Stereoselective inhibition of human, mouse, and horse cholinesterases by bambuterol enantiomers

Bambuterol is a chiral carbamate and a selective inhibitor of butyrylcholinesterase (BChE, EC 3.1.1.8). In order to relate bambuterol selectivity and stereoselectivity of BChE and acetylcholinesterase (AChE, EC 3.1.1.7) of different species, we studied the inhibition of human, mouse, and horse BChE, as well as AChE of human and mouse by (R)- and (S)-bambuterol. AChE and BChE of all studied species were progressively inhibited by both bambuterol enantiomers, with a preference for the (R)-bambuterol whose inhibition rate constants were about five times higher than that of (S)-bambuterol. We observed no significant difference between human and mouse in bambuterol enantiomer BChE inhibition. However, (R)-bambuterol inhibited horse BChE about 14 times slower than human and mouse BChE, and the inhibition rate for (S)-bambuterol was about 18 times slower. Although the primary structure of horse BChE differs from the other two species in 15 amino acids, we presumed that differences in inhibition rates could be attributed to threonine at position 69 located close to the peripheral site of BChE. Since BChE inhibition by bambuterol enantiomers was at least 8000 times faster than that of AChE, both bambuterol enantiomers proved to be selective BChE inhibitors, as was previously shown for racemate.     

Substrate activation of butyrylcholinesterase and substrate inhibition of acetylcholinesterase by 3,3-dimethylbutyl-N-n-butylcarbamate and 2-trimethylsilyl-ethyl-N-n-butylcarbamate

Carbamates are used to treat Alzheimer's disease. These compounds inhibit acetylcholinesterase and butyrylcholinesterase. The goal of this work is to use the substrate analogs of butyrylcholinesterase, 3,3-dimethylbutyl-N-n-butylcarbamate (1) and 2-trimethylsilyl-ethyl-N-n-butylcarbamate (2) to probe the substrate activation mechanism of butyrylcholinesterase. Compounds 1 and 2 are characterized as the pseudo substrate inhibitors of acetylcholinesterase; however, compounds 1 and 2 are characterized as the essential activators of butyrylcholinesterase. Therefore, compounds 1 and 2 mimic the substrate in the acetylcholinesterase-catalyzed reactions, but the behavior of compounds 1 and 2 mimics the substrate activation in the butyrylcholinesterase-catalyzed reactions.     

Design,synthesis and biological evaluation of novel carbamates as potential inhibitors of acetylcholinesterase and butyrylcholinesterase

Rivastigmine, a dual inhibitor of acetylcholinesterase (AChE) and butyrylcholinesterase (BChE), has been approved by U.S. Food and Drug Administration to treat Alzheimer’s disease (AD) and Parkinson’s disease (PD) dementia. In the current work, a bambuterol derivative lacking one of the carbamoyloxy groups on the benzene ring (BMC-1) and its analogues were synthesized using 1-(3-hydroxyphenyl) ethan-1-one and 1-(4-hydroxyphenyl) ethan-1-one as starting materials. In-vitro cholinesterase assay established that nine compounds were more potent to inhibit both electric eel AChE and equine serum BChE than rivastigmine under the same experimental conditions. Further study confirmed that among the nine carbamates, BMC-3 (IC_50(AChE) = 792 nM, IC_50(BChE) = 2.2 nM) and BMC-16 (IC_50(AChE) = 266 nM, IC_50(BChE) = 10.6 nM) were excellent cholinesterase inhibitors with potential of permeating through the blood-brain barrier. These carbamates could be used as potential dual inhibitors of AChE and BChE and to discover novel drugs for the treatment of AD and PD dementia.     

