunsaturated fatty acids (PUFAs), especially those of the n-3 class, has immunosuppressive effects on both innate and adaptive immunity through various mechanisms. In this review, we focus on the PUFA modulation of membrane architecture and its consequent effects on both T cell responses and antigen presentation. We first use data from in vitro and in vivo experiments to make the case that the immunosuppressive effects of PUFAs begin with membrane incorporation and modulation of lipid-protein lateral organization. This in turn inhibits downstream signaling mediated by T cell receptors and suppresses T cell activation and proliferation. Next, we review evidence for PUFA-mediated alteration of major histocompatibility complex class I and II surface expression and antigen presentation. We propose that PUFAs influence the expression of major histocompatibility complex by altering its conformation, orientation, lateral organization, and trafficking, with consequences for recognition by effector T cells. Finally, we present data from model membrane studies to explain the physical principles that make PUFA acyl chains unique in modifying membrane lateral organization and protein function. An important concept to emerge from these studies is that PUFA acyl chains and cholesterol molecules are sterically incompatible. By applying this concept to the T cell activation and signaling model, mechanisms emerge by which PUFAs can modulate membrane lipid-protein lateral organization. Our data-based models show that membrane modification of both effectors and targets is an important, often overlooked, mechanism of immunomodulation by PUFAs.

A second fatty acid amide hydrolase with variable distribution among placental mammals

Fatty acid amides constitute a large and diverse class of lipid transmitters that includes the endogenous cannabinoid anandamide and the sleep-inducing substance oleamide. The magnitude and duration of fatty acid amide signaling are controlled by enzymatic hydrolysis in vivo. Fatty acid amide hydrolase (FAAH) activity in mammals has been primarily attributed to a single integral membrane enzyme of the amidase signature (AS) family. Here, we report the functional proteomic discovery of a second membrane-associated AS enzyme in humans that displays FAAH activity. The gene that encodes this second FAAH enzyme was found in multiple primate genomes, marsupials, and more distantly related vertebrates, but, remarkably, not in a number of lower placental mammals, including mouse and rat. The two human FAAH enzymes, which share 20% sequence identity and are referred to hereafter as FAAH-1 and FAAH-2, hydrolyzed primary fatty acid amide substrates (e.g. oleamide) at equivalent rates, whereas FAAH-1 exhibited much greater activity with N-acyl ethanolamines (e.g. anandamide) and N-acyl taurines. Both enzymes were sensitive to the principal classes of FAAH inhibitors synthesized to date, including O-aryl carbamates and alpha-keto heterocycles. These data coupled with the overlapping, but distinct tissue distributions of FAAH-1 and FAAH-2 suggest that these proteins may collaborate to control fatty acid amide catabolism in primates. The apparent loss of the FAAH-2 gene in some lower mammals should be taken into consideration when extrapolating genetic or pharmacological findings on the fatty acid amide signaling system across species.

Cannabinoid CB1 receptor recognition of endocannabinoids via the lipid bilayer: molecular dynamics simulations of CB1 transmembrane helix 6 and anandamide in a phospholipid bilayer

The phospholipid bilayer plays a central role in the lifecycle of the endogenous cannabinoid, N-arachidonoylethanolamine (anandamide, AEA). Therefore, the orientation and location of AEA in the phospholipid bilayer with respect to key membrane associated proteins, is a central issue in understanding the mechanism of endocannabinoid signaling. In this paper, we report a test of the hypothesis that a betaXXbeta motif (formed by beta branching amino acids, V6.43 and I6.46) on the lipid face of the cannabinoid CB1 receptor in its inactive state may serve as an initial CB1 interaction site for AEA. Eight 6 ns NAMD2 molecular dynamics simulations of AEA were conducted in a model system composed of CB1 transmembrane helix 6 (TMH6) in a 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) bilayer. In addition, eight 6 ns NAMD2 molecular dynamics simulations of a low CB1 affinity (20:2, n-6) analog of AEA were conducted in the same model system. AEA was found to exhibit a higher incidence of V6.43/I6.46 groove insertion than did the (20:2, n-6) analog. In certain cases, AEA established a high energy of interaction with TMH6 by first associating with the V6.43/I6.46 groove and then molding itself to the lipid face of TMH6 to establish a hydrogen bonding interaction with the exposed backbone carbonyl of P6.50. Based upon these results, we propose that the formation of this hydrogen bonded AEA/TMH6 complex may be the initial step in CB1 recognition of AEA in the lipid bilayer.

