Human acid beta-glucosidase. Use of conduritol B epoxide derivatives to investigate the catalytically active normal and Gaucher disease enzymes

Human acid beta-glucosidase (glucosylceramidase; EC 3.2.1.45) cleaves the glycosidic bonds of glucosyl ceramide and synthetic beta-glucosides. Conduritol B epoxide (CBE) and its brominated derivative are mechanism-based inhibitors which bind covalently to the catalytic site of acid beta-glucosidase. Procedures using brominetritiated CBE and monospecific anti-human placental acid beta-glucosidase IgG were developed to determine the molar concentrations of functional acid beta-glucosidase catalytic sites in pure placental enzyme preparations from normal sources; kcat values then were calculated from Vmax = [Et]kcat using glucosyl ceramide substrates with dodecanoyl (2135 +/- 45 min-1) and hexanoyl (3200 +/- 410 min-1) fatty acid acyl chains and 4-alkyl-umbelliferyl beta-glucoside substrates with methyl (2235 +/- 197 min-1), heptyl (1972 +/- 152 min-1), nonyl (2220 +/- 247 min-1), and undecyl (773 +/- 44 min-1) alkyl chains. The respective kcat values for acid beta-glucosidase in a crude normal splenic preparation were about 60% of these values. In comparison, the kcat values of the mutant splenic acid beta-glucosidase from two Type 1 Ashkenazi Jewish Gaucher disease (AJGD) patients were about 1.5-3-fold decreased and had Km values for each substrate which were similar to those for the normal acid beta-glucosidase. The interaction of the normal and Type 1 AJGD enzymes with CBE in a 1:1 stoichiometry conformed to a model with reversible EI complexes formed prior to covalent inactivation. With CBE, the equal kmax values (maximal rate of inactivation) for the normal (0.051 +/- 0.009 min-1) and Type 1 AJGD (0.058 +/- 0.016 min-1) enzymes were consistent with the minor differences in kcat. In contrast, the Ki value (dissociation constant) (839 +/- 64 microM) for the Type 1 AJGD enzymes was about 5 times the normal Ki value (166 +/- 57 microM). These results indicated that the catalytically active Type 1 AJGD acid beta-glucosidase had nearly normal hydrolytic capacity and suggested an amino acid substitution in or near the acid beta-glucosidase active site leading to an in vivo instability of the mutant enzymatic activity.     

Human acid beta-glucosidase: use of inhibitors, alternative substrates and amphiphiles to investigate the properties of the normal and Gaucher disease active sites

Comparative studies with lipoidal inhibitors and alternative substrates were conducted to investigate the properties of the active site of human acid beta-glucosidase (D-glucosyl-N-acylsphingosine glucohydrolase, EC 3.2.1.45) from normal placenta and spleens of Type 1 Ashkenazi Jewish Gaucher disease (AJGD) patients. With the normal enzyme, the inhibitory potencies of series of alkyl(Cn; n = 0-18)amines, alkyl beta-glucosides and alkyl-1-deoxynojirimycins were a biphasic function of increasing chain length: i.e., large decreases in Ki,app or IC50 were found only with n greater than 4 and limiting values were approached with n = 12-14. This biphasic function of alkyl chain length was observed in the presence or absence of detergents and/or negatively charged lipids. In the presence of Triton X-100 concentrations greater than the critical micellar concentration, the relative (to deoxynojirimycin) inhibitory potencies of the N-Cn-deoxynojirimycins (n greater than 4) were decreased about 3-5-fold, due to an energy requirement to extract the inhibitors from Triton X-100 micelles. The Ki,app or IC50 of N-hexylglucosylsphingosine was inversely related to the Triton X-100 concentration and was not affected by the presence of 'co-glucosidase'. The mutual exclusion of glucon, N-Cn-deoxynojirimycin and sphingosine derivatives from the normal enzyme suggested a shared region for binding in the active site. Increasing the fatty-acid acyl chain length of glucosyl ceramide from 1 to 24 carbons had minor effects on Km,app ( = Kis,app) (8-40 microM), but increased Vmax,app up to 13-fold. With the AJGD enzyme, the inhibitor and alternative substrate findings were similar to those with the normal enzyme, except that Kis,app(AJGD)/Kis,app(normal) = 4 to 11 for the Cn-glycons and sphingosine derivatives. These results indicated that (1) the Ki,app or Km,app values for amphiphilic inhibitors or substrates reflect a balance of binding energies for two hydrophobic subsites within the enzyme's active site and Triton X-100 micelles and (2) the abnormal properties of the AJGD enzyme result from an amino-acid alteration(s) within or near a hydrophilic region which is shared by the glycon-binding site and the two hydrophobic sites of the active site.     

