Biochemical characterization and mechanism of

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a member of glycoside hydrolase family 3. ... β-glucosidases catalyse the synthesis of alkyl glycosides by ...... TIM (triose phosphate isomerase) barrel structure ...

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Biochem. J. (2001) 353, 117–127 (Printed in Great Britain)

Biochemical characterization and mechanism of action of a thermostable β-glucosidase purified from Thermoascus aurantiacus Neil J. PARRY*1, David E. BEEVER†, Emyr OWEN†, Isabel VANDENBERGHE‡, Jozef VAN BEEUMEN‡ and Mahalingeshwara K. BHAT*2 *Food Materials Science Division, Institute of Food Research, Norwich Research Park, Colney, Norwich NR4 7UA, U.K., †Department of Agriculture, The University of Reading, Earley Gate, P.O. Box 236, Reading RG6 6AT, U.K., and ‡University of Ghent, Laboratory of Protein Biochemistry and Protein Engineering, K.L. Ledeganckstraat 35, B-9000, Ghent, Belgium

An extracellular β-glucosidase from Thermoascus aurantiacus was purified to homogeneity by DEAE-Sepharose, Ultrogel AcA 44 and Mono-P column chromatography. The enzyme was a homotrimer, with a monomer molecular mass of 120 kDa ; only the trimer was optimally active at 80 mC and at pH 4.5. At 90 mC, the enzyme showed 70 % of its optimal activity. It was stable at pH 5.2 and at temperatures up to 70 mC for 48 h, but stability decreased above 70 mC and at pH values above and below 5.0. The enzyme hydrolysed aryl and alkyl β--glucosides and cello-oligosaccharides, and was specific for substrates with a βglycosidic linkage. The hydroxy groups at positions 2, 4 and 6 of a glucose residue at the non-reducing end of a disaccharide appeared to be essential for catalysis. The enzyme had the lowest Km towards p-nitrophenyl β--glucoside (0.1137 mM) and the highest kcat towards cellobiose and β,β-trehalose (17 052 minV"). It released one glucose unit at a time from the non-reducing end of cello-oligosaccharides, and the rate of hydrolysis decreased

with an increase in chain length. Glucose and -δ-gluconolactone inhibited the β-glucosidase competitively, with Ki values of 0.29 mM and 8.3 nM respectively, while methanol, ethanol and propan-2-ol activated the enzyme. The enzyme catalysed the synthesis of methyl, ethyl and propyl β--glucosides in the presence of methanol, ethanol and propan-2-ol respectively with either glucose or cellobiose, although cellobiose was preferred. An acidic pH favoured hydrolysis and transglycosylation, but high concentrations of alcohols favoured the latter reaction. The stereochemistry of cellobiose hydrolysis revealed that βglucosidase from T. aurantiacus is a retaining glycosidase, while N-terminal amino acid sequence alignment indicated that it is a member of glycoside hydrolase family 3.

INTRODUCTION

β-glucosidases catalyse the synthesis of alkyl glycosides by transglycosylation and play an important role in improving the aroma of wines [9,10]. Thus a detailed study on the biochemical and catalytic properties, together with the mechanism of action, of β-glucosidases is important in order to assess their full potential from both fundamental and applied standpoints. The thermophilic fungus Thermoascus aurantiacus produces relatively high levels of endoglucanase, exoglucanase and βglucosidase when grown on a lignocellulosic carbon source such as corn cob [11]. Also, the cellulase components of this fungus are remarkably stable over a wide range of pH values and temperatures, and appear to be of tremendous commercial interest [12]. The present paper reports on the biochemical and catalytic properties of β-glucosidase from T. aurantiacus, together with its transglycosylation activity, the stereochemistry of cellobiose hydrolysis and N-terminal amino acid sequencing.

Cellulose is the most abundant and renewable source of energy on Earth. Cellulose can be converted into soluble sugars by either acid or enzymic hydrolysis [1,2] ; the latter is preferred due to the high yields of desired products with fewer by-products, and in commercial situations it is considered to be economically favourable [2,3]. Micro-organisms capable of degrading crystalline cellulose extensively produce three main types of enzymes, namely endoglucanase (1,4-β--glucan glucanohydrolase ; EC 3.2.1.4), exoglucanase (1,4-β--glucan cellobiohydrolase ; EC 3.2.1.91) and β-glucosidase (cellobiase or β--glucoside glucohydrolase ; EC 3.2.1.21) [1–3]. Understanding the role and the mechanism of action of cellulase components during the solubilization of crystalline cellulose has been the subject of immense research for the past three decades [4,5]. Previous studies have demonstrated that endo- and exoglucanases act synergistically and promote the solubilization of crystalline cellulose into soluble sugars [4,5], while β-glucosidase completes the hydrolysis by converting cellobiose and cellooligosaccharides into glucose [6]. β-Glucosidase is also considered to be part of a cellulase system, since it stimulates the rate and extent of cellulose hydrolysis by relieving cellobiose-induced inhibition of endo- and exo-glucanases [7,8]. Furthermore, some

Key words : mode of action, N-terminal sequencing, stereochemistry, substrate specificity, transglycosylation.

