Evidence for a single catalytic and two binding sites in the almond emulsin beta-D-glucosidase molecule

Thermostable glucose-tolerant glucoamylase produced by the thermophilic fungus Scytalidium thermophilum

Glucoamylase produced by Scytalidium thermophilum was purified 80-fold by DEAE-cellulose, ultrafiltration and CM-cellulose chromatography. The enzyme is a glycoprotein containing 9.8% saccharide, pI of 8.3 and molar mass of 75 kDa (SDS-PAGE) or 60 kDa (Sepharose 6B). Optima of pH and temperature with starch or maltose as substrates were 5.5/70 degrees C and 5.5/65 degrees C, respectively. The enzyme was stable for 1 h at 55 degrees C and for about 8 d at 4 degrees C, either at pH 7.0 or pH 5.5. Starch, amylopectin, glycogen, amylose and maltose were the substrates preferentially hydrolyzed. The activity was activated by 1 mmol/L Mg2+ (27%), Zn2+ (21%), Ba2+ (8%) and Mn2+ (5%). Km and vlim values for starch and maltose were 0.21 g/L, 62 U/mg protein and 3.9 g/L, 9.0 U/mg protein, respectively. Glucoamylase activity was only slightly inhibited by glucose up to a 1 mol/L concentration.

Purification and characterization of isoforms of beta-galactosidases in mung bean seedlings

Five isoforms of beta-galactosidase (EC 3.2.1.23), designated as beta-galactosidases I-V, were isolated from five-day-old mung bean (Vigna radiata) seedlings. Beta-galactosidases II and III were purified to electrophoretic homogeneity by a procedure involving acid precipitation, ammonium sulfate fractionation, chromatography on diethylaminoethyl-cellulose (DEAE-Cellulose) and con A-Sepharose. and chromatofocusing. Beta-galactosidases I, II and III have the same molecular mass of 87 kDa. comprising two nonidentical subunits with molecular masses of 38 and 48 kDa, while beta-galactosidases IV and V have molecular masses of 45 and 73 kDa, respectively. All the enzymes were active against p-nitrophenyl-beta-D-galactoside, and to a lesser extent, p-nitrophenyl-alpha-L-arabinoside and p-nitrophenyl-beta-D-fucoside. The enzymes were inhibited by D-galactono-1,4-lactone, D-galactose, Hg2+, Ag+ and sodium dodecyl sulfate (SDS). Beta-galactosidases I, II and III were shown to be competitively inhibited by either D-galactono-1, 4-lactone or D-galactose. Isoforms I, II and III have a common optimal pH of 3.6, while isoforms IV and V have pH optima at 3.8 and 4.0, respectively. Isoelectric points of isoforms I, II and III were 7.7, 7.5 and 7.3, respectively. Double immunodiffusion analysis indicated that beta-galactosidases I, II, III and V are immunologically similar to each other, while beta-galactosidase IV shares partially identical antigenic determinants with the other four isoforms. The purified beta-galactosidases II and III were capable of releasing D-galactose residue from the hemicellulose fraction isolated from mung bean seeds.

Catalytic mechanism of a family 3 beta-glucosidase and mutagenesis study on residue Asp-247

A family 3 beta-glucosidase (EC 3.2.1.21) from Flavobacterium meningosepticum has been cloned and overexpressed. The mechanistic action of the enzyme was probed by NMR spectroscopy and kinetic investigations, including substrate reactivity, secondary kinetic isotope effects and inhibition studies. The stereochemistry of enzymic hydrolysis was identified as occurring with the retention of an anomeric configuration, indicating a double-displacement reaction. Based on the k(cat) values with a series of aryl glucosides, a Bronsted plot with a concave-downward shape was constructed. This biphasic behaviour is consistent with a two-step mechanism involving the formation and breakdown of a glucosyl-enzyme intermediate. The large Bronsted constant (beta=-0.85) for the leaving-group-dependent portion (pK(a) of leaving phenols >7) indicates substantial bond cleavage at the transition state. Secondary deuterium kinetic isotope effects with 2,4-dinitrophenyl beta-D-glucopyanoside, o-nitrophenyl beta-D-glucopyanoside and p-cyanophenyl beta-D-glucopyanoside as substrates were 1.17+/-0.02, 1.19+/-0.02 and 1.04+/-0.02 respectively. These results support an S(N)1-like mechanism for the deglucosylation step and an S(N)2-like mechanism for the glucosylation step. Site-directed mutagenesis was also performed to study essential amino acid residues. The activities (k(cat)/K(m)) of the D247G and D247N mutants were 30000- and 200000-fold lower respectively than that of the wild-type enzyme, whereas the D247E mutant retained 20% of wild-type activity. These results indicate that Asp-247 is an essential amino acid. It is likely that this residue functions as a nucleophile in the reaction. This conclusion is supported by the kinetics of the irreversible inactivation of the wild-type enzyme by conduritol-B-epoxide, compared with the much slower inhibition of the D247E mutant and the lack of irreversible inhibition of the D247G mutant.