Effects of the glucosidase inhibitors nojirimycin and deoxynojirimycin on the biosynthesis of membrane and secretory glycoproteins.

The glucosidase inhibitors nojirimycin (NM) and 1-deoxynojirimycin (dNM) interfere with N-linked glycosylation. The effects of NM and dNM on the biosynthesis of secretory glycoproteins (IgD and IgM) and membrane glycoproteins (HLA-A, B, C and -DR antigens) have been examined. Whereas treatment of IgD- and IgM-producing cells with NM results in the transfer of drastically shortened oligosaccharide side chains, treatment with dNM inhibits trimming, most probably through interaction with glucosidase I and/or II. A comparison of NM and dNM with tunicamycin and the mannosidase inhibitor swainsonine (SW) show that each of the inhibitors interferes with N-linked glycosylation in a distinct manner. For both Ig and HLA antigens, the effects of SW are discernible at the final stages of glycan maturation only, whereas the effects of dNM are observed quite early in the biosynthetic process. The secretion of IgD, but not IgM, was blocked in dNM-treated cells. The HLA-A, B, C heavy chains synthesized by the Daudi cell line were degraded in an accelerated fashion in dNM-treated cells, but no effects were seen on the HLA-DR antigens in these cells. Although both SW and dNM interfere with trimming, further modifications of the oligosaccharide side chains occur, and show that the two processes are not obligately coupled. Glucosidase inhibitors such as NM and dNM, as well as the mannosidase inhibitor SW, allow modification of glycan structure, and may be used to study the biological role of glycoprotein oligosaccharides and their modifications.     

Human acid beta-glucosidase: affinity purification of the normal placental and Gaucher disease splenic enzymes on N-alkyl-deoxynojirimycin-sepharose

Two sepharose-bound 1-deoxynojirimycin N-alkyl derivatives, N-(9-carboxynonyl)- and N-(11-carboxyundecyl)-deoxynojirimycin, were used for the affinity purification of acid beta-glucosidase (beta-Glc) from normal and type-1 Ashkenazi Jewish Gaucher disease (AJGD) sources. The capacities of these nondegradable inhibitor supports were 0.5 and 0.75 mg of normal beta-Glc/ml of settled gel, respectively. The purified normal enzyme (14-18% yield) had a specific activity of 1.6 X 10(6) nmol/h/mg protein and was homogeneous as evidenced by a single protein species of Mr = 67,000 on sodium dodecylsulfate-polyacrylamide gel electrophoresis and reverse phase high-performance liquid chromatography (HPLC). Microsequencing demonstrated a single N terminus, and the sequence of the first 22 N-terminal amino acids was colinear with that predicted from the beta-Glc cDNA. Amino acid composition analyses of beta-Glc revealed a high content (35%) of hydrophobic amino acids. The N-decyl-deoxynojirimycin support facilitated the purification of the residual enzyme from type-1 AJGD spleen to about 7,500-fold in four steps with a yield of about 11%. These new affinity supports provided improved stability, capacity and/or specificity compared to other affinity or HPLC methods for purifying this lysosomal glycosidase.     

Computational analysis of glycoside hydrolase family 1 specificities

Glycoside hydrolase family 1 consists of beta-glucosidases, beta-galactosidases, 6-phospho-beta-galactosidases, myrosinases, and other enzymes having similar primary and tertiary structures but diverse specificities. Among these enzymes, beta-glucosidases hydrolyze cellobiose to glucose, and therefore they are key players in any cellulose to glucose process. All family members attack beta-glycosidic bonds between a pyranosyl glycon and an aglycon, but most have little specificity for the aglycon or for the bond configuration. Furthermore, glycon specificity is not absolute. Sixteen family members (six beta-glucosidases, two cyanogenic beta-glucosidases, one 6-phospho-beta-galactosidase, two myrosinases, and five beta-glycosidases) have known tertiary structures. We have used automated docking to computationally bind disaccharides with allopyranosyl, galactopyranosyl, glucopyranosyl, mannopyranosyl, 6-phosphogalactopyranosyl, and 6-phosphoglucopyranosyl glycons, all linked by beta-(1,2), beta-(1,3), beta-(1,4), and beta-(1,6)-glycosidic bonds to beta-glucopyranoside aglycons, along with beta-(1,1-thio)-allopyranosyl, -galactopyranosyl, -glucopyranosyl, and -mannopyranosyl) beta-glucopyranosides, into all of these structures to investigate the structural determinants of their enzyme specificities. The following are the eight active-site residues: Glu191, Thr194, Phe205, Asn285, Arg336, Asn376, Trp378, and Trp465 (Zea mays beta-glucosidase numbering), that control a significant amount of glycon specificity. (c) 2008 Wiley Periodicals, Inc. Biopolymers 89: 1021-1031, 2008.This article was originally published online as an accepted preprint. The "Published Online" date corresponds to the preprint version. You can request a copy of the preprint by emailing the Biopolymers editorial office at biopolymers@wiley.com.     

