Gene/Protein Disease Symptom Drug Enzyme Compound
Pivot Concepts: Gene/Protein Disease Symptom Drug Enzyme Compound   Target Concepts: Gene/Protein Disease Symptom Drug Enzyme Compound

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Query: EC:3.1.1.8 (cholinesterase)
12,691 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Cholinesterases use a Glu-His-Ser catalytic triad to enhance the nucleophilicity of the catalytic serine. We have previously shown by proton NMR that horse serum butyryl cholinesterase, like serine proteases, forms a short, strong hydrogen bond (SSHB) between the Glu-His pair upon binding mechanism-based inhibitors, which form tetrahedral adducts, analogous to the tetrahedral intermediates in catalysis [Viragh, C., et al. (2000) Biochemistry 39, 16200-16205]. We now extend these studies to human acetylcholinesterase, a 136 kDa homodimer. The free enzyme at pH 7.5 shows a proton resonance at 14.4 ppm assigned to an imidazole NH of the active-site histidine, but no deshielded proton resonances between 15 and 21 ppm. Addition of a 3-fold excess of the mechanism-based inhibitor m-(N,N,N-trimethylammonio)trifluoroacetophenone (TMTFA) induced the complete loss of the 14.4 ppm signal and the appearance of a broad, deshielded resonance of equal intensity with a chemical shift delta of 17.8 ppm and a D/H fractionation factor phi of 0.76 +/- 0.10, consistent with a SSHB between Glu and His of the catalytic triad. From an empirical correlation of delta with hydrogen bond lengths in small crystalline compounds, the length of this SSHB is 2.62 +/- 0.02 A, in agreement with the length of 2.63 +/- 0.03 A, independently obtained from phi. Upon addition of a 3-fold excess of the mechanism-based inhibitor 4-nitrophenyl diethyl phosphate (paraoxon) to the free enzyme at pH 7.5, and subsequent deethylation, two deshielded resonances of unequal intensity appeared at 16.6 and 15.5 ppm, consistent with SSHBs with lengths of 2.63 +/- 0.02 and 2.65 +/- 0.02 A, respectively, suggesting conformational heterogeneity of the active-site histidine as a hydrogen bond donor to either Glu-327 of the catalytic triad or to Glu-199, also in the active site. Conformational heterogeneity was confirmed with the methylphosphonate ester anion adduct of the active-site serine, which showed two deshielded resonances of equal intensity at 16.5 and 15.8 ppm with phi values of 0.47 +/- 0.10 and 0.49 +/- 0.10 corresponding to average hydrogen bond lengths of 2.59 +/- 0.04 and 2.61 +/- 0.04 A, respectively. Similarly, lowering the pH of the free enzyme to 5.1 to protonate the active-site histidine (pK(a) = 6.0 +/- 0.4) resulted in the appearance of two deshielded resonances, at 17.7 and 16.4 ppm, consistent with SSHBs with lengths of 2.62 +/- 0.02 and 2.63 +/- 0.02 A, respectively. The NMR-derived distances agree with those found in the X-ray structures of the homologous acetylcholinesterase from Torpedo californica complexed with TMTFA (2.66 +/- 0.28 A) and sarin (2.53 +/- 0.26 A) and at low pH (2.52 +/- 0.25 A). However, the order of magnitude greater precision of the NMR-derived distances establishes the presence of SSHBs at the active site of acetylcholinesterase, and detect conformational heterogeneity of the active-site histidine. We suggest that the high catalytic power of cholinesterases results in part from the formation of a SSHB between Glu and His of the catalytic triad.
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PMID:Short, strong hydrogen bonds at the active site of human acetylcholinesterase: proton NMR studies. 1134 33