Compounds with the dioxopyrimidine cycle inhibit cholinesterases from different groups of animals

We firstly synthesized derivatives of 6-methyluracil, alloxazine, and xanthine, containing omega-tetraalkylammonium (TAA) groups at the N(1) and N(3) atoms in a pyrimidine cycle and assayed their anticholinesterase activities. Compounds with triethylpentylammoniumalkyl groups behaved as typical reversible inhibitors of acetylcholinesterase (AChE) (pI(50) 3.20-6.22) and butyrylcholinesterase (BuChE) (pI(50) 3.05-5.71). Compounds, containing two ethyl residues and a substituted benzyl fragment in the tetraalkylammonium group at N(3) atoms or two similar TAA groups at N(1) and N(3) atoms, possessed very high anticholinesterase activity. Although these compounds displayed the activity of typical irreversible AChE inhibitors (a progressive AChE inactivation; k(i) 7.6x10(8) to 3.5x10(9)M(-1)min(-1)), they were reversible inhibitors of BuChE (pI(50) 3.9-6.9). The efficiency of AChE inhibition by some of these compounds was more than 10(4) times higher than the efficiency of BuChE inhibition. Several synthesized TAA derivates of 6-methyluracil reversibly inhibited electric eel and cobra venom AChEs and horse serum BuChE. However, depending on their structure, the tested compounds possessed the time-progressing inhibition of mammalian erythrocyte AChE, typically of irreversible inhibitors. As shown upon dialysis and gel-filtration, the formed mammalian AChE-inhibitor complex was stable. Thus, a new class of highly active, selective, and irreversible inhibitors of mammalian AChE was described. In contrast to classical phosphorylating or carbamoylating AChE inhibitors, these compounds are devoid of acylating functions. Probably, these inhibitors interact with certain amino acid residues at the entrance to the active-site gorge.     

Amino acids defining the acyl pocket of an invertebrate cholinesterase

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 π–π or cation–π 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.     

Microcalorimetric study of the inhibition of butyrylcholinesterase by carbamates

The inhibition of horse serum butyrylcholinesterase (EC 3.1.1.8) by three carbamates (eserine, neostigmine, and rivastigmine) was studied by flow microcalorimetry at 37 degrees C in Tris buffer (pH 7.5). The kinetics of carbamylation was studied in the absence or presence of the substrate, butyrylcholine, using an extension of the model described by Stojan and coworkers (FEBS Lett. 440 (1998) 85-88). The model was fitted to the data by a nonlinear regression procedure using simulated annealing followed by Marquardt's method. The affinity of the carbamates for the free enzyme increased in the order neostigmine相似文献   

Development of acetophenone ligands as potential neuroimaging agents for cholinesterases

Association of cholinesterase with β-amyloid plaques and tau neurofibrillary tangles in Alzheimer’s disease offers an opportunity to detect disease pathology during life. Achieving this requires development of radiolabelled cholinesterase ligands with high enzyme affinity. Various fluorinated acetophenone derivatives bind to acetylcholinesterase with high affinity, including 2,2,2-trifluoro-1-(3-dimethylaminophenyl)ethanone (1) and 1-(3-tert-butylphenyl)-2,2,2-trifluoroethanone (2). Such compounds also offer potential for incorporation of radioactive fluorine (¹⁸F) for Positron Emission Tomography (PET) imaging of cholinesterases in association with Alzheimer’s disease pathology in the living brain. Here we describe the synthesis of two meta-substituted chlorodifluoroacetophenones using a Weinreb amide strategy and their rapid conversion to the corresponding trifluoro derivatives through nucleophilic substitution by fluoride ion, in a reaction amenable to incorporating ¹⁸F for PET imaging. In vitro kinetic analysis indicates tight binding of the trifluoro derivatives to cholinesterases. Compound 1 has a K_i value of 7 nM for acetylcholinesterase and 1300 nM for butyrylcholinesterase while for compound 2 these values are 0.4 nM and 26 nM, respectively. Tight binding of these compounds to cholinesterase encourages their development for PET imaging detection of cholinesterase associated with Alzheimer’s disease pathology.     