Beta-glycosylamidine as a ligand for affinity chromatography tailored to the glycon substrate specificity of beta-glycosidases

An affinity adsorbent for beta-glycosidases has been prepared by using beta-glycosylamidine as a ligand. beta-Glucosylamidine and beta-galactosylamidine, highly potent and selective inhibitors of beta-glucosidases and beta-galactosidases, respectively, were immobilized by a novel one-pot procedure involving the addition of a beta-glycosylamine and 2-iminothiolane.HCl simultaneously to a matrix modified with maleimido groups via an appropriate spacer to give an affinity adsorbent for beta-glucosidases and beta-galactosidases, respectively. This one-pot procedure enables various beta-glycosylamidine ligands to be formed and immobilized conveniently according to the glycon substrate specificities of the enzymes. A crude enzyme extract from tea leaves (Camellia sinensis) and a beta-galactosidase from Penicillium multicolor were chromatographed directly on each affinity adsorbent to give a beta-glucosidase and a beta-galactosidase to apparent homogeneity in one step by eluting the column with glucose or by a gradient NaCl elution, respectively. The beta-glucosidase and beta-galactosidase were inhibited competitively by a soluble form of the corresponding beta-glycosylamidine ligand with an inhibition constant (K(i)) of 2.1 and 0.80 microM, respectively. Neither enzyme was bound to the adsorbent with a mismatched ligand, indicating that the binding of the glycosidases was of specific nature that corresponds to the glycon substrate specificity of the enzymes. The ease of preparation and the selective nature of the affinity adsorbent should promise a large-scale preparation of the affinity adsorbent for the purification and removal of specific glycosidases according to their glycon substrate specificities.     

Role of sugar hydroxyl groups in glycoside hydrolysis. Cleavage mechanism of deoxyglucosides and related substrates by beta-glucosidase A3 from Aspergillus wentii

The contribution of the hydroxyl groups at C-2 and C-4 and of the hydroxy-methyl group at C-5 of beta-glucopyranosides to their hydrolysis by beta-glucosidase A3 (beta-D-glucoside glucohydrolase, EC 3.2.1.21) from Aspergillus wentii was investigated with 4-methylumbelliferyl-beta-glucosides with appropriate structural modifications. Relative hydrolysis rates expressed by kcat/kcat (glucoside) are: 2-deoxy, 4. 10(-6); 2-deoxy-2-amino, 2.4 . 10(-4); 2-deoxy-2-ammonio, less than 1 . 10(-6); 4-deoxy, 1.8 . 10(-4); xyloside, 6.3 . 10(4); galactoside, less than 1 . 10(-6). Binding to the active site as measured by the Km value of these substrates or by the Ki value of the appropriate inhibitors is only moderately decreased by the above modifications. A temperature study with the 2-deoxyglucoside showed that the decrease in kcat is not due to a change in delta H but to a more negative delta S. The steady-state hydrolysis of the 2-deoxyglucoside is approached with a "burst" (rate constant 0.13 min-1) at pH 6 and 1 mM substrate; deglycosylation of the enzyme is partially rate-limiting. Rate constants for glycosylation and deglycosylation calculated from pre-steady-state kinetics were in good agreement with the constants calculated from experiments where the 2-deoxyglucoside was used as an inhibitor for the hydrolysis of the glucoside and where a slow approach to the steady state of the inhibited reaction is observed.     