EXPERIMENTAL Materials T. aurantiacus IMI 216529 was from the Centre for Agriculture and Biosciences International (CABI), Egham, Surrey, U.K. All chemicals and reagents were from either Sigma or BDH, while

Abbreviations used : pNP, p-nitrophenol or p-nitrophenyl ; pNPGlc, p-nitrophenyl β-D-glucoside ; oNPGlc, o-nitrophenyl β-D-glucoside ; pNPXyl, pnitrophenyl β-D-xyloside ; oNPXyl, o-nitrophenyl β-D-xyloside ; pNPGlc2, p-nitrophenyl β-D-cellobioside ; pNPGlc3, p-nitrophenyl β-D-cellotrioside ; pNPGlc4, p-nitrophenyl β-D-cellotetraoside ; MeUmb, 4-methylumbelliferone or 4-methylumbelliferyl ; MeUmbGlc, 4-methylumbelliferyl β-D-glucoside ; MeUmbGlc2, 4-methylumbelliferyl β-D-cellobioside ; MeUmbMan, 4-methylumbelliferyl β-D-mannoside ; MeUmbLac, 4-methylumbelliferyl β-D-lactoside. 1 Present address : Unilever Research, Colworth Laboratory, Colworth House, Sharnbrook, Bedford MK44 1LQ, U.K. 2 To whom correspondence should be addressed (e-mail Mahalingeshwara.Bhat!bbsrc.ac.uk). # 2001 Biochemical Society

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electrophoresis solutions and chromatography media were from Anachem and Pharmacia respectively.

Production and purification of β-glucosidase A crude cellulase-rich fraction of T. aurantiacus (supplied by Cayla, Toulouse, France) was prepared as follows. A 10 litre batch of medium [13] supplemented with glucose (30 g:lV"), yeast extract (2.5 g:lV") and wheat bran (5 g:lV") was inoculated with 750 ml of 30 h pre-culture grown on the same medium without wheat bran at 50 mC using an orbital shaker. The culture was grown for 48 h at 50 mC and used to inoculate a fermenter containing 300 litres of medium [13] containing paper waste (20 g:lV"), wheat bran (20 g:lV") and yeast extract (5 g:lV"), with the pH adjusted to 5.0. The fermentation was carried out for 6 days at 50 mC with agitation at 400 rev.\min. The culture was harvested, and the supernatant was fractionated by ultrafiltration (using 200 000 and 15 000 kDa cut-off membranes) into cellulaseand hemicellulase-rich fractions, before concentrating by acetone (50 %, v\v) precipitation. The freeze-dried cellulase-rich fraction (15 g) dissolved in 100 ml of distilled water was subjected to 80 % (NH ) SO %# % saturation. The precipitated protein containing endoglucanase, exoglucanase and β-glucosidase was dissolved in 10 mM sodium acetate buffer and desalted on a Bio-Gel P6DG column (XK ; 2.6 cmi100 cm). This was concentrated, equilibrated with 50 mM sodium acetate buffer (pH 5.0) and chromatographed on a DEAE-Sepharose column (XK ; 2.6 cmi90 cm) equilibrated with the same buffer. The bound endoglucanase, exoglucanase and β-glucosidase were eluted using a 0–0.5 M NaCl gradient prepared in 50 mM sodium acetate buffer (pH 5.0). Fractions containing β-glucosidase were pooled, concentrated, equilibrated with 50 mM sodium acetate buffer (pH 5.0) and desalted on a Bio-Gel P6DG column (XK ; 2.6 cmi100 cm). The desalted sample was concentrated to 25 ml and chromatographed on an Ultrogel AcA 44 column (XK ; 1.6 cmi100 cm) at a flow rate of 1 ml:minV". The β-glucosidase was separated from endo- and exo-glucanases, and further purified by chromatofocusing on a Mono-P column (HR ; 0.5 cmi20 cm) using Polybuffer 74 in the pH range between 3.4 and 5.5.