The active center gorge of human acetylcholinesterase (HuAChE) is lined by 14 aromatic residues, whereas in the closely related human butyrylcholinesterase (HuBChE) 3 of the aromatic active center residues (Phe295, Phe297, Tyr337) as well as 3 of the residues at the gorge entrance (Tyr72, Tyr124, Trp286) are replaced by aliphatic amino acids. To investigate whether this structural variability can account for the reactivity differences between the two enzymes, gradual replacement of up to all of the 6 aromatic residues in HuAChE by the corresponding residues in HuBChE was carried out. The affinities of the hexamutant (Y72N/Y124Q/W286A/F295L/F297V/Y337A) toward tacrine, decamethonium, edrophonium, huperzine A, or BW284C51 differed by about 5-, 80-, 170-, 25000-, and 17000-fold, respectively, from those of the wild-type HuAChE. For most of these prototypical noncovalent active center and peripheral site ligands, the hexamutant HuAChE displayed a reactivity phenotype closely resembling that of HuBChE. These results support the accepted view that the active center architectures of AChE and BChE differ mainly by the presence of a larger void space in BChE. Nevertheless, reactivity of the hexamutant HuAChE toward the substrates acetylthiocholine and butyrylthiocholine, or covalent ligands such as phosphonates and the transition state analogue m-(N,N,N-trimethylammonio)trifluoroacetophenone (TMTFA), is about 45-170-fold lower than that of HuBChE. Most of this reduction in reactivity can be related to the combined replacements of the three aromatic residues at the active center, Phe295, Phe297, and Tyr337. We propose that the hexamutant HuAChE, unlike BChE, is impaired in its capacity to accommodate certain tetrahedral species in the active center. This impairment may be related to the enhanced mobility of the catalytic histidine His447, which is observed in molecular dynamics simulations of the hexamutant and the F295L/F297V/Y337A HuAChE enzymes but not in the wild-type HuAChE.
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PMID:Does "butyrylization" of acetylcholinesterase through substitution of the six divergent aromatic amino acids in the active center gorge generate an enzyme mimic of butyrylcholinesterase? 1141 96

A patient was a fifty-year-old man, who had a 35 year-history of facioscapulohumeral muscular dystrophy (FSHD). He was admitted to our hospital because of acute progressive weakness involving his lower extremities without any fluctuation in the recent 3 weeks. We clinically followed him for 30 years and he was able to do all daily activities, walked alone, drove a car and climbed stairs with a handrail. His 76-year-old mother had about 60 year-history of FSHD and could walk with support. On admission, neurological examination revealed moderate to marked muscle weakness and atrophy of the face, limb-girdle and all extremities, predominantly in the upper proximal limbs. He could hardly stand and needed a stick for walking. He had no blepharoptosis or ocular movement disturbance, and did not complain of difficulties in swallowing and chewing. CK values and other laboratory data were normal, and serum anti-Jo-1 antibody, anti-SSA/Ro antibody and anticardiolipin IgG antibody were negative. Because EMG examination revealed myopathic changes and an X-ray examination of the lumbar spine was normal. Thus, polymyositis and neurologenic disorders were ruled out. Disturbance in chewing and swallowing, that were uncommon in FSHD, appeared about a month after admission. Repetitive stimulation test revealed typical waning pattern. Edrophonium chloride injection was effective for decreased waning and the clinical symptoms. The titer of serum anti-ACh receptor antibody was 97 nmol/l, confirming the diagnosis of myasthenia gravis. Because of fluctuated dyspnea, thymectomy was done and his condition gradually relieved after administration of corticosteroid and choline esterase inhibitor. From this experience, we learned that we have to consider other neuromuscular disorders, even rare ones, if there existed unusual weakness of underlying muscular dystrophy.
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PMID:[A patient with facioscapulohumeral muscular dystrophy accompanied by myasthenia gravis]. 1167 58

We present a 73-year-old man with probable dementia with Lewy bodies(DLB). At 65 years of age, he gradually developed bradykinesia, gait disturbance and mild amnesia. At 71 years of age, he noted resting tremor in bilateral hands, and amnesia and disorientation were exacerbated. He was diagnosed as having parkinsonism and took L-dopa/carbidopa at 100 mg/day. Since he developed hallucination and abnormal behavior 2 days after the initiation of the drug, he stopped taking L-dopa and was admitted to our hospital. A neurological examination on admission revealed moderate amnesia, disorientation, finger agnosia, constitutional apraxia, mask-like face, cogwheel rigidity, resting tremor in bilateral hands, and bradykinesia. Brain MRI showed mild brain atrophy, and single photon emission computerized tomography(SPECT) showed diffuse moderate hypoperfusion in bilateral cerebral cortex. As he had fluctuating cognitive dysfunction and parkinsonism, he was diagnosed to have probable DLB. As his dementia was exacerbated by trihexyphenidyl, an anti-cholinergic agent, at 2 mg/day, we treated him with donepezil, an anti-choline esterase agent, at 3-5 mg/day. His parkinsonism, including rigidity and bradykinesia, was markedly improved his dementia, consisting of amnesia and disorientation. Electroencephalography (EEG) improved in the organization of the dominant rhythm. The SPECT improved in the blood perfusion of the bilateral frontal lobe as well as cognitive function and parkinsonism were maintained by donepezil for 6 months after discharge. A therapeutic efficacy of donepezil for DLB has recently been reported. It is notable that donepezil was beneficial not only for cognitive dysfunction but also for parkinsonism in the present case with probable DLB.
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PMID:[A patient with probable dementia with Lewy bodies, who showed improvement of dementia and parkinsonism by the administratim of donepezil]. 1180 50

