Gene_locus Report for: human-ACHE

Recent events demonstrated that organophosphorus nerve agents are a serious threat for civilian and military populations. The current therapy includes a pyridinium aldoxime reactivator to restore the enzymatic activity of acetylcholinesterase located in the central nervous system and neuro-muscular junctions. One major drawback of these charged acetylcholinesterase reactivators is their poor ability to cross the blood-brain barrier. In this study, we propose to evaluate glucoconjugated oximes devoid of permanent charge as potential central nervous system reactivators. We determined their in vitro reactivation efficacy on inhibited human acetylcholinesterase, the crystal structure of two compounds in complex with the enzyme, their protective index on intoxicated mice, and their pharmacokinetics. We then evaluated their endothelial permeability coefficients with a human in vitro model. This study shed light on the structural restrains of new sugar oximes designed to reach the central nervous system through the glucose transporter located at the blood-brain barrier.

Acetylcholinesterase (EC 3.1.1.7; AChE), a key acetylcholine-hydrolyzing enzyme in cholinergic neurotransmission, is present in a variety of states in situ, including monomers, C-terminally disulfide-linked homodimers, homotetramers, and up to three tetramers covalently attached to structural subunits. Could oligomerization that ensures high local concentrations of catalytic sites necessary for efficient neurotransmission, be affected by environmental factors? Using small-angle X-ray scattering (SAXS) and cryo-EM, we demonstrate that homodimerization of recombinant monomeric human AChE (hAChE) in solution occurs through a C-terminal 4-helix bundle (4HB) at micromolar concentrations. We show that diethylphosphorylation of the active serine in the catalytic gorge or isopropylmethylphosphonylation by the R(P) enantiomer of sarin promotes a ten-fold increase in homodimer dissociation. We also demonstrate the dissociation of organophosphate (OP)-conjugated dimers is reversed by structurally diverse oximes 2PAM, HI6 or RS194B, as demonstrated by SAXS of diethylphosphoryl-hAChE. However, binding of oximes to the native ligand-free hAChE, binding of high-affinity reversible ligands, or formation of a S(P)-sarin-hAChE conjugate had no effect on homodimerization. Dissociation monitored by time-resolved SAXS (TR-SAXS) occurs in milliseconds, consistent with rates of hAChE covalent inhibition. OP-induced dissociation was not observed in the SAXS profiles of the double-mutant Y337A/F338A, where the active center gorge volume is larger than in wild-type hAChE. These observations suggest a key role of the tightly packed acyl pocket in allosterically triggered OP-induced dimer dissociation, with the potential for local reduction of acetylcholine-hydrolytic power in situ. Computational models predict allosteric correlated motions extending from the acyl pocket towards the 4HB dimerization interface 25 A away.

Profound synaptic dysfunction contributes to early loss of short-term memory in Alzheimer's disease. This study was set up to analyze possible neuroprotective effects of two dual binding site inhibitors of acetylcholinesterase (AChE), a new 6-methyluracil derivative, C-35, and the clinically used inhibitor donepezil. Crystal structure of the complex between human AChE and C-35 revealed tight contacts of ligand along the enzyme active site gorge. Molecular dynamics simulations indicated that the external flexible part of the ligand establishes multiple transient interactions with the enzyme peripheral anionic site. Thus, C-35 is a dual binding site inhibitor of AChE. In transgenic mice, expressing a chimeric mouse/human amyloid precursor protein and a human presenilin-1 mutant, C-35 (5mg/kg, i.p) and donepezil (0.75mg/kg, i.p) partially reversed synapse loss, decreased the number of amyloid plaques, and restored learning and memory. To separate temporal symptomatic therapeutic effects, associated with the increased lifetime of acetylcholine in the brain, from possible disease-modifying effect, an experimental protocol based on drug withdrawal from therapy was performed. When administration of C-35 and donepezil was terminated three weeks after the trial started, animals that were receiving C-35 showed a much better ability to learn than those who received vehicle or donepezil. Our results provide additional evidence that dual binding site inhibitors of AChE have Alzheimer's disease-modifying action.

Recent events demonstrated that organophosphorus nerve agents are a serious threat for civilian and military populations. The current therapy includes a pyridinium aldoxime reactivator to restore the enzymatic activity of acetylcholinesterase located in the central nervous system and neuro-muscular junctions. One major drawback of these charged acetylcholinesterase reactivators is their poor ability to cross the blood-brain barrier. In this study, we propose to evaluate glucoconjugated oximes devoid of permanent charge as potential central nervous system reactivators. We determined their in vitro reactivation efficacy on inhibited human acetylcholinesterase, the crystal structure of two compounds in complex with the enzyme, their protective index on intoxicated mice, and their pharmacokinetics. We then evaluated their endothelial permeability coefficients with a human in vitro model. This study shed light on the structural restrains of new sugar oximes designed to reach the central nervous system through the glucose transporter located at the blood-brain barrier.