Assay of β-glucosidase A microtitre plate method was developed using p-nitrophenyl β-glucoside (pNPGlc). A 25 µl portion of culture filtrate or 20 ng (25 µl) of pure enzyme was mixed with 25 µl of 200 mM sodium acetate buffer (pH 5.0) and 50 µl of distilled water, and preincubated at 50 mC for 5 min. The reaction was initiated by adding 25 µl of 10 mM pNPGlc and continued for 15 min at 50 mC, before being terminated by the addition of 100 µl of glycine buffer, pH 10.8. The colour developed was read at 405 nm, and translated to µmol of p-nitrophenol (pNP) using a standard graph prepared under the same conditions. The unit of β-glucosidase activity is expressed as the amount of enzyme required to release 1 µmol of pNP per min under the above assay conditions.

Biochemical characterization of β-glucosidase Homogeneity and molecular mass These were determined by SDS\PAGE [15] and gel filtration on Sephacryl 300. The time course of denaturation was studied by SDS\PAGE using 25 µg of β-glucosidase mixed with SDStreatment buffer and boiled for 1, 2, 4, 6, 8 and 10 min. The possible quaternary structure was analysed under reduced and non-reduced conditions by incubating 25 µg of β-glucosidase at 100 mC for 0, 4 and 8 min. Activity staining was performed by incubating the SDS\polyacrylamide gel with 50 ml of 0.26 µM 4-methylumbelliferyl β--glucoside (MeUmbGlc) after washing the gel with 50 mM sodium acetate buffer at pH 4.0. The β-glucosidase activity was visualized under UV light.

pH and temperature optima The pH and temperature profiles of β-glucosidase were constructed by determining its activity on pNPGlc, as described above, in the pH range 2.8–6.8 (citrate\phosphate buffer, 0.1 M) and in the temperature range 40–90 mC.

Stability Stability was determined at pH 2.8, 4.0, 5.2 and 6.8 and at temperatures of 50, 60, 70 and 80 mC for 48 h. A 300 µl portion of β-glucosidase (2 µg protein\ml) was incubated at different pH values, and a 15 µl sample was withdrawn after 0.5, 1, 2, 4, 8, 18, 28 and 48 h and assayed for β-glucosidase using pNPGlc.

Substrate specificity This was determined using pNP glycosides, disaccharides, oligosaccharides and polysaccharides. Activity on pNP glycosides was determined by measuring pNP release, as described above. All compounds were used at 0.5, 1, 1.5 and 2 mM, at pH 4.5 and 60 mC, and were incubated with 20 ng of β-glucosidase for 5 min. Hydrolysis of disaccharides and oligosaccharides was monitored by the release of glucose [16] using same reaction conditions as for pNP glycosides, except with a 2 ml reaction volume. β-Glucosidase activity on polysaccharides was determined using 0.5–2.5 mg of substrate, 20 ng of enzyme and 50 mM sodium acetate buffer, pH 4.5, in a 2 ml reaction volume at 60 mC for 5 min, and by measuring the reducing sugars by either the Somogyi [17] or the dinitrosalicylic acid [18] method.

Kinetics Km, kcat and the Km\kcat ratio for the hydrolysis of pNPGlc, o-nitrophenyl β--glucoside (oNPGlc), gentiobiose, sophorose, cellobiose, β,β-trehalose and n-octyl β--glucopyranoside by β-glucosidase were determined using 0–4 mM substrate and by analysing the data by reciprocal plots using an Enzfitter program.

Effects of glucose and D-δ-gluconolactone These were studied by determining the activity of β-glucosidase towards pNPGlc in the presence of 0–10 mM glucose and 0–0.1 µM -δ-gluconolactone. The Ki values given are the averages of two separate experiments performed in duplicate.

Determination of protein

Effects of alcohols

Protein was determined using bicinchoninic acid [14]. Absorbance at 280 nm was used to monitor the protein content in the column fractions.

The effects of methanol, ethanol and propan-2-ol (0–100 %, v\v) on the hydrolysis of pNPGlc were studied using the pNPGlc assay, with 20 ng of β-glucosidase.

# 2001 Biochemical Society

β-Glucosidase from Thermoascus aurantiacus Hydrolysis of 4-methylumbelliferone (MeUmb)-substituted sugars A Perkin–Elmer spectrofluorimeter (model LS50) was used to detect MeUmb released from MeUmb-substituted sugars, with excitation at 397 nm and emission at 448 nm. A 3 ml quartz cuvette containing 1.5 ml of substrate (0–200 µM) and 1.496 ml of 100 mM sodium acetate buffer (pH 4.5) was preincubated at 60 mC for 5 min prior to the addition of 4 µl of β-glucosidase (50 ng of protein). The release of MeUmb was followed for 15 min, at 10 s intervals. Rates of bond cleavage were determined for MeUmbGlc and 4-methylumbelliferyl β--cellobioside (MeUmbGlc ). β-Glucosidase activity towards 4-methylumbelli# feryl β--mannoside (MeUmbMan), 4-methylumbelliferyl β-lactoside (MeUmbLac) and methylumbelliferyl α--glucoside was determined using 100 µM substrate, under the same conditions.