Rivastigmine, a carbamate inhibitor of acetylcholinesterase, is already in use for treatment of Alzheimer's disease under the trade name of Exelon. Rivastigmine carbamylates Torpedo californica acetylcholinesterase very slowly (k(i) = 2.0 M(-1) min(-1)), whereas the bimolecular rate constant for inhibition of human acetylcholinesterase is >1600-fold higher (k(i) = 3300 M(-1) min(-1)). For human butyrylcholinesterase and for Drosophila melanogaster acetylcholinesterase, carbamylation is even more rapid (k(i) = 9 x 10(4) and 5 x 10(5) M(-1) min(-1), respectively). Spontaneous reactivation of all four conjugates is very slow, with <10% reactivation being observed for the Torpedo enzyme after 48 h. The crystal structure of the conjugate of rivastigmine with Torpedo acetylcholinesterase was determined to 2.2 A resolution. It revealed that the carbamyl moiety is covalently linked to the active-site serine, with the leaving group, (-)-S-3-[1-(dimethylamino)ethyl]phenol, being retained in the "anionic" site. A significant movement of the active-site histidine (H440) away from its normal hydrogen-bonded partner, E327, was observed, resulting in disruption of the catalytic triad. This movement may provide an explanation for the unusually slow kinetics of reactivation.
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PMID:Kinetic and structural studies on the interaction of cholinesterases with the anti-Alzheimer drug rivastigmine. 1188 71

While substitution of the aromatic residues (Phe295, Phe338), located in the vicinity of the catalytic His447 in human acetylcholinesterase (HuAChE) had little effect on catalytic activity, simultaneous replacement of both residues by aliphatic amino acids resulted in a 680-fold decrease in catalytic activity. Molecular simulations suggested that the activity decline is related to conformational destabilization of His447, similar to that observed for the hexamutant HuAChE which mimics the active center of butyrylcholinesterase. On the basis of model structures of other cholinesterases (ChEs), we predicted that catalytically nonproductive mobility of His447 could be restricted by introduction of aromatic residue in a different location adjacent to this histidine (Val407). Indeed, the F295A/F338A/V407F enzyme is 170-fold more reactive than the corresponding double mutant and only 3-fold less reactive than the wild-type HuAChE. However, analogous substitution of Val407 in the hexamutant HuAChE (generating the heptamutant Y72N/Y124Q/W286A/F295L/F297V/Y337A/V407F) did not enhance catalytic activity. Reactivity of these double, triple, hexa, and hepta mutant HuAChEs was monitored toward covalent ligands such as organophosphates and the transition state analogue TMFTA, which probe, respectively, the facility of the enzymes to accommodate Michaelis complexes and to undergo the acylation process. The findings suggest that in the F295A/F338A mutant the two His447 conformational states, which are essential for the different stages of the catalytic process, seem to be destabilized. On the other hand, in the F295A/F338A/V407F mutant only the state involved in acylation is impaired. Such differential effects on the His447 conformational properties demonstrate the general role of aromatic residues in cholinesterases, and probably in other serine hydrolases, in "trapping" of the catalytic histidine and thereby in optimization of catalytic activity.
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PMID:The aromatic "trapping" of the catalytic histidine is essential for efficient catalysis in acetylcholinesterase. 1208 73

Butyrylcholinesterase is a serine esterase, closely related to acetylcholinesterase. Both enzymes employ a catalytic triad mechanism for catalysis, similar to that used by serine proteases such as alpha-chymotrypsin. Enzymes of this type are generally considered to be inactive at pH values below 5, because the histidine member of the catalytic triad becomes protonated. We have found that butyrylcholinesterase retains activity at pH <or= 5, under conditions of excess substrate activation. This low-pH activity appears with wild-type butyrylcholinesterase as well as with all mutants we examined: A328G, A328I, A328F, A328Y, A328W, E197Q, L286W, V288W and Y332A (residue A328 is at the bottom of the active-site gorge, near the pi-cation-binding site; E197 is next to the active-site serine S198; L286 and V288 form the acyl-binding pocket; and Y332 is a component of the peripheral anionic site). For example, the kcat value at pH 5.0 for activity in the presence of excess substrate was 32900 +/- 4400 min(-1) for wild-type, 55200 +/- 1600 min(-1) for A328F, and 28 700 +/- 700 min(-1) for A328W. This activity is titratable, with pKa values of 6.0-6.6, suggesting that the catalytic histidine is protonated at pH 5. The existence of activity when the catalytic histidine is protonated indicates that the catalytic-triad mechanism of butyrylcholinesterase does not operate for catalysis at low pH. The mechanism explaining the catalytic behaviour of butyrylcholinesterase at low pH in the presence of excess substrate remains to be elucidated.
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PMID:High activity of human butyrylcholinesterase at low pH in the presence of excess butyrylthiocholine. 1260 82