Acetylcholinesterase (EC 3.1.1.7; AChE), a key acetylcholine-hydrolyzing enzyme in cholinergic neurotransmission, is present in a variety of states in situ, including monomers, C-terminally disulfide-linked homodimers, homotetramers, and up to three tetramers covalently attached to structural subunits. Could oligomerization that ensures high local concentrations of catalytic sites necessary for efficient neurotransmission, be affected by environmental factors? Using small-angle X-ray scattering (SAXS) and cryo-EM, we demonstrate that homodimerization of recombinant monomeric human AChE (hAChE) in solution occurs through a C-terminal 4-helix bundle (4HB) at micromolar concentrations. We show that diethylphosphorylation of the active serine in the catalytic gorge or isopropylmethylphosphonylation by the R(P) enantiomer of sarin promotes a ten-fold increase in homodimer dissociation. We also demonstrate the dissociation of organophosphate (OP)-conjugated dimers is reversed by structurally diverse oximes 2PAM, HI6 or RS194B, as demonstrated by SAXS of diethylphosphoryl-hAChE. However, binding of oximes to the native ligand-free hAChE, binding of high-affinity reversible ligands, or formation of a S(P)-sarin-hAChE conjugate had no effect on homodimerization. Dissociation monitored by time-resolved SAXS (TR-SAXS) occurs in milliseconds, consistent with rates of hAChE covalent inhibition. OP-induced dissociation was not observed in the SAXS profiles of the double-mutant Y337A/F338A, where the active center gorge volume is larger than in wild-type hAChE. These observations suggest a key role of the tightly packed acyl pocket in allosterically triggered OP-induced dimer dissociation, with the potential for local reduction of acetylcholine-hydrolytic power in situ. Computational models predict allosteric correlated motions extending from the acyl pocket towards the 4HB dimerization interface 25 A away.

Corrected : Organophosphate (OP) intoxications from nerve agent and OP pesticide exposures are managed with pyridinium aldoxime-based therapies whose success rates are currently limited. The pyridinium cation hampers uptake into the central nervous system (CNS). Furthermore, it frequently binds to aromatic residues of OP-inhibited acetylcholinesterase (AChE) in orientations that are non-productive for AChE reactivation, and the structural diversity of OPs impedes efficient reactivation. Improvements of OP antidotes need to include much better access of AChE reactivators to the CNS and optimized orientation of the antidotes' nucleophile within the AChE active-center gorge. On the basis of X-ray structures of a CNS-penetrating reactivator, monoxime RS194B, reversibly bound to native and venomous agent X (VX)-inhibited human AChE (hAChE), here we created seven uncharged acetamido bis-oximes as candidate antidotes. Both oxime groups in these bis-oximes were attached to the same central, saturated heterocyclic core. Diverse protonation of the heterocyclic amines and oxime groups of the bis-oximes resulted in equilibration among up to 16 distinct ionization forms, including uncharged forms capable of diffusing into the CNS and multiple zwitterionic forms optimal for reactivation reactions. Conformationally diverse zwitterions that could act as structural antidote variants significantly improved in vitro reactivation of diverse OP-hAChE conjugates. Oxime group re-orientation of one of the bis-oximes, forcing it to point into the active center for reactivation, was confirmed by X-ray structural analysis. Our findings provide detailed structure-activity properties of several CNS-directed, uncharged aliphatic bis-oximes holding promise for use as protonation-dependent, conformationally adaptive, "smart" accelerated antidotes against OP toxicity.

Serving a critical role in neurotransmission, human acetylcholinesterase (hAChE) is the target of organophosphate nerve agents. Hence, there is an active interest in studying the mechanism of inhibition and recovery of enzymatic activity, which could lead to better countermeasures against nerve agents. As hAChE is found in different oligomeric assemblies, certain approaches to studying it have been problematic. Herein, we examine the biochemical and structural impact of monomerizing hAChE by using two mutations: L380R/F535K. The activities of monomeric hAChE L380R/F535K and dimeric hAChE were determined to be comparable utilizing a modified Ellman's assay. To investigate the influence of subunit-subunit interactions on the structure of hAChE, a 2.1 A X-ray crystallographic structure was determined. Apart from minor shifts along the dimer interface, the overall structure of the hAChE L380R/F535K mutant is similar to that of dimeric hAChE. To probe whether the plasticity of the active site was overtly impacted by monomerizing hAChE, the kinetic constants of (PR/S ) - VX (ethyl({2-[bis(propan-2-yl)amino]ethyl}sulfanyl)(methyl)phosphinate) inhibition and subsequent rescue of hAChE L380R/F535K activity with HI-6 (1-(2'-hydroxyiminomethyl-1'-pyridinium)-3-(4'-carbamoyl-1-pyridinium)) were determined and found to be comparable to those of dimeric hAChE. Thus, hAChE L380R/F535K could be used as a substitute for dimeric hAChE when experimentally probing the ability of the hAChE active site to accommodate future nerve agent threats or judge the ability of new therapeutics to access the active site.