Mode of action This was studied using unsubstituted and pNP-substituted cellooligosaccharides. A 300 µl reaction mixture containing 100 µg of substrate, 20 ng of β-glucosidase, 150 µl of 20 mM sodium acetate buffer (pH 5.0) and distilled water was incubated at 50 mC for 2 h. Samples of 25 µl were withdrawn at different time intervals, mixed with 25 µl of acetonitrile and analysed on an NH # Spherisorb column connected to a Gynkotech HPLC system using acetonitrile\water (7 : 3, v\v) as a mobile phase. Initial hydrolytic rates for pNP cello-oligosaccharides were determined relative to the release of glucose.

Transglycosylation activity

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products were determined by one-dimensional "H and "$C NMR spectroscopy. The reaction products were also analysed by HPLC. For this purpose, a 100 µl sample was diluted 2-fold with acetonitrile, and 10 µl of this was injected on to an NH -Spherisorb column # (15 cmi0.4 cm), and eluted using acetonitrile\water (8 : 2, v\v) V at a flow rate of 250 µl:min ". The products were identified using standard α and β methyl glucosides.

N-terminal protein sequencing A 50 µg freeze-dried sample of β-glucosidase was dissolved in 30 µl of distilled water, of which 15 µl was used for the analysis. Sequence analysis was performed on a model 477A protein sequencer (Applied Biosystems), working in the pulsed-liquid mode. Related sequences in the GenBank and SwissProt Databases were searched for using the program BLAST [19], and sequence alignment was performed using the GCG sequence alignment program.

RESULTS Purification of β-glucosidase An extracellular β-glucosidase from T. aurantiacus was purified by DEAE-Sepharose, Ultrogel AcA 44 and Mono-P column chromatography. During DEAE-Sepharose column chromatography, the major β-glucosidase peak was eluted with an NaCl gradient (0–0.5 M) ; this peak also contained endo- and exoglucanase activities. Fractionation of this peak on an Ultrogel AcA 44 column separated the β-glucosidase from the endo- and exo-glucanase activities. The final purification of β-glucosidase

This was monitored using either glucose or cellobiose as a donor and methanol as an acceptor. A 300 µl reaction mixture containing 100 ng of β-glucosidase, 50 µl of glucose or cellobiose (2 mg:mlV"), 25 or 50 % (v\v) methanol and distilled water was incubated at 50 mC. Samples of 25 µl were withdrawn at different time intervals (0.25–24 h) and mixed with 25 µl of acetonitrile, and 20 µl of this mixture was analysed on an NH -Spherisorb # column connected to a Gynkotech HPLC system, using acetonitrile\water (7 : 3, v\v) as the mobile phase. The data were analysed using a Gynkosoft program. The effect of pH (3.5, 5.0 and 6.5) on the synthesis of alkyl glycosides was studied using 40 and 80 % (v\v) methanol, ethanol or propan-2-ol. A 1 ml reaction mixture containing 100 ng of βglucosidase, 1 mg of cellobiose, 0.25 ml of 100 mM buffer of the required pH value, the required amount of alcohol and distilled water was incubated at 50 mC for 10 h. Samples of 25 µl were taken at different time intervals and analysed by HPLC as described above.

Stereochemical course of cellobiose hydrolysis This was determined by assessing the transglycosylation activity of β-glucosidase by "H and "$C NMR. A 5 ml reaction mixture containing 2.5 ml of 50 mM sodium acetate buffer (pH 5.0), 50 mg of cellobiose and 2 µg of β-glucosidase in 40 % (v\v) methanol was incubated for 12, 24 and 36 h at 50 mC. The reaction was terminated by boiling the sample for 10 min. A 100 µl sample was retained for HPLC analysis. The remaining 4.9 ml was concentrated to dryness (Buchi RE111 Rotor vapour equipped with a Buchi 461 water bath) at 35 mC. The dried sample was dissolved in 2 ml of #H O and placed in an HP 507 # NMR tube. The NMR spectra were recorded using a JEOL EX270 Fourier Transform Spectrophotometer at 67.8 MHz ("$C) and 270.05 MHz ("H). Chemical structures of the reaction