A high throughput fluorescent assay for measuring the activity of fatty acid amide hydrolase

Fatty acid amide hydrolase (FAAH) is the enzyme responsible for the rapid degradation of fatty acid amides such as the endocannabinoid anandamide. Inhibition of FAAH activity has been suggested as a therapeutic approach for the treatment of chronic pain, depression and anxiety, through local activation of the cannabinoid receptor CB1. We have developed a high throughput screening assay for identification of FAAH inhibitors using a novel substrate, decanoyl 7-amino-4-methyl coumarin (D-AMC) that is cleaved by FAAH to release decanoic acid and the highly fluorescent molecule 7-amino-4-methyl coumarin (AMC). This assay gives an excellent signal window for measuring FAAH activity and, as a continuous assay, inherently offers improved sensitivity and accuracy over previously reported endpoint assays. The assay was validated using a panel of known FAAH inhibitors and purified recombinant human FAAH, then converted to a 384 well format and used to screen a large library of compounds (>600,000 compounds) to identify FAAH inhibitors. This screen identified numerous novel FAAH inhibitors of diverse chemotypes. These hits confirmed using a native FAAH substrate, anandamide, and had very similar rank order potency to that obtained using the D-AMC substrate. Collectively these data demonstrate that D-AMC can be successfully used to rapidly and effectively identify novel FAAH inhibitors for potential therapeutic use.

Delineation of a fundamental alpha-ketoheterocycle substituent effect for use in the design of enzyme inhibitors

The synthesis and examination of a systematic series of 5-substituted 2-keto oxazoles as inhibitors of fatty acid amide hydrolase (FAAH) defined a fundamental substituent effect that led to the discovery of inhibitors with Ki's as low as 400 pM. The intrinsic basis of the relationship (-log Ki vs sigmap), which relates Ki with the Hammett sigmap constant of the substituent, the magnitude of the effect (rho = 3.01), and its predictive value (R2 = 0.91) suggest a widespread applicability in studies beyond FAAH.

Subcellular localization and tissue specific expression of amidase 1 from Arabidopsis thaliana

Amidase 1 (AMI1) from Arabidopsis thaliana converts indole-3-acetamide (IAM), into indole-3-acetic acid (IAA). AMI1 is part of a small isogene family comprising seven members in A. thaliana encoding proteins which share a conserved glycine- and serine-rich amidase-signature. One member of this family has been characterized as an N-acylethanolamine-cleaving fatty acid amidohydrolase (FAAH) and two other members are part of the preprotein translocon of the outer envelope of chloroplasts (Toc complex) or mitochondria (Tom complex) and presumably lack enzymatic activity. Among the hitherto characterized proteins of this family, AMI1 is the only member with indole-3-acetamide hydrolase activity, and IAM is the preferred substrate while N-acylethanolamines and oleamide are not hydrolyzed significantly, thus suggesting a role of AMI1 in auxin biosynthesis. Whereas the enzymatic function of AMI1 has been determined in vitro, the subcellular localization of the enzyme remained unclear. By using different GFP-fusion constructs and an A. thaliana transient expression system, we show a cytoplasmic localization of AMI1. In addition, RT-PCR and anti-amidase antisera were used to examine tissue specific expression of AMI1 at the transcriptional and translational level, respectively. AMI1-expression is strongest in places of highest IAA content in the plant. Thus, it is concluded that AMI1 may be involved in de novo IAA synthesis in A. thaliana.