Illuminating the binding interactions of galactonoamidines during the inhibition of β-galactosidase (E. coli)

Several galactonoamidines were previously identified as very potent competitive inhibitors that exhibit stabilizing hydrophobic interactions of the aglycon in the active site of β-galactosidase (Aspergillus oryzae). To elucidate the contributions of the glycon to the overall inhibition ability of the compounds, three glyconoamidine derivatives with alteration in the glycon at C-2 and C-4 were synthesized and evaluated herein. All amidines are competitive inhibitors of β-galactosidase (Escherichia coli) and show significantly reduced inhibition ability when compared to the parent. The results highlight strong hydrogen-bonding interactions between the hydroxyl group at C-2 of the amidine glycon and the active site of the enzyme. Slightly weaker H-bonds are promoted through the hydroxyl group at C-4. The inhibition constants were determined to be picomolar for the parent galactonoamidine, and nanomolar for the designed derivatives rendering all glyconoamidines very potent inhibitors of glycosidases albeit the derivatized amidines show up to 700-fold lower inhibition activity than the parent.     

Reversible inhibitors of beta-glucosidase

A variety of reversible inhibitors of sweet almond beta-glucosidase were examined. These included simple sugars and sugar derivatives, amines and phenols. With respect to the sugar inhibitors and, indeed, the various glycoside substrates, the enzyme has what can be considered a "relaxed specificity". No single substituent on glucose, for example, is essential for binding. Replacement of a hydroxyl group with an anionic substituent reduces the affinity while substitution with a cationic (amine) substituent enhances the affinity. Amines, in general, are good inhibitors, binding more tightly than the corresponding alcohols: pKiRNH3+ = 0.645pKiROH + 1.77 (n = 9, r = 0.97). The affinity of a series of 10 primary amines was found to be strongly influenced by substituent hydrophobicity: pKi = 0.52 pi + 1.32 (r = 0.95). The major binding determinant of the glycoside substrates is the aglycon moiety. Thus, the Ki values of phenols are similar in magnitude to the Ks values of the corresponding aryl beta-glucoside. The pH dependence for the inhibition by various phenols indicates that it is the un-ionized phenol which binds to the enzyme when an enzymic group of pKa = 6.8 (+/- 0.1) is protonated. The affinity of the phenol inhibitor is dependent on its basicity with a Br?nsted coefficient for binding of beta = -0.26 (n = 14, r = 0.98). The pH dependence of the binding of two particularly potent beta-glucosidase inhibitors was also examined. 1-Deoxynojirimycin (1,5-dideoxy-1,5-imino-D-glucitol) has a pH-corrected Ki = 6.5 microM, and D-glucono-1,5-lactam has a pH-corrected Ki = 29 microM.(ABSTRACT TRUNCATED AT 250 WORDS)     

Studies on the mechanism of castanospermine inhibition of alpha- and beta-glucosidases

Castanospermine (1,6,7,8-tetrahydroxyoctahydroindolizine) is an indolizidine alkaloid that was isolated from the Australian plant, Castanospermum australe. This alkaloid was found to be a potent inhibitor of lysosomal alpha- and beta-glucosidases. In this report, the mechanism of inhibition of amyloglucosidase (an exo-1,4-alpha-glucosidase) and almond emulsin beta-glucosidase was examined. Castanospermine proved to be a competitive inhibitor of amyloglucosidase at both pH 4.5 and 6.0 when assayed with the p-nitrophenyl-alpha-D-glucoside. It was also a competitive inhibitor of almond emulsin beta-glucosidase at pH 6.5, but in this case previous studies had shown that inhibition was of the mixed type at pH 4.5 to 5.0. Th pH of the incubation mixture had a marked effect on the inhibition. Thus, in all cases, castanospermine was a much better inhibitor at pH 6.0 to 6.5 than it was at lower pH values. The pK for castanospermine was found to be 6.09, indicating that the alkaloid was probably more active in the unprotonated form. This was also suggested by the fact that the N-oxide of castanospermine, while still a competitive inhibitor, was 50 to 100 times less active than was castanospermine, and its activity was not markedly altered by pH. These results probably explain why castanospermine is a good inhibitor of the glycoprotein processing enzyme, glucosidase I, since this is a neutral enzyme.     