Structure-guided design of novel pharmacologically active molecules relies at least in part on functionally relevant accuracy of macromolecular structures for template based drug design. Currently, about 95% of all macromolecular X-ray structures available in the PDB (Protein Data Bank) were obtained from diffraction experiments at low, cryogenic temperatures. However, it is known that functionally relevant conformations of both macromolecules and pharmacological ligands can differ at higher, physiological temperatures. We describe in this article development and properties of new human acetylcholinesterase (AChE) crystals of space group P31 and a new unit cell, amenable for room-temperature X-ray diffraction studies. We co-crystallized hAChE in P31 unit cell with the reversible inhibitor 9-aminoacridine that binds at the base of the active center gorge in addition to inhibitors that span the full length of the gorge, donepezil (Aricept, E2020) and AChE specific inhibitor BW284c51. Their new low temperature P31 space group structures appear similar to those previously obtained in the different P3121 unit cell. Successful solution of the new room temperature 3.2 A resolution structure of BW284c51*hAChE complex from large P31 crystals enables us to proceed with studying room temperature structures of lower affinity complexes, such as oxime reactivators bound to hAChE, where temperature related conformational diversity could be expected in both oxime and hAChE, which could lead to better informed structure-based design under closer-to-physiological temperature conditions.

Exposure to organophosphorus compounds (OPs) may be fatal if untreated, and a clear and present danger posed by nerve agent OPs has become palpable in recent years. OPs inactivate acetylcholinesterase (AChE) by covalently modifying its catalytic serine. Inhibited AChE cannot hydrolyze the neurotransmitter acetylcholine leading to its build-up at the cholinergic synapses and creating an acute cholinergic crisis. Current antidotes, including oxime reactivators that attack the OP-AChE conjugate to free the active enzyme, are inefficient. Better reactivators are sought, but their design is hampered by a conformationally rigid portrait of AChE extracted exclusively from 100K X-ray crystallography and scarcity of structural knowledge on human AChE (hAChE). Here, we present room temperature X-ray structures of native and VX-phosphonylated hAChE with an imidazole-based oxime reactivator, RS-170B. We discovered that inhibition with VX triggers substantial conformational changes in bound RS-170B from a "nonproductive" pose (the reactive aldoxime group points away from the VX-bound serine) in the reactivator-only complex to a "semi-productive" orientation in the VX-modified complex. This observation, supported by concurrent molecular simulations, suggested that the narrow active-site gorge of hAChE may be significantly more dynamic than previously thought, allowing RS-170B to reorient inside the gorge. Furthermore, we found that small molecules can bind in the choline-binding site hindering approach to the phosphorous of VX-bound serine. Our results provide structural and mechanistic perspectives on the reactivation of OP-inhibited hAChE and demonstrate that structural studies at physiologically relevant temperatures can deliver previously overlooked insights applicable for designing next-generation antidotes.

The threat of a deliberate release of chemical nerve agents has underscored the need to continually improve field effective treatments for these types of poisonings. The oxime containing HLo-7 is a potential second-generation therapeutic reactivator. A synthetic process for HLo-7 is detailed with improvements to the DIBAL reduction and ion exchange steps. HLo-7 was visualized for the first time within the active site of human acetylcholinesterase and its relative ex vivo potency confirmed against various nerve agents using a phrenic nerve hemidiaphragm assay.

Tabun represents the phosphoramidate class of organophosphates that are covalent inhibitors of acetylcholinesterase (AChE), an essential enzyme in neurotransmission. Currently used therapy in counteracting excessive cholinergic stimulation consists of a muscarinic antagonist (atropine) and an oxime reactivator of inhibited AChE, but the classical oximes are particularly ineffective in counteracting tabun exposure. In a recent publication (Kovarik et al., 2019), we showed that several oximes prepared by the Huisgen 1,3 dipolar cycloaddition and related precursors efficiently reactivate the tabun-AChE conjugate. Herein, we pursue the antidotal question further and examine a series of lead precursor molecules, along with triazole compounds, as reactivators of two AChE mutant enzymes. Such studies should reveal structural subtleties that reside within the architecture of the active center gorge of AChE and uncover intimate mechanisms of reactivation of alkylphosphate conjugates of AChE. The designated mutations appear to minimize steric constraints of the reactivating oximes within the impacted active center gorge. Indeed, after initial screening of the triazole oxime library and its precursors for the reactivation efficacy on Y337A and Y337A/F338A human AChE mutants, we found potentially active oxime-mutant enzyme pairs capable of degrading tabun in cycles of inhibition and reactivation. Surprisingly, the most sensitive ex vivo reactivation of mutant AChEs occurred with the alkylpyridinium aldoximes. Hence, although the use of mutant enzyme bio-scavengers in humans may be limited in practicality, bioscavenging and efficient neutralization of tabun itself or phosphoramidate mixtures of organophosphates might be achieved efficiently in vitro or ex vivo with these mutant AChE combinations.