The endocannabinoid system as an emerging target of pharmacotherapy

The recent identification of cannabinoid receptors and their endogenous lipid ligands has triggered an exponential growth of studies exploring the endocannabinoid system and its regulatory functions in health and disease. Such studies have been greatly facilitated by the introduction of selective cannabinoid receptor antagonists and inhibitors of endocannabinoid metabolism and transport, as well as mice deficient in cannabinoid receptors or the endocannabinoid-degrading enzyme fatty acid amidohydrolase. In the past decade, the endocannabinoid system has been implicated in a growing number of physiological functions, both in the central and peripheral nervous systems and in peripheral organs. More importantly, modulating the activity of the endocannabinoid system turned out to hold therapeutic promise in a wide range of disparate diseases and pathological conditions, ranging from mood and anxiety disorders, movement disorders such as Parkinson's and Huntington's disease, neuropathic pain, multiple sclerosis and spinal cord injury, to cancer, atherosclerosis, myocardial infarction, stroke, hypertension, glaucoma, obesity/metabolic syndrome, and osteoporosis, to name just a few. An impediment to the development of cannabinoid medications has been the socially unacceptable psychoactive properties of plant-derived or synthetic agonists, mediated by CB(1) receptors. However, this problem does not arise when the therapeutic aim is achieved by treatment with a CB(1) receptor antagonist, such as in obesity, and may also be absent when the action of endocannabinoids is enhanced indirectly through blocking their metabolism or transport. The use of selective CB(2) receptor agonists, which lack psychoactive properties, could represent another promising avenue for certain conditions. The abuse potential of plant-derived cannabinoids may also be limited through the use of preparations with controlled composition and the careful selection of dose and route of administration. The growing number of preclinical studies and clinical trials with compounds that modulate the endocannabinoid system will probably result in novel therapeutic approaches in a number of diseases for which current treatments do not fully address the patients' need. Here, we provide a comprehensive overview on the current state of knowledge of the endocannabinoid system as a target of pharmacotherapy.

The putative endocannabinoid transport blocker LY2183240 is a potent inhibitor of FAAH and several other brain serine hydrolases

How lipid transmitters move within and between cells to communicate signals remains an important and largely unanswered question. Integral membrane transporters, soluble lipid-binding proteins, and metabolic enzymes have all been proposed to collaboratively regulate lipid signaling dynamics in vivo. Assignment of the relative contributions made by each of these classes of proteins requires selective pharmacological agents to perturb their individual functions. Recently, LY2183240, a heterocyclic urea inhibitor of the putative endocannabinoid (EC) transporter, was shown to disrupt the cellular uptake of the lipid EC anandamide and promote analgesia in vivo. Here, we show that LY2183240 is a potent, covalent inhibitor of the EC-degrading enzyme fatty acid amide hydrolase (FAAH). LY2183240 inactivates FAAH by carbamylation of the enzyme's serine nucleophile. More global screens using activity-based proteomic probes identified several additional serine hydrolases that are also inhibited by LY2183240. These results indicate that the blockade of anandamide uptake observed with LY2183240 may be due primarily to the inactivation of FAAH, providing further evidence that this enzyme serves as a metabolic driving force that promotes the diffusion of anandamide into cells. More generally, the proteome-wide target promiscuity of LY2183240 designates the heterocyclic urea as a chemotype with potentially excessive protein reactivity for drug design.

Jekyll and hyde: two faces of cannabinoid signaling in male and female fertility

Mammalian reproduction is a complicated process designed to diversify and strengthen the genetic complement of the offspring and to safeguard regulatory systems at various steps for propagating procreation. An emerging concept in mammalian reproduction is the role of endocannabinoids, a group of endogenously produced lipid mediators, that bind to and activate cannabinoid receptors. Although adverse effects of cannabinoids on fertility have been implicated for years, the mechanisms by which they exert these effects were not clearly understood. With the identification of cannabinoid receptors, endocannabinoid ligands, their key synthetic and hydrolytic pathways, and the generation of mouse models missing cannabinoid receptors, a wealth of information on the significance of cannabinoid/endocannabinoid signaling in spermatogenesis, fertilization, preimplantation embryo development, implantation, and postimplantation embryonic growth has been generated. This review focuses on various aspects of the endocannabinoid system in male and female fertility. It is hoped that a deeper insight would lead to potential clinical applications of the endocannabinoid signaling as a target for correcting infertility and improving reproductive health in humans.