Analysis of mandelonitrile lyase and beta-glucosidase from sweet almonds by combined electrophoretic techniques

Almonds are a rich source of mandelonitrile lyase (oxynitrilase) and beta-glucosidase. The isolation of these two enzymes from sweet almonds requires fractional ammonium sulfate precipitation followed by ion-exchange chromatography on diethylaminoethyl-(DEAE) and carboxymethylcellulose (CMC) columns. In the present investigation different electrophoretic techniques such as sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), isoelectric focusing in immobilized pH gradients (IEF-IPG), and capillary electrophoresis were used to characterize these two enzymes. For the first time, beta-glucosidase and oxynitrilase were separated in an immobilized pH gradient of one pH unit. Capillary zone electrophoresis (CZE) was an excellent tool for analysis of the purity of enzyme preparations, achieving complete separation of various protein constituents in only 15 min. CZE showed a resolving capacity for the separation of enzyme forms comparable to that of isoelectric focusing in an immobilized pH gradient.

Primary structures of the N-linked carbohydrate chains from honeybee venom phospholipase A2

The N-linked carbohydrate chains of phospholipase A2 from honeybee (Apis mellifera) were released from glycopeptides with peptide-N-glycanase A and reductively aminated with 2-aminopyridine. The fluorescent derivatives were separated by size-fractionation and reverse-phase HPLC, yielding 14 fractions. Structural analysis was accomplished by compositional and methylation analyses, by comparison of the HPLC elution patterns with reference oligosaccharides, by stepwise exoglycosidase digestions which were monitored by HPLC, and, where necessary, by 500-MHz 1H-NMR spectroscopy. Ten oligosaccharides consisted of mannose, N-acetylglucosamine and fucose alpha 1-6 and/or alpha 1-3 linked to the innermost N-acetylglucosamine. Four compounds, which comprised 10% of the oligosaccharide pool from phospholipase A2, contained a rarely found terminal element with N-acetylgalactosamine. The structures of the 14 N-glycans from honeybee phospholipase A2 can be arranged into the following three series: [formula: see text]

Kinetic investigation of the substrate specificity of the cyanogenic beta-D-glucosidase (linamarase) of white clover

Partially purified linamarase from Trifolium repens (genotype Lili acac) plants was kinetically characterized. Kinetic evidence was found to support the assumption that this cyanogenic beta-D-glucosidase has a broad substrate spectrum. p-Nitrophenyl-beta-D-xylopyranoside and p-nitrophenyl-alpha-L-arabinopyranoside substrates bound almost as tightly to the active center of the enzyme as the glucono(1----5)lactone transition-state analog inhibitor. Substrate specificity investigation also indicated that positions C-4 and C-6 on the pyranoside ring play an essential role in both substrate orientation and splitting. Recently very similar kinetic characteristics were reported on some mammalian cytosolic beta-D-glucosidases and a possible physiological interpretation of this coincidence is discussed. Inhibition studies with glucono(1----5)lactone revealed that the carbohydrate moiety of each substrate attached to the same binding site in the active center. Inhibition experiments with 1-thio substrate analogs demonstrated that the aglycon and the angular arrangement around the glycosidic linkage were the major determinants in the observed substrate specificity.