Profound synaptic dysfunction contributes to early loss of short-term memory in Alzheimer's disease. This study was set up to analyze possible neuroprotective effects of two dual binding site inhibitors of acetylcholinesterase (AChE), a new 6-methyluracil derivative, C-35, and the clinically used inhibitor donepezil. Crystal structure of the complex between human AChE and C-35 revealed tight contacts of ligand along the enzyme active site gorge. Molecular dynamics simulations indicated that the external flexible part of the ligand establishes multiple transient interactions with the enzyme peripheral anionic site. Thus, C-35 is a dual binding site inhibitor of AChE. In transgenic mice, expressing a chimeric mouse/human amyloid precursor protein and a human presenilin-1 mutant, C-35 (5mg/kg, i.p) and donepezil (0.75mg/kg, i.p) partially reversed synapse loss, decreased the number of amyloid plaques, and restored learning and memory. To separate temporal symptomatic therapeutic effects, associated with the increased lifetime of acetylcholine in the brain, from possible disease-modifying effect, an experimental protocol based on drug withdrawal from therapy was performed. When administration of C-35 and donepezil was terminated three weeks after the trial started, animals that were receiving C-35 showed a much better ability to learn than those who received vehicle or donepezil. Our results provide additional evidence that dual binding site inhibitors of AChE have Alzheimer's disease-modifying action.

Over 50 years ago, the toxicity of irreversible organophosphate inhibitors targeting human acetylcholinesterase (hAChE) was observed to be stereospecific. The therapeutic reversal of hAChE inhibition by reactivators has also been shown to depend on the stereochemistry of the inhibitor. To gain clarity on the mechanism of stereospecific inhibition, the X-ray crystallographic structures of hAChE inhibited by a racemic mixture of VX (P R/S) and its enantiomers were obtained. Beyond identifying hAChE structural features that lend themselves to stereospecific inhibition, structures of the reactivator HI-6 bound to hAChE inhibited by VX enantiomers of varying toxicity, or in its uninhibited state, were obtained. Comparison of hAChE in these pre-reactivation and post-reactivation states along with enzymatic data reveals the potential influence of unproductive reactivator poses on the efficacy of these types of therapeutics. The recognition of structural features related to hAChE's stereospecificity toward VX shed light on the molecular influences of toxicity and their effect on reactivators. In addition to providing a better understanding of the innate issues with current reactivators, an avenue for improvement of reactivators is envisioned.

Organophosphorus nerve agents interfere with cholinergic signaling by covalently binding to the active site of the enzyme acetylcholinesterase (AChE). This inhibition causes an accumulation of the neurotransmitter acetylcholine, potentially leading to overstimulation of the nervous system and death. Current treatments include the use of antidotes that promote the release of functional AChE by an unknown reactivation mechanism. We have used diffusion trap cryocrystallography and density functional theory (DFT) calculations to determine and analyze prereaction conformers of the nerve agent antidote HI-6 in complex with Mus musculus AChE covalently inhibited by the nerve agent sarin. These analyses reveal previously unknown conformations of the system and suggest that the cleavage of the covalent enzyme-sarin bond is preceded by a conformational change in the sarin adduct itself. Together with data from the reactivation kinetics, this alternate conformation suggests a key interaction between Glu202 and the O-isopropyl moiety of sarin. Moreover, solvent kinetic isotope effect experiments using deuterium oxide reveal that the reactivation mechanism features an isotope-sensitive step. These findings provide insights into the reactivation mechanism and provide a starting point for the development of improved antidotes. The work also illustrates how DFT calculations can guide the interpretation, analysis, and validation of crystallographic data for challenging reactive systems with complex conformational dynamics.

Irreversible inhibition of the essential nervous system enzyme acetylcholinesterase by organophosphate nerve agents and pesticides may quickly lead to death. Oxime reactivators currently used as antidotes are generally less effective against pesticide exposure than nerve agent exposure, and pesticide exposure constitutes the majority of cases of organophosphate poisoning in the world. The current lack of published structural data specific to human acetylcholinesterase organophosphate-inhibited and oxime-bound states hinders development of effective medical treatments. We have solved structures of human acetylcholinesterase in different states in complex with the organophosphate insecticide, paraoxon, and oximes. Reaction with paraoxon results in a highly perturbed acyl loop that causes a narrowing of the gorge in the peripheral site that may impede entry of reactivators. This appears characteristic of acetylcholinesterase inhibition by organophosphate insecticides but not nerve agents. Additional changes seen at the dimer interface are novel and provide further examples of the disruptive effect of paraoxon. Ternary structures of paraoxon-inhibited human acetylcholinesterase in complex with the oximes HI6 and 2-PAM reveals relatively poor positioning for reactivation. This study provides a structural foundation for improved reactivator design for the treatment of organophosphate intoxication. Proteins 2016; 84:1246-1256. (c) 2016 Wiley Periodicals, Inc.