Purin-6-yl 6-deoxy-1-thio-beta-D-glucopyranoside. Enzymology, chemistry, and cytotoxicity

Purin-6-yl 6-deoxy-1-thio-beta-D-glucopyranoside (4) is a substrate for almond beta-glucosidase and a weak competitive inhibitor of bovine liver beta-D-glucuronidase (Ki approximately 20mM). Both 4 and purine-protonated 4 undergo hydrolysis catalyzed by dilute acid in the pH range 0.17-2.59. These results are compared with those previously obtained with ammonium (purin-6-yl 1-thio-beta-D-glucopyranosid)uronate, (purin-6-yl 1-thio-beta-D-glucopyranosid)uronamide, purin-6-yl 1-thio-beta-D-glucopyranoside, and purin-6-yl 2-deoxy-1-thio-beta-D-glucopyranoside, and it is concluded that the data support an involvement of substituents at C-5 in producing productive Michaelis-complex conformers. The 6-deoxyglucoside is more active than the D-glucosiduronic acid in an L1210 mouse screen.     

Induction of Cellulase by Gentiobiose and Its Sulfur-Containing Analog in Penicillium purpurogenum

Cellulase induction by beta-glucodisaccharides was investigated by using non-cellulase-induced mycelia of Penicillium purpurogenum P-26, a highly-cellulase-producing fungus. Gentiobiose induced significant amounts of cellulase compared with cellobiose when nojirimycin was added to the induction medium to inhibit extracellular beta-glucosidase activity. Thiogentiobiose (6-S-beta-d-glucopyranosyl-6-thio-d-glucose), a sulfur-containing analog of gentiobiose, was more effective for cellulase induction than gentiobiose even in the absence of nojirimycin. Thiogentiobiose appeared to be a gratuitous inducer since it was not metabolized during cellulase induction. Gentiobiose was formed from cellobiose by the intracellular beta-glucosidase of P. purpurogenum. These findings indicate that gentiobiose is an active inducer of cellulase for this fungus and may possibly be formed by intracellular beta-glucosidase from cellobiose.     

Involvement of a conidial endoglucanase and a plasma-membrane-bound beta-glucosidase in the induction of endoglucanase synthesis by cellulose in Trichoderma reesei

The induction of endo-1,4-beta-glucanase synthesis by Trichoderma reesei QM 9414 was investigated in conidia, mycelia and protoplasts. Cellulose induced endoglucanase synthesis only in conidia, but not in glucose-grown mycelia or protoplasts. Cellooligosaccharides and sophorose induced endoglucanase synthesis in mycelia, conidia and protoplasts. Only conidia exhibited detectable basal endoglucanase levels, whereas beta-glucosidase activity was found in conidia, mycelia and protoplasts. The beta-glucosidase was inhibited in vitro by nojirimycin and glucono-delta-lactone. Addition of either of these inhibitors to the induction medium blocked de noro synthesis of endo-1,4-beta-glucanase with cellulose (conidia) or cellooligosaccharides (protoplasts and mycelia) as inducer, whereas induction by sophorose remained unaffected. The results are consistent with the assumption that basal constitutive levels of endoglucanase and beta-glucosidase are involved in the induction of cellulase synthesis by cellulose in T. reesei.     

Slow inhibition of almond beta-glucosidase by azasugars: determination of activation energies for slow binding

The thermodynamic and activation energies of the slow inhibition of almond beta-glucosidase with a series of azasugars were determined. The inhibitors studied were isofagomine ((3R,4R,5R)-3,4-dihydroxy-5-hydroxymethylpiperidine, 1), isogalactofagomine ((3R,4S,5R)-3,4-dihydroxy-5-hydroxymethylpiperidine, 2), (-)-1-azafagomine ((3R,4R,5R)-4,5-dihydroxy-3-hydroxymethylhexahydropyridazine, 3), 3-amino-3-deoxy-1-azafagomine (4) and 1-deoxynojirimycin (5). It was found that the binding of 1 to the enzyme has an activation enthalpy of 56.1 kJ/mol and an activation entropy of 25.8 J/molK. The dissociation of the enzyme-1 complex had an activation enthalpy of -2.5 kJ/mol and an activation entropy of -297 J/molK. It is suggested that the activation enthalpy of association is due to the breaking of bonds to water, while the large negative activation entropy of dissociation is due at least in part to the resolvation of the enzyme with water molecules. For the association of 1 DeltaH(0) is 58.6 kJ/mol and DeltaS(0) is 323.8 J/molK. Inhibitor 3 has an activation enthalpy of 39.3 kJ/mol and an activation entropy of -17.9 J/molK for binding to the enzyme, and an activation enthalpy of 40.8 kJ/mol and an activation entropy of -141.0 J/molK for dissociation of the enzyme-inhibitor complex. For the association of 3 DeltaH(0) is -1.5 kJ/mol and DeltaS(0) is 123.1 J/molK. Inhibitor 5 is not a slow inhibitor, but its DeltaH(0) and DeltaS(0) of association are -30 kJ/mol and -13.1 J/molK. The large difference in DeltaS(0) of association of the different inhibitors suggests that the anomeric nitrogen atom of inhibitors 1-4 is involved in an interaction that results in a large entropy increase.     