Figure 1

SDS/PAGE of purified β-glucosidase from T. aurantiacus

(a) Denaturation of β-glucosidase at 100 mC as a function of time ; (b) mobility of βglucosidase, denatured under reduced and non-reduced conditions for different time periods ; (c) activity staining of β-glucosidase, denatured under reduced and non-reduced conditions for different time periods. Standard protein markers in the order of increasing molecular mass are : carbonic anhydrase (31.0 kDa), lactate dehydrogenase (36.5 kDa), glutamate dehydrogenase (55.4 kDa), BSA (66.3 kDa), phosphorylase b (97.4 kDa), β-galactosidase (116.3 kDa) and myosin (200 kDa). Arrows in (a) indicate (from bottom to top) the monomer, dimer and trimer of β-glucosidase, and that in (c) indicates β-glucosidase activity staining. # 2001 Biochemical Society

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N. J. Parry and others Table 1

Kinetic constants for β-glucosidase from T. aurantiacus

Activity was assayed using 0–4 mM substrate, at pH 4.5 and 60 mC, with 20 ng of enzyme. The release of pNP, o-nitrophenol or glucose was measured as described in the Experimental section. The kinetic constants were determined by analysing the data by reciprocal plots using an Enzfitter program. All values are the averages of two separate experiments done in duplicate.

Figure 2

pH/temperature profile of β-glucosidase from T. aurantiacus

Activity was determined using pNPGlc, at pH values between 2.8 and 6.8 and at temperatures between 40 and 90 mC, as described in Experimental section. The activity at different temperatures is plotted as a function of pH. All values are the averages of two separate experiments done in duplicate.

was achieved by chromatofocusing on a Mono-P column in the pH region from 3.4 to 5.5. A total of 1 mg of β-glucosidase was obtained with a specific activity of 190i10$ µmol:minV":mg of proteinV", when measured at pH 4.5 and 60 mC.

Homogeneity and molecular mass Purified β-glucosidase from T. aurantiacus was homogeneous, with a denatured molecular mass of 120 kDa (Figure 1a). The time course of denaturation and subsequent analysis by SDS\ PAGE showed the presence of three protein bands, corresponding to the monomer, dimer and trimer of β-glucosidase (Figure 1a). Nevertheless, the gel filtration showed that the native enzyme had a molecular mass of " 350 kDa and appeared to be a homotrimer. SDS\PAGE under native and denatured conditions (with and without mercaptoethanol) showed that the β-glucosidase was denatured only under reduced conditions and by boiling (Figure 1b). Under non-reduced conditions, the enzyme remained as a trimer even after boiling for 8 min (Figure 1b). Also, at time zero and under reduced conditions, the enzyme was present as a trimer (Figure 1b). The activity staining showed that only the native enzyme was fully active, and enzyme treated with mercaptoethanol at 0 min was less active (Figure 1c).

Temperature and pH optima β-Glucosidase was optimally active at 80 mC and at pH 4.5 (Figure 2). At 90 mC, the enzyme showed "70 % of its optimal activity. Up to 60 mC, the activity of β-glucosidase was low ; it then increased rapidly to reach a maximum at 80 mC. With respect to pH, the enzyme showed relatively high activity between pH 3.8 and 5.0 (Figure 2).

Stability β-Glucosidase was stable up to 70 mC at pH 5.2. At 80 mC and pH 5.2, the enzyme lost 90 % of its original activity in 30 min. At all other pH and temperatures tested the enzyme was less stable, except at 50 mC and pH 6.8, under which conditions it was stable for up to 48 h. # 2001 Biochemical Society

Substrate

Km (mM)

kcat (minV1)

10V3ikcat/Km (mMV1:minV1)

pNPGlc oNPGlc n-Octyl β-D-glucoside β,β-Trehalose Cellobiose Gentiobiose Sophorose

0.1137 0.2500 0.2718 0.4340 0.6370 0.2095 1.3390

14 532 14 112 14 088 17 052 17 052 8444 8184

127.81 56.45 51.83 39.29 26.77 41.01 6.11

Substrate specificity of β-glucosidase Activity on pNP glycosides The β-glucosidase showed highest activity (190i 10$ µmol:minV":mg of proteinV") on pNPGlc. The presence of the nitro group at the ortho position decreased the activity, as observed with oNPGlc. The β-glucosidase activity on pNP β--cellobioside (pNPGlc ) was 10 times lower than that on # pNPGlc, while the activity on pNP β--lactoside was at least 5-fold lower than that on pNPGlc . The β-glucosidase activity # on pNP α--galactoside, pNP β--galactoside and o-nitrophenyl β--galactoside was negligible. The enzyme showed low activity on pNP β--xyloside (pNPXyl) and o-nitrophenyl β--xyloside (oNPXyl), but its activity was higher when the nitro group was in the ortho position. The enzyme hydrolysed pNPgentiobiose, pNPmaltose, pNPlactose, pNPmelibiose, pNPmannose and pNPfucose to a limited extent. Replacement of the hydroxy group at the C-2 position with an NH group, as in the case of # pNPgalactosamine, resulted in no activity. Also, the replacement of a glucosidic bond by a thiol linkage abolished the β-glucosidase activity, as with pNP 1-thio-β--glucoside and pNP 1-thio-β-galactoside.