Upon heterologous overexpression, many proteins misfold or aggregate, thus resulting in low functional yields. Human acetylcholinesterase (hAChE), an enzyme mediating synaptic transmission, is a typical case of a human protein that necessitates mammalian systems to obtain functional expression. We developed a computational strategy and designed an AChE variant bearing 51 mutations that improved core packing, surface polarity, and backbone rigidity. This variant expressed at approximately 2,000-fold higher levels in E. coli compared to wild-type hAChE and exhibited 20 degrees C higher thermostability with no change in enzymatic properties or in the active-site configuration as determined by crystallography. To demonstrate broad utility, we similarly designed four other human and bacterial proteins. Testing at most three designs per protein, we obtained enhanced stability and/or higher yields of soluble and active protein in E. coli. Our algorithm requires only a 3D structure and several dozen sequences of naturally occurring homologs, and is available at http://pross.weizmann.ac.il.

The high toxicity of organophosphorus compounds originates from covalent inhibition of acetylcholinesterase (AChE), an essential enzyme in cholinergic neurotransmission. Poisonings that lead to life-threatening toxic manifestations require immediate treatment that combines administration of anticholinergic drugs and an aldoxime as a reactivator of AChE. An alternative approach to reduce the in vivo toxicity of OPs focuses on the use of bioscavengers against the parent organophosphate. Our previous research showed that AChE mutagenesis can enable aldoximes to substantially accelerate the reactivation of OP-enzyme conjugates, while dramatically slowing down rates of OP-conjugate dealkylation (aging). Herein, we demonstrate an efficient HI-6-assisted VX detoxification, both ex vivo in human blood and in vivo in mice by hAChE mutants modified at the choline binding site (Y337A and Y337A/F338A). The catalytic scavenging of VX in mice improved therapeutic outcomes preventing lethality and resulted in a delayed onset of toxicity symptoms.

Acetylcholinesterase is a critical enzyme that regulates neurotransmission by degrading the neurotransmitter acetylcholine in synapses of the nervous system. It is an important target for both therapeutic drugs that treat Alzheimers disease and chemical warfare agents that cripple the nervous system and cause death through paralysis. The enzyme has both catalytic and peripheral sites to which inhibitors may bind. Structures of recombinant human acetylcholinesterase in complex with the natural product inhibitors dihydrotanshinone I and territrem B reveal dihydrotanshinone I binding that is specific to only the peripheral site and territrem B binding that spans both sites and distorts the protein backbone in the peripheral site. These inhibitors may function as important molecular templates for therapeutics used for treatment of disease and protection against nerve agents.

The multifunctional nature of Alzheimer's disease calls for MTDLs (multitarget-directed ligands) to act on different components of the pathology, like the cholinergic dysfunction and amyloid aggregation. Such MTDLs are usually on the basis of cholinesterase inhibitors (e.g. tacrine or huprine) coupled with another active molecule aimed at a different target. To aid in the design of these MTDLs, we report the crystal structures of hAChE (human acetylcholinesterase) in complex with FAS-2 (fasciculin 2) and a hydroxylated derivative of huprine (huprine W), and of hBChE (human butyrylcholinesterase) in complex with tacrine. Huprine W in hAChE and tacrine in hBChE reside in strikingly similar positions highlighting the conservation of key interactions, namely, pi-pi/cation-pi interactions with Trp86 (Trp82), and hydrogen bonding with the main chain carbonyl of the catalytic histidine residue. Huprine W forms additional interactions with hAChE, which explains its superior affinity: the isoquinoline moiety is associated with a group of aromatic residues (Tyr337, Phe338 and Phe295 not present in hBChE) in addition to Trp86; the hydroxyl group is hydrogen bonded to both the catalytic serine residue and residues in the oxyanion hole; and the chlorine substituent is nested in a hydrophobic pocket interacting strongly with Trp439. There is no pocket in hBChE that is able to accommodate the chlorine substituent.

Human acetylcholinesterase (AChE) is a significant target for therapeutic drugs. Here we present high resolution crystal structures of human AChE, alone and in complexes with drug ligands; donepezil, an Alzheimer's disease drug, binds differently to human AChE than it does to Torpedo AChE. These crystals of human AChE provide a more accurate platform for further drug development than previously available.

Tabun is a warfare agent that inhibits human acetylcholinesterase (hAChE) by rapid phosphylation of the catalytic serine. A time-dependent reaction occurs on the tabun adduct, leading to an "aged" enzyme, resistant to oxime reactivators. The aging reaction may proceed via either dealkylation or deamidation, depending on the stereochemistry of the phosphoramidyl adduct. We solved the X-ray structure of aged tabun-hAChE complexed with fasciculin II, and we show that aging proceeds through O-dealkylation, in agreement with the aging mechanism that we determined for tabun-inhibited human butyrylcholinesterase and mouse acetylcholinesterase. Noteworthy, aging and binding of fasciculin II lead to an improved thermostability, resulting from additional stabilizing interactions between the two subdomains that face each other across the active site gorge. This first structure of hAChE inhibited by a nerve agent provides structural insight into the inhibition and aging mechanisms and a structural template for the design of molecules capable of reactivating aged hAChE.