Human acid beta-glucosidase: use of sphingosyl and N-alkyl-glucosylamine inhibitors to investigate the properties of the active site

Human acid beta-glucosidase (D-glucosyl-N-acylsphingosine glucohydrolase, EC 3.2.1.45) cleaves the beta-glucosidic bonds of glucosylceramide and synthetic beta-glucosides. The specificity of binding to the active site of this enzyme was evaluated using series of inhibitors including synthetic sphingosines, N-alkyl(Cn)-deoxynojirimycins (1,5-dideoxy-5-iminoglucose) and N-Cn-glucosylamines. The sphingosines were rapidly reversible inhibitors with maximal potency (IC50 approximately 78-150 micro M) at chain lengths of 14-18 carbons. The presence of unsaturation between C4 and C5 was required for inhibition of enzyme activity. Neither the nature of this bond (double or triple bond) nor the presence of erythro or threo configurations at C2 influenced inhibitory potency. The N-C10- to N-C14-deoxynojirimycins were rapidly reversible inhibitors with Ki approximately 8.5 nM. In comparison, the 1-amino glucose derivatives, i.e., N-Cn-glucosylamines (n = 12-18), were more potent (IC50 approximatley 0.3-3 nM) and their maximal inhibitory potencies were dependent on time as well as enzyme and substrate concentrations: i.e., the N-C12- to N-C18-glucosylamines were competitive, slow-tight binding inhibitors. Analyses of progress curves at various N-Cn-glucosylamine (n = 14-18) concentrations indicated the formation of rapidly dissociating initial EI collison complex which then undergoes a conformational change to a slowly reversible EI complex. These results were consistent with the long chain N-Cn-glucosylamines being reaction intermediate analogues and with this enzyme's hydrolytic mechanism requiring a conformational change during the transition state.     

Phosphorylation of the alpha 1,2-linked mannosylsaccharide by N-acetylglucosamine-1-phosphotransferase from fibroblasts occurs at the terminal mannose

UDP-GlcNAc: Lysosomal enzyme precursor N-acetylglucosamine-1-phosphotransferase activity from normal fibroblasts was measured using methyl 2-O-(alpha-D-mannopyranosyl) 6-deoxy-6-fluoro-alpha-D-mannopyranoside and methyl 2-O-(6-deoxy-6-fluoro-alpha-D-mannopyranosyl) alpha-D-mannopyranoside as acceptors. The results indicate that the phosphorylation in man alpha 1----2 man sequence occurs at the C-6 position of the terminal mannose residue.     

Characterization of calf liver glucosidase I and its inhibition by basic sugar analogs

Glucosidase I, the enzyme catalyzing the first step of N-linked oligosaccharide processing, has been purified from calf liver crude membranes [H. Hettkamp, G. Legler, and E. Bause, (1984) Eur. J. Biochem. 142, 85-90]. Binding experiments with concanavalin A-Sepharose suggest that glucosidase I is a glycoprotein with high-mannose carbohydrate chain(s). The enzyme has a subunit molecular mass of approximately 83 kDa and specifically hydrolyzes the terminal alpha-1,2-linked glucose residue from the natural Glc3-Man9-GlcNAc2 oligosaccharide. Studies with a variety of substrates modified in the aglycon moiety suggest that the Glc2 branch rather than the more distant domains of the substrate molecule are important for binding and hydrolysis. Glucosidase I does not require metal ions for activity and is strongly inhibited by 1-deoxynojirimycin (dNM) and its N-alkyl derivatives. Ki values range from 0.07 microM for N-methyl-dNM to 1.0 microM for dNM, measured at the pH-optimum of enzyme activity. The pH dependence of inhibition indicates that the cationic form of the inhibitors is the active species. Comparison of the Ki for N-decanoyl-dNM (approximately 70 microM) with that of N-decyl-dNM (approximately 0.4 microM) suggests that electrostatic interactions at the catalytic site of the enzyme are important for inhibitor binding. 1-Deoxymannojirimycin, previously assumed to be a specific mannosidase inhibitor, as well as its N-methyl and N-5-carboxypentyl derivatives, inhibit glucosidase I with Ki values around 190, 17, and 100 microM, respectively. This apparent lack of specificity shows that in vivo experiments on N-glycoprotein processing as well as the interpretation of results with these mannosidase inhibitors may give misleading results when these compounds are used in the millimolar range.     