Activity on disaccharides and other sugar derivatives β-Glucosidase rapidly hydrolysed both cellobiose and β,βtrehalose, with comparable rates of hydrolysis. The enzyme was less active on sophorose and gentiobiose than on cellobiose, but inactive on lactose (α and β anomers), melibiose, maltose, sucrose, α,α-trehalose, gentiobiose octa-acetate, raffinose and cyclic α1,4-linked dextrins. β-Glucosidase hydrolysed arbutin, esculin, methyl β-glucoside and salicin to a limited extent. It was more active on n-octyl β--glucopyranoside than on cellobiose, but less active than on pNPGlc. It rapidly released MeUmb from MeUmbGlc, but there was a delay in the release of MeUmb from MeUmbGlc , # since the enzyme first released MeUmbGlc and glucose from this substrate. Determination of the kinetic constants revealed that the enzyme had an approx. 30 % lower Km and a 6-fold higher kcat\Km towards MeUmbGlc than towards MeUmbGlc . The # enzyme was not active on MeUmbLac, MeUmbMan or α-linked MeUmbGlc.

Activity on polysaccharides The β-glucosidase hydrolysed CM-cellulose and laminarin, but the specific activity was considerably lower than that with disaccharides. The enzyme showed the highest activity on

β-Glucosidase from Thermoascus aurantiacus

Figure 3

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Hydrolysis of cello-oligosaccharides by β-glucosidase from T. aurantiacus as a function of time

For details, see the Experimental section. Panels (A), (B), (C) and (D) show the products released from cellobiose, cellotriose, cellotetraose and cellopentaose respectively, as a function of time. The solid and broken lines in (A) correspond to reaction products released in the presence of distilled water and in sodium acetate buffer (pH 5.0) respectively. Each point is the mean of two separate experiments done in duplicate. Symbols : , glucose ; #, cellobiose ; $, cellotriose ; =, cellotetraose ; 4, cellopentaose.

laminarin, but this was  100 times lower than that recorded for cellobiose.

Determination of the kinetic constants for preferred substrates Table 1 shows the kinetic constants of β-glucosidase for preferred aryl β--glucosides and disaccharides. The enzyme had the lowest and highest Km for pNPGlc and sophorose respectively. The highest kcat values were obtained with β,β-trehalose and cellobiose. The kcat\Km ratios for the hydrolysis of β,β-trehalose and cellobiose were lower than that for pNPGlc hydrolysis, but the Km\kcat ratio for pNPGlc hydrolysis was 2-fold higher than that for oNPGlc and n-octyl β--glucoside.

Inhibition by glucose and D-δ-gluconolactone Glucose and -δ-gluconolactone inhibited β-glucosidase competitively, with Ki values of 0.29 mM and 8.3 nM respectively, indicating that -δ-gluconolactone is a stronger inhibitor of β-glucosidase than glucose.

Effects of alcohols Methanol, ethanol and propan-2-ol activated the β-glucosidase at concentrations below 50 % (v\v), and were inhibitory at higher concentrations. Propan-2-ol activated the enzyme 2-fold

at 20 % (v\v) ; at a concentration of 30 % (v\v), methanol and ethanol activated the enzyme by 30 % and 40 % respectively. The enzyme was more active in methanol than in ethanol and propan2-ol at concentrations higher than 60 %. Propan-2-ol at between 90 and 100 % (v\v) resulted in the greatest inhibition (50 %) of this enzyme.

Mode of action This was studied using unsubstituted and pNP-substituted cellooligosaccharides.