By rapid hydrolysis of the neurotransmitter, acetylcholine, acetylcholinesterase terminates neurotransmission at cholinergic synapses. Acetylcholinesterase is a very fast enzyme, functioning at a rate approaching that of a diffusion-controlled reaction. The powerful toxicity of organophosphate poisons is attributed primarily to their potent inhibition of acetylcholinesterase. Acetylcholinesterase inhibitors are utilized in the treatment of various neurological disorders, and are the principal drugs approved thus far by the FDA for management of Alzheimer's disease. Many organophosphates and carbamates serve as potent insecticides, by selectively inhibiting insect acetylcholinesterase. The determination of the crystal structure of Torpedo californica acetylcholinesterase permitted visualization, for the first time, at atomic resolution, of a binding pocket for acetylcholine. It also allowed identification of the active site of acetylcholinesterase, which, unexpectedly, is located at the bottom of a deep gorge lined largely by aromatic residues. The crystal structure of recombinant human acetylcholinesterase in its apo-state is similar in its overall features to that of the Torpedo enzyme; however, the unique crystal packing reveals a novel peptide sequence which blocks access to the active-site gorge.

Human chromosome 7 has historically received prominent attention in the human genetics community, primarily related to the search for the cystic fibrosis gene and the frequent cytogenetic changes associated with various forms of cancer. Here we present more than 153 million base pairs representing 99.4% of the euchromatic sequence of chromosome 7, the first metacentric chromosome completed so far. The sequence has excellent concordance with previously established physical and genetic maps, and it exhibits an unusual amount of segmentally duplicated sequence (8.2%), with marked differences between the two arms. Our initial analyses have identified 1,150 protein-coding genes, 605 of which have been confirmed by complementary DNA sequences, and an additional 941 pseudogenes. Of genes confirmed by transcript sequences, some are polymorphic for mutations that disrupt the reading frame.

A 2.91-billion base pair (bp) consensus sequence of the euchromatic portion of the human genome was generated by the whole-genome shotgun sequencing method. The 14.8-billion bp DNA sequence was generated over 9 months from 27,271,853 high-quality sequence reads (5.11-fold coverage of the genome) from both ends of plasmid clones made from the DNA of five individuals. Two assembly strategies-a whole-genome assembly and a regional chromosome assembly-were used, each combining sequence data from Celera and the publicly funded genome effort. The public data were shredded into 550-bp segments to create a 2.9-fold coverage of those genome regions that had been sequenced, without including biases inherent in the cloning and assembly procedure used by the publicly funded group. This brought the effective coverage in the assemblies to eightfold, reducing the number and size of gaps in the final assembly over what would be obtained with 5.11-fold coverage. The two assembly strategies yielded very similar results that largely agree with independent mapping data. The assemblies effectively cover the euchromatic regions of the human chromosomes. More than 90% of the genome is in scaffold assemblies of 100,000 bp or more, and 25% of the genome is in scaffolds of 10 million bp or larger. Analysis of the genome sequence revealed 26,588 protein-encoding transcripts for which there was strong corroborating evidence and an additional approximately 12,000 computationally derived genes with mouse matches or other weak supporting evidence. Although gene-dense clusters are obvious, almost half the genes are dispersed in low G+C sequence separated by large tracts of apparently noncoding sequence. Only 1.1% of the genome is spanned by exons, whereas 24% is in introns, with 75% of the genome being intergenic DNA. Duplications of segmental blocks, ranging in size up to chromosomal lengths, are abundant throughout the genome and reveal a complex evolutionary history. Comparative genomic analysis indicates vertebrate expansions of genes associated with neuronal function, with tissue-specific developmental regulation, and with the hemostasis and immune systems. DNA sequence comparisons between the consensus sequence and publicly funded genome data provided locations of 2.1 million single-nucleotide polymorphisms (SNPs). A random pair of human haploid genomes differed at a rate of 1 bp per 1250 on average, but there was marked heterogeneity in the level of polymorphism across the genome. Less than 1% of all SNPs resulted in variation in proteins, but the task of determining which SNPs have functional consequences remains an open challenge.