Amphioxus (Branchiostoma floridae) cholinesterase 2 (ChE2) hydrolyzes acetylthiocholine (AsCh) almost exclusively. We constructed a homology model of ChE2 on the basis of Torpedo californica acetylcholinesterase (AChE) and found that the acyl pocket of the enzyme resembles that of Drosophila melanogaster AChE, which is proposed to be comprised of Phe330 (Phe290 in T. californica AChE) and Phe440 (Val400), rather than Leu328 (Phe288) and Phe330 (Phe290), as in vertebrate AChE. In ChE2, the homologous amino acids are Phe312 (Phe290) and Phe422 (Val400). To determine if these amino acids define the acyl pocket of ChE2 and its substrate specificity, and to obtain information about the hydrophobic subsite, partially comprised of Tyr352 (Phe330) and Phe353 (Phe331), we performed site-directed mutagenesis and in vitro expression. The aliphatic substitution mutant F312I ChE2 hydrolyzes AsCh preferentially but also butyrylthiocholine (BsCh), and the change in substrate specificity is due primarily to an increase in k(cat) for BsCh; K(m) and K(ss) are also altered. F422L and F422V produce enzymes that hydrolyze BsCh and AsCh equally due to an increase in k(cat) for BsCh and a decrease in k(cat) for AsCh. Our data suggest that Phe312 and Phe422 define the acyl pocket. We also screened mutants for changes in sensitivity to various inhibitors. Y352A increases the sensitivity of ChE2 to the bulky inhibitor ethopropazine. Y352A decreases inhibition by BW284c51, consistent with its role as part of the choline-binding site. Aliphatic replacement mutations produce enzymes that are more sensitive to inhibition by iso-OMPA, presumably by increasing access to the active site serine. Y352A, F353A and F353V make ChE2 considerably more resistant to inhibition by eserine and neostigmine, suggesting that binding of these aromatic inhibitors is mediated by pi-pi or cation-pi interactions at the hydrophobic site. Our results also provide information about the aromatic trapping of the active site histidine and the inactivation of ChE2 by sulfhydryl reagents.
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PMID:Amino acids defining the acyl pocket of an invertebrate cholinesterase. 1466 5

A grand mal seizure is an unexpected, rare adverse event in a child receiving eye drops to dilate the pupils for an eye examination. A case is reported of a convulsion in a 23-month-old boy after he received Cyclogyl (cyclopentolate) and Neo-Synephrine (phenylephrine) eye drops before ophthalmoscopy. His serum sodium was 125 mEq/L, and he had low plasma pseudocholinesterase activity. Children exposed to organophosphate insecticides and other pseudocholinesterase inhibitors may be at risk for cyclopentolate toxicity.
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PMID:Grand mal seizure in a child 30 minutes after Cyclogyl (cyclopentolate hydrochloride) and 10% Neo-Synephrine (phenylephrine hydrochloride) eye drops were instilled. 1512 94

Tosylphenylalanine chloromethyl ketone (TPCK) and tosyllysine chloromethyl ketone (TLCK) are irreversible modifiers of histidine which is located in the catalytic triad of chymotrypsin and trypsin, respectively. The effects of TPCK and TLCK on the histidine in the catalytic triad of the desensitized butyrylcholinesterase (BChE), prepared from human serum by heating at 45 degrees C for 24 h, were investigated in detail. It is found that these reagents do not modify, but reversibly inhibit the desensitized enzyme as a function of time. Just as it is for the native enzyme, TPCK is a hyperbolic mixed-type inhibitor of the desensitized BChE with Ki, alpha and beta values of 0.017 +/- 0.003 mM, 3.942 +/- 1.125 and 0.524 +/- 0.070, respectively. However, TLCK is the pure competitive inhibitor of the desensitized BChE with a Ki value of 0.008 +/- 0.000 mM, while it is hyperbolic mixed-type inhibitor of the native form. These findings show that the conformation of the active site cavity of desensitized BChE is different from that of the native enzyme.
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PMID:Do alkylating agents modify the histidine residue of the desensitized butyrylcholinesterase? 1564 71


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