Hydrolysis of unsubstituted cello-oligosaccharides Time courses of hydrolysis of cello-oligosaccharides by βglucosidase are shown in Figures 3(A)–3(D). The enzyme rapidly hydrolysed cellobiose to glucose ; this reaction was more favoured in distilled water than in a buffered system (Figure 3A). Hydrolysis of cellotriose resulted initially in the formation of glucose and cellobiose, and the latter was hydrolysed further to glucose (Figure 3B). The action of β-glucosidase on cellotetraose resulted in the formation of glucose, cellobiose and cellotriose, with the subsequent hydrolysis of cellotriose to cellobiose and glucose (Figure 3C). From cellopentaose, the enzyme initially released glucose, cellobiose, cellotriose and cellotetraose (Figure 3D). As the reaction progressed, the levels of cellobiose, cellotriose and cellotetraose decreased in reverse order, while the # 2001 Biochemical Society

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Figure 4

N. J. Parry and others

Hydrolysis of pNP cello-oligosaccharides by β-glucosidase from T. aurantiacus as a function of time

For details, see the Experimental section. Panels (A), (B), (C) and (D) show the products released from pNPGlc, pNPGlc2, pNPGlc3 and pNPGlc4 respectively as a function of time. Each point is the mean of two separate experiments done in duplicate. Symbols : , glucose ; , pNPGlc ; 5, pNPGlc2, 4, pNPGlc3 ; W, pNPGlc4.

level of glucose increased. Thus the β-glucosidase appeared to hydrolyse cello-oligosaccharides in a stepwise manner, releasing one glucose unit at a time.

Hydrolysis of pNP-substituted cello-oligosaccharides Time courses of hydrolysis of pNP-substituted cellooligosaccharides by β-glucosidase are shown in Figures 4(A)– 4(D). The pNP was not detected by the mass detector under the analysis conditions used. During the hydrolysis of pNPGlc its concentration decreased, with a corresponding increase in the glucose concentration (Figure 4A). The hydrolysis of pNPGlc # resulted in the formation of pNPGlc and glucose (Figure 4B). The concentrations of these products increased up to 60 min of the reaction, after which the level of pNPGlc decreased, followed by an increase in the glucose concentration. The hydrolysis of pNP β--cellotrioside (pNPGlc ) resulted initially in the form$ ation of pNPGlc and glucose (Figure 4C). Conversion of # pNPGlc into pNPGlc and glucose was detected only after 90 # and 120 min of incubation. During the hydrolysis of pNP β-cellotetraoside (pNPGlc ), there was initial formation of pNPGlc % $ and glucose, followed by formation of pNPGlc after 30 min of # incubation, but no pNPGlc was detected during the 120 min reaction time (Figure 4D). Clearly the β-glucosidase attacked # 2001 Biochemical Society

pNP cello-oligosaccharides from the non-reducing end, by releasing one glucose unit at a time. The rate of hydrolysis of pNP cello-oligosaccharides by βglucosidase was determined by measuring the glucose released as a function of time. The enzyme released 1.7, 1.4, 0.2 and 0.1 µg of glucose per h from pNPGlc, pNPGlc , pNPGlc and # $ pNPGlc respectively. Thus the rate of hydrolysis of pNP cello% oligosaccharides decreased markedly as the chain length increased.

Transglycosylation activity This was demonstrated using either glucose or cellobiose as a donor and methanol as an acceptor. With 25 % (v\v) methanol, low levels of glucose coincided with an increase in methyl β-glucoside formation over the 24 h incubation period (Figure 5A). With 50 % (v\v) methanol, the level of glucose fluctuated, and only small amounts of methyl β--glucoside were formed during the 24 h reaction period (Figure 5B). When cellobiose was used with 25 % (v\v) methanol, there was a rapid decrease in the cellobiose concentration, with a steady increase in the concentrations of methyl β--glucoside and glucose in equimolar amounts (Figure 5C). With 50 % (v\v) methanol, as the cellobiose concentration decreased, an increase in the concentrations of both methyl β--glucoside and glucose

β-Glucosidase from Thermoascus aurantiacus

Figure 5

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Transglycosylation activity of β-glucosidase from T. aurantiacus

For details, see the Experimental section. Panels (A) and (B) show transglycosylation activity with glucose at 25 % and 50 % (v/v) methanol respectively. Panels (C) and (D) show transglycosylation activity with cellobiose at 25 % and 50 % (v/v) methanol respectively. Each point is the mean of two separate experiments done in duplicate. Symbols : , glucose ; $, cellobiose ; #, methyl β-D-glucoside.

was noted, with the concentration of methyl β--glucoside being higher than that of glucose (Figure 5D).