Interactions of butane, but-2-ene or xylene-like linked bispyridinium para-aldoximes with native and tabun-inhibited human cholinesterases

Kinetic parameters were evaluated for inhibition of native and reactivation of tabun-inhibited human erythrocyte acetylcholinesterase (AChE, EC 3.1.1.7) and human plasma butyrylcholinesterase (BChE, EC 3.1.1.8) by three bispyridinium para-aldoximes with butane (K074), but-2-ene (K075) or xylene-like linker (K114). Tested aldoximes reversibly inhibited both cholinesterases with the preference for binding to the native AChE. Both cholinesterases showed the highest affinity for K114 (K_i was 0.01 mM for AChE and 0.06 mM for BChE). The reactivation of tabun-inhibited AChE was efficient by K074 and K075. Their overall reactivation rate constants were around 2000 min⁻¹ M⁻¹, which is seven times higher than for the classical bispyridinium para-aldoxime TMB-4. The reactivation of tabun-inhibited AChE assisted by K114 was slow and reached 90% after 20 h. Since the aldoxime binding affinity of tabun-inhibited AChE was similar for all tested aldoximes (and corresponded to their K_i), the rate of the nucleophilic displacement of the phosphoryl-moiety from the active site serine was the limiting factor for AChE reactivation. On the other hand, none of the aldoximes displayed a significant reactivation of tabun-inhibited BChE. Even after 20 h, the reactivation maximum was 60% for 1 mM K074 and K075, and only 20% for 1 mM K114. However, lower BChE affinities for K074 and K075 compared to AChE suggest that the fast tabun-inhibited AChE reactivation by these compounds would not be obstructed by their interactions with BChE in vivo.     

Amino acid residues involved in the interaction of acetylcholinesterase and butyrylcholinesterase with the carbamates Ro 02-0683 and bambuterol, and with terbutaline.

In order to identify amino acids involved in the interaction of acetylcholinesterase (AChE; EC 3.1.1.7) and butyrylcholinesterase (BChE; EC 3.1.1.8) with carbamates, the time course of inhibition of the recombinant mouse enzymes BChE wild-type (w.t.), AChE w.t. and of 11 site-directed AChE mutants by Ro 02-0683 and bambuterol was studied. In addition, the reversible inhibition of cholinesterases by terbutaline, the leaving group of bambuterol, was studied. The bimolecular rate constant of AChE w.t. inhibition was 6.8 times smaller by Ro 02-0683 and 16000 times smaller by bambuterol than that of BChE w.t. The two carbamates were equipotent BChE inhibitors. Replacement of tyrosine-337 in AChE with alanine (resembling the choline binding site of BChE) resulted in 630 times faster inhibition by bambuterol. The same replacement decreased the inhibition by Ro 02-0683 ten times. The difference in size of the choline binding site in the two w.t. enzymes appeared critical for the selectivity of bambuterol and terbutaline binding. Removal of the charge with the mutation D74N caused a reduction in the reaction rate constants for Ro 02-0683 and bambuterol. Substitution of tyrosine-124 with glutamine in the AChE peripheral site significantly increased the inhibition rate for both carbamates. Substitution of phenylalanine-297 with alanine in the AChE acyl pocket decreased the inhibition rate by Ro 02-0683. Computational docking of carbamates provided plausible orientations of the inhibitors inside the active site gorge of mouse AChE and human BChE, thus substantiating involvement of amino acid residues in the enzyme active sites critical for the carbamate binding as derived from kinetic studies.     

Synthesis,structural characterization,docking, lipophilicity and cytotoxicity of 1-[(1R)-1-(6-fluoro-1,3-benzothiazol-2-yl)ethyl]-3-alkyl carbamates,novel acetylcholinesterase and butyrylcholinesterase pseudo-irreversible inhibitors

In the current study, sixteen novel derivatives of (R)-1-(6-fluorobenzo[d]thiazol-2-yl)ethanamine were synthesized as acetylcholinesterase (AChE) and butyrylcholinesterase (BChE) inhibitors. Chemical structures together with purity of the synthesized compounds were substantiated by IR, ¹H, ¹³C, ¹⁹F NMR, high resolution mass spectrometry and elemental analysis. The optical activities were confirmed by optical rotation measurements. The synthesized compounds were evaluated for their AChE and BChE inhibitory activities. In addition, the cytotoxicity of the most active compounds was investigated against human cell lines employing XTT tetrazolium salt reduction assay and xCELLigence system allowing a label-free assessment of the cells proliferation. Our results demonstrated that the inhibitory mechanism was confirmed to be pseudo-irreversible, in line with previous studies on carbamates. Compounds indicated as 3b, 3d, 3l and 3n showed the best AChE inhibitory activity of all the evaluated compounds and were up to tenfold more potent than standard drug rivastigmine. The binding mode was determined using state-of-the-art covalent docking and scoring methodology. The obtained data clearly demonstrated that 3b, 3d, 3l and 3n benzothiazole carbamates possess high inhibitory activity against AChE and BChE and concurrently negligible cytotoxicity. In conclusion, our results indicate, that these derivatives could be promising in an effective therapeutic intervention for Alzheimer’s disease.     