Binding mode of new (thio)hydantoin inhibitors of fatty acid amide hydrolase: comparison with two original compounds, OL-92 and JP104

Substituted (thio)hydantoins (2-thioxoimidazolidinones and imidazolidinediones) were reported as new potential reversible inhibitors of fatty acid amide hydrolase (FAAH). Their binding mode to FAAH was explored to rationalize their activity and give idea to design highly active inhibitors. Starting from the crystal structure of one of these molecules, docking studies provide us with rational basis for the design of new inhibitors within the thiohydantoin family.

A monotopic membrane protein goes solo

Carnitine palmitoyltransferases (CPTs) are part of the enzymatic system that imports fatty acids into mitochondria. The crystal structure of rat CPT-2 by Rufer et al. (2006) (this issue of Structure) reveals a Y-shaped tunnel for binding the CoA and acyl-carnitine substrates and a hydrophobic insert mediating membrane association.

A novel scintillation proximity assay for fatty acid amide hydrolase compatible with inhibitor screening

A binding assay for human fatty acid amide hydrolase (FAAH) using the scintillation proximity assay (SPA) technology is described. This SPA uses the specific interactions of [3H]R(+)-methanandamide (MAEA) and FAAH expressing microsomes to evaluate the displacement activity of FAAH inhibitors. We observed that a competitive nonhydrolyzed FAAH inhibitor, [3H]MAEA, bound specifically to the FAAH microsomes. Coincubation with an FAAH inhibitor, URB-597, competitively displaced the [3H]MAEA on the FAAH microsomes. The released radiolabel was then detected through an interaction with the SPA beads. The assay is specific for FAAH given that microsomes prepared from cells expressing the inactive FAAH-S241A mutant or vector alone had no significant ability to bind [3H]MAEA. Furthermore, the binding of [3H]MAEA to FAAH microsomes was abolished by selective FAAH inhibitors in a dose-dependent manner, with IC50 values comparable to those seen in a functional assay. This novel SPA has been validated and demonstrated to be simple, sensitive, and amenable to high-throughput screening.

Pharmacological characterization of endocannabinoid transport and fatty acid amide hydrolase inhibitors

: 1. The mechanism of anandamide uptake and disposal has been an issue of considerable debate in the cannabinoid field. Several compounds have been reported to inhibit anandamide uptake or fatty acid amide hydrolase (FAAH; the primary catabolic enzyme of anandamide) activity with varying degrees of potency and selectivity. We recently reported the first evidence of a binding site involved in the uptake of endocannabinoids that is independent from FAAH. There are no direct comparisons of purported selective inhibitory compounds in common assay conditions measuring anandamide uptake, FAAH activity and binding activity. 2. A subset of compounds reported in the literature were tested in our laboratory under common assay conditions to measure their ability to (a) inhibit [(14)C]-anandamide uptake in cells containing (RBL-2H3) or cells lacking (HeLa) FAAH, (b) inhibit purified FAAH hydrolytic activity, and (c) inhibit binding to a putative binding site involved in endocannabinoid transport in both RBL and HeLa cell membranes. 3. Under these conditions, nearly all compounds tested inhibited (a) uptake of [(14)C]-anandamide, (b) enzyme activity in purified FAAH preparations, and (c) radioligand binding of [(3)H]-LY2183240 in RBL and HeLa plasma membrane preparations. General rank order potency was preserved within the three assays. However, concentration response curves were right-shifted for functional [(14)C]-anandamide uptake in HeLa (FAAH(-/-)) cells. 4. A more direct comparison of multiple inhibitors could be made in these three assay systems performed in the same laboratory, revealing more information about the selectivity of these compounds and the relationship between the putative endocannabinoid transport protein and FAAH. At least two separate proteins appear to be involved in uptake and degradation of anandamide. The most potent inhibitory compounds were right-shifted when transport was measured in HeLa (FAAH(-/-)) cells suggesting a requirement for a direct interaction with the FAAH protein to maintain high affinity binding of anandamide or inhibitors to the putative anandamide transport protein.