Chromosome 7q22 has been the focus of many cytogenetic and molecular studies aimed at delineating regions commonly deleted in myeloid leukemias and myelodysplastic syndromes. We have compared a gene-dense, GC-rich sub-region of 7q22 with the orthologous region on mouse chromosome 5. A physical map of 640 kb of genomic DNA from mouse chromosome 5 was derived from a series of overlapping bacterial artificial chromosomes. A 296 kb segment from the physical map, spanning ACHE: to Tfr2, was compared with 267 kb of human sequence. We identified a conserved linkage of 12 genes including an open reading frame flanked by ACHE: and Asr2, a novel cation-chloride cotransporter interacting protein Cip1, Ephb4, Zan and Perq1. While some of these genes have been previously described, in each case we present new data derived from our comparative sequence analysis. Adjacent unfinished sequence data from the mouse contains an orthologous block of 10 additional genes including three novel cDNA sequences that we subsequently mapped to human 7q22. Methods for displaying comparative genomic information, including unfinished sequence data, are becoming increasingly important. We supplement our printed comparative analysis with a new, Web-based program called Laj (local alignments with java). Laj provides interactive access to archived pairwise sequence alignments via the WWW. It displays synchronized views of a dot-plot, a percent identity plot, a nucleotide-level local alignment and a variety of relevant annotations. Our mouse-human comparison can be viewed at http://web.uvic.ca/~bioweb/laj.html. Laj is available at http://bio.cse.psu.edu/, along with online documentation and additional examples of annotated genomic regions.

Structures of recombinant wild-type human acetylcholinesterase and of its E202Q mutant as complexes with fasciculin-II, a 'three-finger' polypeptide toxin purified from the venom of the eastern green mamba (Dendroaspis angusticeps), are reported. The structure of the complex of the wild-type enzyme was solved to 2.8 A resolution by molecular replacement starting from the structure of the complex of Torpedo californica acetylcholinesterase with fasciculin-II and verified by starting from a similar complex with mouse acetylcholinesterase. The overall structure is surprisingly similar to that of the T. californica enzyme with fasciculin-II and, as expected, to that of the mouse acetylcholinesterase complex. The structure of the E202Q mutant complex was refined starting from the corresponding wild-type human acetylcholinesterase structure, using the 2.7 A resolution data set collected. Comparison of the two structures shows that removal of the charged group from the protein core and its substitution by a neutral isosteric moiety does not disrupt the functional architecture of the active centre. One of the elements of this architecture is thought to be a hydrogen-bond network including residues Glu202, Glu450, Tyr133 and two bridging molecules of water, which is conserved in other vertebrate acetylcholinesterases as well as in the human enzyme. The present findings are consistent with the notion that the main role of this network is the proper positioning of the Glu202 carboxylate relative to the catalytic triad, thus defining its functional role in the interaction of acetylcholinesterase with substrates and inhibitors.

Hypersensitivity to acetylcholinesterase inhibitors (anti-AChEs) causes severe nervous system symptoms under low dose exposure. In search of direct genetic origin(s) for this sensitivity, we studied six regions in the extended 22 kb promoter of the ACHE gene in individuals who presented adverse responses to anti-AChEs and in randomly chosen controls. Two contiguous mutations, a T-->A substitution, disrupting a putative glucocorticoid response element, and a 4-bp deletion, abolishing one of two adjacent HNF3 binding sites, were identified 17 kb upstream of the transcription start site. Allele frequencies for these mutations were 0.006 and 0.012, respectively, in 333 individuals of various ethnic origins, with a strong linkage between the deletion and the biochemically neutral H322N mutation in the coding region of ACHE. Heterozygous carriers of the deletion included a proband who presented with acute hypersensitivity to the anti-AChE pyridostigmine and another with unexplained excessive vomiting during a fourth pregnancy following three spontaneous abortions. Electromobility shift assays, transfection studies and measurements of AChE levels in immortalized lymphocytes as well as in peripheral blood from both carriers and non-carriers, revealed functional relevance for this mutation both in vitro and in vivo and showed it to increase AChE expression, probably by alleviating competition between the two hepatocyte nuclear factor 3 binding sites. Moreover, AChE-overexpressing transgenic mice, unlike normal FVB/N mice, displayed anti-AChE hypersensitivity and failed to transcriptionally induce AChE production following exposure to anti-AChEs. Our findings point to promoter polymorphism(s) in the ACHE gene as the dominant susceptibility factor(s) for adverse responses to exposure or to treatment with anti-AChEs.

Huperzine A (HUP), a natural, potent, 'slow,' reversible inhibitor of antiacetylcholinesterase (AChE), has been suggested to be superior to antiacetylcholinesterase drugs now being used for management of Alzheimer's disease. To delineate the binding site of human AChE (HuAChE) for HUP, the biochemical constants kon, koff, and Ki were determined for complexes formed between HUP and single-site (Y337F, Y337A, F295A, W286A, and E202Q) or double-site (F295L/F297V) mutants of recombinant HuAChE (rHuAChE). The kinetic and dissociation constants were compared with those obtained for wild-type rHuAChE and AChE from Torpedo californica. Results demonstrate that the inhibition of AChE by HUP occurs through association with residues located inside the active site 'gorge,' rather than at the rim of the gorge. Tyrosine at position 337 (Y337) is essential for inhibition of rHuAChE by HUP (Ki = 26 nM). An aromatic array constituted from residues Y337, F295, and probably W86 is likely to offer a multicontact subsite that interacts with the ammonium group and with both the exo-and endocyclic double bond moieties of HUP. Lack of the aromatic side chain in the position homologous to Y337 explains the poor inhibitory potency of HUP toward human butyrylcholinesterase (Ki > 20,000 nM). Replacement of the carboxylate-containing E202 by glutamine had only marginal effect on the stability of the complex formed between HUP and rHuAChE. The pH-rate profiles suggest that destabilization of the complex after proton gain cannot be attributed solely to protonation of E202. These findings are expected to establish HUP as a lead compound for the design of new anti-AChE drugs.