Stereo-specific inhibition of acetyl- and butyryl-cholinesterases by enantiomers of cis,cis-decahydro-2-naphthyl-N-n-butylcarbamate

Enantiomers of cis,cis-decahydro-2-naphthyl-N-n-butylcarbamate show stereo-specific inhibition for acetylcholinesterase and butyrylcholinesterase. For both inhibition reaction, (2S,4aR,8aS)-cis,cis-decahydro-2-naphthyl-N-n- butylcarbamate is more potent than (2R,4aS,8aR)-cis,cis-decahydro-2-naphthyl-N-n-butylcarbamate. Optically pure (2S,4aR,8aS)-(-)- and (2R,4aS,8aR)-(+)-cis,cis-decahydro-2-naphthols are resolved by the porcine pancreatic lipase-catalyzed acetylation of decahydro-2-naphthols with vinyl acetate. Absolute configurations and the enantiomeric excess values of (2S,4aR,8aS)-(-)- and (2R,4aS,8aR)-(+)-cis,cis-decahydro-2-naphthols are determined from the (19)F NMR spectra of their Mosher's ester derivatives. We fail to resolve (2S,4aR,8aR)- and (2R,4aS,8aS)-trans,cis-decahydro-2-naphthols from the porcine pancreatic lipase-catalyzed acetylation of decahydro-2-naphthols with vinyl acetate.     

Identification of amino acid residues involved in the binding of Huperzine A to cholinesterases.

Huperzine A, a potential agent for therapy in Alzheimer's disease and for prophylaxis of organophosphate toxicity, has recently been characterized as a reversible inhibitor of cholinesterases. To examine the specificity of this novel compound in more detail, we have examined the interaction of the 2 stereoisomers of Huperzine A with cholinesterases and site-specific mutants that detail the involvement of specific amino acid residues. Inhibition of fetal bovine serum acetylcholinesterase by (-)-Huperzine A was 35-fold more potent than (+)-Huperzine A, with KI values of 6.2 nM and 210 nM, respectively. In addition, (-)-Huperzine A was 88-fold more potent in inhibiting Torpedo acetylcholinesterase than (+)-Huperzine A, with KI values of 0.25 microM and 22 microM, respectively. Far larger KI values that did not differ between the 2 stereoisomers were observed with horse and human serum butyrylcholinesterases. Mammalian acetylcholinesterase, Torpedo acetylcholinesterase, and mammalian butyrylcholinesterase can be distinguished by the amino acid Tyr, Phe, or Ala in the 330 position, respectively. Studies with mouse acetylcholinesterase mutants, Tyr 337 (330) Phe and Tyr 337 (330) Ala yielded a difference in reactivity that closely mimicked the native enzymes. In contrast, mutation of the conserved Glu 199 residue to Gln in Torpedo acetylcholinesterase produced only a 3-fold increase in KI value for the binding of Huperzine A.(ABSTRACT TRUNCATED AT 250 WORDS)     

Divergent Regulation of Acetylcholinesterase and Butyrylcholinesterase in Tissues of the Rat