Fatty Acid Amide Hydrolase Inhibitors from Virtual Screening of the Endocannabinoid System

The endocannabinoid system consists of two cannabinoid receptors (CB1 and CB2), endogenous ligands (endocannabinoids), and the enzymes involved in the metabolism of the endocannabinoids, including fatty acid amide hydrolase (FAAH) and monoglyceride lipase (MGL). In the present study, virtual screening of MGL inhibitors was performed by utilizing a comparative model of the human MGL enzyme. All hit molecules were tested for their potential MGL inhibitory activity, but no compounds were found capable of inhibiting MGL-like enzymatic activity in rat cerebellar membranes. However, these compounds were also tested for their potential FAAH inhibitory activity and five compounds (2-6) inhibiting FAAH were found with IC50 values between 4 and 44 microM. In addition, the hit molecules from the virtual screening of CB2 receptor ligands (reported previously in Salo et al. J. Med. Chem. 2005, 48, 7166) were also tested in our FAAH assay, and four active compounds (7-10) were found with IC50 values between 0.52 and 22 microM. Additionally, compound 7 inhibited MGL-like enzymatic activity with an IC50 value of 31 microM.

High-Throughput Screening for the Discovery of Inhibitors of Fatty Acid Amide Hydrolase Using a Microsome-Based Fluorescent Assay

Fatty acid amide hydrolase (FAAH) is a membrane-associated enzyme that catalyzes the hydrolysis of several endogenous bioactive lipids, including anandamide (AEA), N-palmitoylethanolamine (PEA), oleamide, and N-oleoylethanolamine (OEA). These fatty acid amides participate in many physiological activities such as analgesia, anxiety, sleep modulation, anti inflammatory responses, and appetite suppression. Because FAAH plays an essential role in controlling the tone and activity of these endogenous bioactive lipids, this enzyme has been implicated to be a drug target for the therapeutic management of pain, anxiety, and other disorders. In an effort to discover FAAH inhibitors, the authors have previously reported the development of a novel fluorescent assay using purified FAAH microsomes as an enzyme source and a fluorogenic substrate, arachidonyl 7-amino, 4-methyl coumarin amide (AAMCA). Herein, the authors have adapted this assay to a high-throughput format and have screened a large library of small organic compounds, identifying a number of novel FAAH inhibitors. These data further verify that this fluorescent assay is sufficiently robust, efficient, and low-cost for the identification of FAAH inhibitory molecules and open this class of enzymes for therapeutic exploration.

The transmembrane domain provides nucleotide binding specificity to the bacterial conjugation protein TrwB

In order to understand the functional significance of the transmembrane domain of TrwB, an integral membrane protein involved in bacterial conjugation, the protein was purified in the native, and also as a truncated soluble form (TrwBDeltaN70). The intact protein (TrwB) binds preferentially purine over pyrimidine nucleotides, NTPs over NDPs, and ribo- over deoxyribonucleotides. In contrast, TrwBDeltaN70 binds uniformly all tested nucleotides. The transmembrane domain has the general effect of making the nucleotide binding site(s) less accessible, but more selective. This is in contrast to other membrane proteins in which most of the protein mass, including the catalytic domain, is outside the membrane, but whose activity is not modified by the presence or absence of the transmembrane segment.