Substrate specificity determinants of human acetylcholinesterase (HuAChE) were identified by combination of molecular modeling and kinetic studies with enzymes mutated in residues Trp-86, Trp-286, Phe-295, Phe-297, Tyr-337, and Phe-338. The substitution of Trp-86 by alanine resulted in a 660-fold decrease in affinity for acetythiocholine but had no effect on affinity for the isosteric uncharged substrate (3,3-dimethylbutylthioacetate). The results demonstrate that residue Trp-86 is the anionic site which binds, through cation-pi interactions, the quaternary ammonium of choline, and that of active center inhibitors such as edrophonium. The results also suggest that in the non-covalent complex, charged and uncharged substrates with a common acyl moiety (acetyl) bind to different molecular environments. The hydrophobic site for the alcoholic portion of the covalent adduct (tetrahedral intermediate) includes residues Trp-86, Tyr-337, and Phe-338, which operate through nonpolar and/or stacking interactions, depending on the substrate. Substrates containing choline but differing in the acyl moiety (acetyl, propyl, and butyryl) revealed that residues Phe-295 and Phe-297 determine substrate specificity of the acyl pocket for the covalent adducts. Phe-295 also determines substrate specificity in the non-covalent enzyme substrate complex and thus, the HuAChE F295A mutant exhibits over 130-fold increase in the apparent bimolecular rate constant for butyrylthiocholine compared with wild type enzyme. Reactivity toward specific butyrylcholinesterase inhibitors is similarly dependent on the nature of residues at positions 295 and 297. Amino acid Trp-286 at the rim of the active site "gorge" and Trp-86, in the active center, are essential elements in the mechanism of inhibition by propidium, a peripheral anionic site ligand. Molecular modeling and kinetic data suggest that a cross-talk between Trp-286 and Trp-86 can result in reorientation of Trp-86 which may then interfere with stabilization of substrate enzyme complexes. It is proposed that the conformational flexibility of aromatic residues generates a plasticity in the active center that contributes to the high efficiency of AChE and its ability to respond to external stimuli.

To study the primary structure of human acetylcholinesterase (AcChoEase; EC 3.1.1.7) and its gene expression and amplification, cDNA libraries from human tissues expressing oocyte-translatable AcChoEase mRNA were constructed and screened with labeled oligodeoxynucleotide probes. Several cDNA clones were isolated that encoded a polypeptide with greater than or equal to 50% identically aligned amino acids to Torpedo AcChoEase and human butyrylcholinesterase (BtChoEase; EC 3.1.1.8). However, these cDNA clones were all truncated within a 300-nucleotide-long G + C-rich region with a predicted pattern of secondary structure having a high Gibbs free energy (-117 kcal/mol) downstream from the expected 5' end of the coding region. Screening of a genomic DNA library revealed the missing 5' domain. When ligated to the cDNA and constructed into a transcription vector, this sequence encoded a synthetic mRNA translated in microinjected oocytes into catalytically active AcChoEase with marked preference for acetylthiocholine over butyrylthiocholine as a substrate, susceptibility to inhibition by the AcChoEase inhibitor BW284C51, and resistance to the BtChoEase inhibitor tetraisopropylpyrophosphoramide. Blot hybridization of genomic DNA from different individuals carrying amplified AcChoEase genes revealed variable intensities and restriction patterns with probes from the regions upstream and downstream from the predicted G + C-rich structure. Thus, the human AcChoEase gene includes a putative G + C-rich attenuator domain and is subject to structural alterations in cases of AcChoEase gene amplification.

A single step immunoaffinity purification procedure for human erythrocyte acetylcholinesterase is described which permitted the isolation of milligram quantities of enzyme from 10 U of erythrocytes, with 113,000-fold purification and a yield of about 22%. In SDS-PAGE analysis, the enzyme corresponds to a disulfide linked dimer of 140 kDa which is converted to a 70 kDa monomer upon disulfide reduction. The tryptic peptides generated from purified enzyme were separated by reverse-phase HPLC. Five of these peptides were analysed to determine the amino acid sequences. The obtained sequences showed no homology to the already known amino acid sequences for human serum and brain butyrylcholinesterase and Torpedo californica acetylcholinesterase.