Abstract: Investigating the possibility that acetylcholinesterase (AChE) and butyrylcholinesterase (BuChE) are regulated in a coordinated manner, we have examined the natural variation in activity of these two enzymes in several tissues of adult male Sprague-Dawley, Fischer-344, and Wistar-Furth rats. Both enzymes varied greatly in mean activity among brain, diaphragm, atria, serum, superior cervical ganglia, and liver. In Sprague-Dawley rats there were also large individual variations with up to a fivefold range of AChE activities and up to a 100-fold range of BuChE activities in a given tissue. Individual variations in cholinesterase activities appeared to be smaller in the inbred Fischer-344 or Wistar-Furth rats. Experiments with internal standards of partially purified AChE and BuChE indicated that the individual variations probably reflected differences in the intrinsic content or specific activity of the tissue enzymes. Comparison of the AChE activities in different tissues of a given group of rats failed to reveal statistically significant correlations in any strain (i.e., the relative activity of any one tissue was no guide to the relative activity of any other tissue in the same rat). This result indicates that the regulation of AChE is tissue-specific. By contrast, BuChE activity showed highly significant correlations among the majority of the tissues examined in the Sprague-Dawley rats, implying that widely dispersed factors can affect the regulation of this enzyme. Body-wide regulation is not necessarily the rule, however, since only a single tissue pair in the inbred Fischer rats and none of the pairs in the Wistar-Furth rats showed significant correlations of BuChE activity. In general, AChE and BuChE activities were not correlated with each other to a statistically significant degree. We conclude that the control of these enzymes normally involves different mechanisms and is strongly affected by the genetic background of the sample population.     

Amino acid residues involved in organic solvent-stability of the LST-03 lipase

LST-03 lipase from Pseudomonas aeruginosa LST-03 is highly active and stable in the presence of various organic solvents. To further characterize and improve the organic solvent-stability of the LST-03 lipase, residues that potentially provide this stability were identified and mutated to other amino acids in an effort to increase the organic solvent-stability of the protein. S155L, G157R, S164K, S194R, and D209N mutations were found to improve the organic solvent-stability of the wild-type LST-03 lipase. Such mutations were found to induce structural changes, including the formation of a salt bridge, hydrogen bonds, lead to an improved packing of the hydrophobic core, and pI shift of side chain. These changes increased the stability of the protein, thereby improving the organic solvent-stability of the wild-type LST-03 lipase. In addition, a single mutation was found to stabilize the lipase by single or multiple factors.     

Potent inhibition of acetylcholinesterase by sargachromanol I from Sargassum siliquastrum and by selected natural compounds

Six hundred forty natural compounds were tested for acetylcholinesterase (AChE) and butyrylcholinesterase (BChE) inhibitory activities. Of those, sargachromanol I (SCI) and G (SCG) isolated from the brown alga Sargassum siliquastrum, dihydroberberine (DB) isolated from Coptis chinensis, and macelignan (ML) isolated from Myristica fragrans, potently and effectively inhibited AChE with IC₅₀ values of 0.79, 1.81, 1.18, and 4.16 µM, respectively. SCI, DB, and ML reversibly inhibited AChE and showed mixed, competitive, and noncompetitive inhibition, respectively, with K_i values of 0.63, 0.77, and 4.46 µM, respectively. Broussonin A most potently inhibited BChE (IC₅₀ = 4.16 µM), followed by ML, SCG, and SCI (9.69, 10.79, and 13.69 µM, respectively). In dual-targeting experiments, ML effectively inhibited monoamine oxidase B with the greatest potency (IC₅₀ = 7.42 µM). Molecular docking simulation suggested the binding affinity of SCI (−8.6 kcal/mol) with AChE was greater than those of SCG (−7.9 kcal/mol) and DB (−8.2 kcal/mol). Docking simulation indicated SCI interacts with AChE at Trp81, and that SCG interacts at Ser119. No hydrogen bond was predicted for the interaction between AChE and DB. This study suggests SCI, SCG, DB, and ML be viewed as new reversible AChE inhibitors and useful lead compounds for the development for the treatment of Alzheimer’s disease.     