A computational protocol for the integration of the monotopic protein prostaglandin H2 synthase into a phospholipid bilayer

Prostaglandin H2 synthase (PGHS) synthesizes PGH2, a prostaglandin precursor, from arachidonic acid and was the first monotopic enzyme to have its structure experimentally determined. Both isozymes of PGHS are inhibited by nonsteroidal antiinflammatory drugs, an important class of drugs that are the primary means of relieving pain and inflammation. Selectively inhibiting the second isozyme, PGHS-2, minimizes the gastrointestinal side-effects. This had been achieved by the new PGHS-2 selective NSAIDs (i.e., COX-2 inhibitors) but it has been recently suggested that they suffer from additional side-effects. The design of these drugs only made use of static structures from x-ray crystallographic experiments. Investigating the dynamics of both PGHS-1 and PGHS-2 using classical molecular dynamics is expected to generate new insight into the differences in behavior between the isozymes, and therefore may allow improved PGHS-2 selective inhibitors to be designed. We describe a molecular dynamics protocol that integrates PGHS monomers into phospholipid bilayers, thereby producing in silico atomistic models of the PGHS system. Our protocol exploits the vacuum created beneath the protein when several lipids are removed from the top leaflet of the bilayer. The protein integrates into the bilayer during the first 5 ns in a repeatable process. The integrated PGHS monomer is stable and forms multiple hydrogen bonds between the phosphate groups of the lipids and conserved basic residues (Arg, Lys) on the protein. These interactions stabilize the system and are similar to interactions observed for transmembrane proteins.

Identification of a novel amidase motif in neutral ceramidase

Neutral CDases (ceramidases) are newly identified enzymes with important roles in cell regulation, but little is known about their catalytic mechanisms. In the present study the full-length human neutral CDase was cloned and expressed in the yeast double-knockout strain Dypc1Dydc1, which lacks the yeast CDases YPC1p and YDC1p. Biochemical characterization of the human neutral CDase showed that the enzyme exhibited classical Michaelis-Menten kinetics, with an optimum activity at pH 7.5. Activity was enhanced by Na+ and Ca2+. Mg2+ and Mn2+ were somewhat stimulatory, but Zn2+, Cu2+ and Fe2+ inhibited the enzyme. Dithiothreitol and 2-mercaptoethanol dose-dependently inhibited neutral CDase. In order to identify which amino acids were involved in the catalytic action of neutral CDase, the purified enzyme was subjected to chemical modifications. It was observed that the serine residue modifier di-isopropyl fluorophosphate dose-dependently inhibited activity, implicating a serine residue in the catalytic action. From an alignment of the sequences of the neutral CDases from different species, all conserved serine residues were selected for site-directed mutagenesis. Of the six aligned serine residues that were mutated to alanine, only the S354A mutant lost its activity totally. Ser354 falls within a very highly conserved hexapeptide sequence GDVSPN, which itself was in the middle of a larger conserved sequence, namely NXGDVSPNXXGP/XXC. Moreover, mutations of Asp352 and Cys362 in the consensus sequence to alanine resulted in loss of activity of neutral CDase. Hence the present study identified a novel amidase sequence containing a critical serine residue that may function as a nucleophile in the hydrolytic attack on the amide bond present in ceramide.

Oligomerization of Sulfolobus solfataricus signature amidase is promoted by acidic pH and high temperature

The recombinant amidase from the hyperthermophylic archaeon Sulfolobus solfataricus (SSAM) a signature amidase, was cloned, purified and characterized. The enzyme is active on a large number of aliphatic and aromatic amides over the temperature range 60-95 degrees C and at pH values between 4.0 and 9.5, with an optimum at pH 5.0. The recombinant enzyme is in the form of a dimer of about 110 kD that reversibly associates into an octamer in a pH-dependent reaction. The pH dependence of the state of association was studied using gel permeation chromatography, analytical ultracentrifugation and dynamic light scattering techniques. At pH 7.0 all three techniques show the presence of two species, in about equal amounts, which is compatible with the existence of a dimeric and an octameric form. In decreasing pH, the dimers formed the octameric species and in increasing pH, the octameric species was converted to dimers. Above pH 8.0, only dimers were present, below pH 3.0 only octamers were present. The association of dimers into octamers decreased in non-polar solvents and increased with temperature. A mutant (Y41C) was obtained that did not show this behavior.