Synthesis of 1,4-dideoxy-1,4-imino-D-glucitol, a glucosidase inhibitor

1,2:5,6-Di-O-isopropylidene-D-glucitol was converted via its 1,4-dimethanesulfonate into the 1-azido-4-methanesulfonate which, after deprotection and treatment with barium hydroxide, afforded a 9:1 mixture of the corresponding 3,4- and 4,5-anhydro derivatives. Reduction of this mixture by transfer hydrogenation using ammonium formate in methanol and Pd/C as catalyst afforded 1,4-dideoxy-1,4-imino-D-glucitol (4), the structure of which was proved after acetylation by 1H-n.m.r. spectroscopy. Compound 4 is a potent alpha-D-glucosidase inhibitor (Ki 7 X 10(-4)M) and a less potent beta-D-glucosidase inhibitor (Ki 1.25 X 10(-4)M), and inhibits beta-D-galactosidase non-competitively.

Characterization and kinetics of beta-D-gluco/fuco/galactosidase from sheep liver

This enzyme shows beta-D-glucosidase, beta-D-fucosidase and beta-D-galactosidase activities, all associated in a single peak in Sephadex G-200, DEAE-cellulose, concanavalin A-Sepharose chromatographies, and in high resolution isoelectric focusing (pI 4.56), having the optimal pH in the range 4.5-5.5. The enzyme is very stable under different conditions: (i) at pH in the range 5.5-7.0; (ii) in successive freezing-thawing cycles; (iii) at 4 degrees C; (iv) after exhaustive ultrasonic treatment. It is not stable beyond 40 degrees C, and in the presence of urea, Triton X-100, SDS or mercaptoethanol. HgCl2, KCN, Tris, maltose and the lactones were inhibitors of the enzyme. With glucose, fucose and galactose the inhibition is competitive. In addition, a transglycosylation mechanism seems to occur. The kinetic studies suggest a substrate-activation model and the presence of two primary active sites: fuco-gluco and galacto.

Beta-Xylosidases and a nonspecific wall-bound beta-glucosidase of the yeast Cryptococcus albidus

Cryptococcus albidus grown on wood xylans possesses a soluble intracellular beta-xylosidase (EC 3.2.1.37) as an additional constituent of the xylan-degrading enzyme system of this yeast. The enzyme attacks linear 1,4-beta-xylooligosaccharides in an exo-fashion, liberating xylose from the non-reducing ends. The activity of the enzyme increases in the cells during growth on xylan and incubation with xylobiose or methyl beta-D-xylopyranoside which are the best inducers of extracellular beta-xylanase (EC 3.2.1.8). Various alkyl-,alkyl-1-thio- and aryl beta-D-xylopyranosides were excellent inducers of a different beta-xylosidase of Cryptococcus albidus. This enzyme is localized outside the plasma membrane and is principally associated with cell walls. Unlike the soluble intracellular beta-xylosidase, the wall-bound enzyme does not hydrolyze xylooligosaccharides. Evidence has been obtained that beta-xylosidase activity in the cell walls is not due to the presence of a specific aryl beta-xylosidase, but is exhibited by a nonspecific beta-glucosidase (EC 3.2.1.21) inducible by beta-D-xylopyranosides. The ratio of beta-glucosidase and beta-xylosidase activity in the cells and isolated cell walls from yeast induced by various beta-xylopyranosides and beta-glucopyranosides was very similar. Both wall-bound activities were inhibited in a similar pattern by inhibitors of beta-glucosidases, 1,5-gluconolactone and nojirimycin. This bifunctional enzyme does not bear any relationship to the utilization of xylans in Cryptococcus albidus.

Study on the role of tyrosine side-chains at the active centre of emulsin beta-D-glucosidase

The role of exposed tyrosine side-chains in enzyme-catalyzed reaction by sweet almond emulsin beta-D-glucosidase (beta-D-glucoside glucohydrolase, EC 3.2.1.21) has been studied using N-acetylimidazole as the specific reagent. The changes in activity, binding affinity and kinetic parameters (Km, V) as a result of acetylation of the phenolic hydroxyl groups have been determined. The acetylation increased the Km values of both beta-glucosidase and beta-galactosidase activities, whereas V remained unchanged. Similarly, the binding affinity for immobilized phenyl beta-D-glucopyranoside decreased appreciably. After the removal of the acetyl groups the enzyme regained 96% of the original activity. It is concluded that the tyrosine moieties, located in the active centre of the enzyme, have both glucoside and galactoside binding functions.