Amino acid residues involved in substrate recognition of the Escherichia coli Orf135 protein

The Escherichia coli Orf135 protein, a MutT-type enzyme, hydrolyzes mutagenic 2-hydroxy-dATP (2-OH-dATP) and 8-hydroxy-dGTP, in addition to dCTP and 5-methyl-dCTP, and its deficiency causes increases in both the spontaneous and H(2)O(2)-induced mutation frequencies. To identify the amino acid residues that interact with these nucleotides, the Glu-33, Arg-72, Arg-77, and Asp-118 residues of Orf135, which are candidates for residues interacting with the base, were substituted, and the enzymatic activities of these mutant proteins were examined. The mutant proteins with a substitution at the 33rd, 72nd, and 118th amino acid residues displayed activities affected to various degrees for each substrate, suggesting the involvement of these residues in substrate binding. On the other hand, the mutant protein with a substitution at the 77th Arg residue had activitiy similar to that of the wild-type protein, excluding the possibility that this Arg side chain is involved in base recognition. In addition, the expression of some Orf135 mutants in orf135(-) E. coli reduced the level of formation of rpoB mutants elicited by H(2)O(2). These results reveal the residues involved in the substrate binding of the E. coli Orf135 protein.     

Amino acid residues of Escherichia coli acyl carrier protein involved in heterologous protein interactions

Acyl carrier protein (ACP) is a small, highly conserved protein with an essential role in a myriad of reactions throughout lipid metabolism in plants and bacteria where it interacts with a remarkable diversity of proteins. The nature of the proper recognition and precise alignment between the protein moieties of ACP and its many interactive proteins is not understood. Residues conserved among ACPs from numerous plants and bacteria were considered as possibly being crucial to ACP's function, including protein-protein interaction, and a method of identifying amino acid residue clusters of high hydrophobicity on ACP's surface was used to estimate residues possibly involved in specific ACP-protein interactions. On the basis of this information, single-site mutation analysis of multiple residues, one at a time, of ACP was used to probe the identities of potential contact residues of ACPSH or acyl-ACP involved in specific interactions with selected enzymes. The roles of particular ACP residues were more precisely defined by site-directed fluorescence analyses of various myristoyl-mutant-ACPs upon specific interaction with the Escherichia coli hemolysin-activating acyltransferase, HlyC. This was done by selectively labeling each mutated site, one at a time, with an environmentally sensitive fluoroprobe and observing its fluorescence behavior in the absence and presence of HlyC. Consequently, a picture of the portion of ACP involved in selected macromolecular interaction has emerged.     

Amino acid residues involved in substrate binding and catalysis in an insect digestive beta-glycosidase

A beta-glycosidase (M(r) 50000) from Spodoptera frugiperda larval midgut was purified, cloned and sequenced. It is active on aryl and alkyl beta-glucosides and cellodextrins that are all hydrolyzed at the same active site, as inferred from experiments of competition between substrates. Enzyme activity is dependent on two ionizable groups (pK(a1)=4.9 and pK(a2)=7.5). Effect of pH on carbodiimide inactivation indicates that the pK(a) 7.5 group is a carboxyl. k(cat) and K(m) values were obtained for different p-nitrophenyl beta-glycosides and K(i) values were determined for a range of alkyl beta-glucosides and cellodextrins, revealing that the aglycone site has three subsites. Binding data, sequence alignments and literature beta-glycosidase 3D data supported the following conclusions: (1) the groups involved in catalysis were E(187) (proton donor) and E(399) (nucleophile); (2) the glycone moiety is stabilized in the transition state by a hydrophobic region around the C-6 hydroxyl and by hydrogen bonds with the other equatorial hydroxyls; (3) the aglycone site is a cleft made up of hydrophobic amino acids with a polar amino acid only at its first (+1) subsite.     

Four critical aspartic acid residues potentially involved in the catalytic mechanism of Escherichia coli K-12 WaaR

Escherichia coli K-12 WaaR is a non-processive alpha-1,2 glucosyltransferase, involved in the synthesis of the R-core of lipopolysaccharide. WaaR possesses the four conserved structural regions I, II, III and IV, each presumably involved in the mechanistic function in catalysis. Regions I and III contain the pair of strictly conserved Asp residues. Asp-129, 131 (region I) and 215, 217 (region III) of WaaR were individually converted to Asn by the site-directed mutagenesis of the waaR gene. All mutated enzymes were inactive, supporting the model for an alpha-glycosyl transfer reaction where the pair of strictly conserved aspartic acid residues in regions I and III play a critical role in the catalytic function.