Glycosidase-catalyzed Deoxy Oligosaccharide Synthesis. Practical Synthesis of Monodeoxy Analogs of Ethyl β-Thioisomaltoside Using Aspergillus niger α-Glucosidase

Enzymatic transglycosylation using four possible monodeoxy analogs of p-nitrophenyl α-D-glucopyranoside (Glcα-O-pNP), modified at the C-2, C-3, C-4, and C-6 positions (2D-, 3D-, 4D-, and 6D-Glcα-O-pNP, respectively), as glycosyl donors and six equivalents of ethyl β-D-thioglucopyranoside (Glcβ-S-Et) as a glycosyl acceptor, to yield the monodeoxy derivatives of glucooligosaccharides were done. The reaction was catalyzed using purified Aspergillus niger α-glucosidase in a mixture of 50 mM sodium acetate buffer (pH 4.0)/CH₃CN (1: 1 v/v) at 37°C. High activity of the enzyme was observed in the reaction between 2D-Glcα-O-pNP and Glcβ-S-Et to afford the monodeoxy analogs of ethyl β-thiomaltoside and ethyl β-thioisomaltoside that contain a 2-deoxy α-D-glucopyranose moiety at their glycon portions, namely ethyl 2-deoxy-α-D-arabino-hexopyranosyl-(1,4)-β-D-thioglucopyranoside and ethyl 2-deoxy-α-D-arabino-hexopyranosyl-(1,6)-β-D-thioglucopyranoside, in 6.72% and 46.6% isolated yields (based on 2D-Glcα-O-pNP), respectively. Moreover, from 3D-Glcα-O-pNP and Glcβ-S-Et, the enzyme also catalyzed the synthesis of the 3-deoxy analog of ethyl β-thioisomaltoside that was modified at the glycon α-D-glucopyranose moiety, namely ethyl 3-deoxy-α-D-ribo-hexopyranosyl-(1,6)-β-D-thioglucopyranoside, in 23.0% isolated yield (based on 3D-Glcα-O-pNP). Products were not obtained from the enzymatic reactions between 4D- or 6D-Glcα-O-pNP and Glcβ-S-Et.     

Active site directed inhibition of a cytosolic beta-glucosidase from calf liver by bromoconduritol B epoxide and bromoconduritol F

Hydrolysis of p-nitrophenyl-beta-D-glucoside by cytosolic beta-glucosidase proceeds with retention of the anomeric configuration. Whereas inactivation of the enzyme by the glucosidase inhibitor conduritol B epoxide (CBE) was extremely slow (ki(max)/Ki 0.57 M-1 min-1) it reacted 130 times more rapidly with 6-bromo-6-deoxy-CBE (Br-CBE). The beta-glucosidase could be labeled with [3H]Br-CBE; incorporation of 1 mol inhibitor/mol enzyme resulted in complete loss of activity. Most of the bound inhibitor was released after denaturation and treatment with ammonia as (1,3,4/2,5,6)-6-bromocyclohexanepentol, thus demonstrating the formation of an ester bond with an active site carboxylate by trans-diaxial opening of the epoxide ring. It was concluded from the Ki values for the epoxide inhibitors and for coduritol B with the cytosolic enzyme and corresponding data for the lysosomal beta-glucosidase that the unusually low reactivity with CBE and Br-CBE is probably due to the inability of the cytosolic enzyme to effectively donate a proton to the epoxide oxygen. An extremely rapid inactivation of the cytosolic beta-glucosidase was caused by bromoconduritol F ((1,2,4/3)-1-bromo-2,3,4-trihydroxycyclohex-5-ene) with ki(max)/Ki 10(5) M-1 min-1. In contrast with the Br-CBE-inhibited enzyme the beta-glucosidase inhibited by bromoconduritol F was subject to spontaneous reactivation with t1/2 approximately 20 min.