Effect of pH on the transglycosylation activity of β-glucosidase with methanol, ethanol and propan-2-ol Methanol The effect of pH (3.5, 5.0 and 6.5) on transglycosylation activity was studied using cellobiose with 40 and 80 % (v\v) methanol ; these concentrations activated and inhibited the hydrolytic activity of β-glucosidase respectively. With 40 % (v\v) methanol, a rapid increase in the production of methyl β--glucoside was noted at all pH values. The highest amount of methyl β-glucoside (1200 µg\ml) was produced at pH 5.0, and the lowest amount (200 µg\ml) at pH 6.5. The amount of methyl β-glucoside formed in the presence of 80 % (v\v) methanol was low

compared with that formed with 40 % (v\v) methanol at all pH values. The amounts of glucose released in the presence of 40 and 80 % (v\v) methanol were highest at pH 3.5 and 5.0, and lowest at pH 6.5. At pH 3.5 and 5.0, other transglycosylated products with retention times higher than that of glucose and lower than that of cellobiose were also formed. At pH 6.5, no such products were found with either 40 or 80 % (v\v) methanol. Nevertheless, 40 % (v\v) methanol and pH 5.0 were found to be ideal for the production of high levels of methyl β--glucoside from cellobiose.

Ethanol The effects of 30 % and 80 % (v\v) ethanol on the production of ethyl β--glucoside were studied at pH 3.5, 5.0 and 6.5 using # 2001 Biochemical Society

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Figure 6

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Time-course 1H NMR spectra of cellobiose hydrolysis by β-glucosidase from T. aurantiacus in the presence of methanol

For details, see the Experimental section. T is reaction time (h). Notations : α2-Glc2, proton signal at the C-1 position of the terminal glucose residue of cellobiose in an α-conformation (chemical shift 5.3 p.p.m.) ; β2β1-Glc2, proton signals at the C-1 positions of both glucose residues of cellobiose in the β-conformation (chemical shifts of 4.62 and 4.5 p.p.m. respectively) ; β1-MeβDGlc, proton signal at the C-1 position of methyl β-D-glucoside in the β-conformation (chemical shift 4.35 p.p.m.).

cellobiose as a donor. These two concentrations of ethanol respectively activated and inhibited β-glucosidase activity. With 30 % (v\v) ethanol, the enzyme produced the highest amount of ethyl β--glucoside (250 µg\ml) at pH 5.0 ; at pH 3.5 and 6.5, the amounts formed were 125 and 200 µg\ml respectively. The amount of glucose produced from cellobiose with 30 % (v\v) ethanol at all pH values was at least four times the amount of ethyl β--glucoside formed. The β-glucosidase produced the highest amount of ethyl β--glucoside (400 µg\ml) with 80 % (v\v) ethanol, at pH 3.5 ; however, at pH 5.0 ethyl β--glucoside production was decreased to 50 µg\ml, while at pH 6.5 the enzyme was totally inactive. In addition, the amounts of glucose produced at pH 3.5 and 5.0 with 80 % (v\v) ethanol were comparable with the amount of ethyl β--glucoside formed. Thus the low concentrations of ethanol favoured the hydrolysis # 2001 Biochemical Society

of cellobiose, while high concentrations promoted the transglycosylation reaction. Also, 80 % (v\v) ethanol and pH 3.5 were found to be ideal conditions for the production of high levels of ethyl β--glucoside from cellobiose. The formation of other transglycosylated products was observed only at pH 3.5 and 5.0, at both ethanol concentrations.

Propan-2-ol The effects of 20 and 80 % (v\v) propan-2-ol on propyl β-glucoside production were studied at pH 3.5, 5.0 and 6.5 with cellobiose as the donor. At 20 % (v\v) propan-2-ol, the amount of propyl β--glucoside formed was less than 40 µg\ml at all three pH values, and the enzyme hydrolysed cellobiose rapidly irrespective of pH. With 80 % (v\v) propan-2-ol, the enzyme

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the C-1 position of glucose residues of cellobiose were observed. After 12 h, the depletion of the β configuration of cellobiose, together with a rapid decrease in the β-glucosidic bond signal, were observed. Also, a new signal corresponding to methyl β-glucoside was detected. As the reaction progressed, the signal of the β-glycosidic bond (β β ) of the cellobiose decreased, while # " that of the α-cellobiose (α ) did not change. In addition, the # analysis of samples by "$C NMR at times 0 h and 36 h demonstrated the production of methyl β--glucoside only.

HPLC analysis HPLC analysis of the α and β forms of methyl -glucoside showed only a partial separation with a double peak when the two forms were present in the reaction mixture. However, HPLC analysis of reaction samples taken at various time intervals contained a single peak, corresponding to methyl β--glucoside. In addition, the levels of methyl β--glucoside and glucose increased simultaneously as the level of cellobiose decreased (Figure 7). Figure 7 Quantitative HPLC analysis of cellobiose hydrolysis by βglucosidase from T. aurantiacus in the presence of methanol as a function of time For details, see the Experimental section. Each value is the mean of two separate experiments done in duplicate. Black, grey and white bars correspond to methyl β-D-glucoside, glucose and cellobiose respectively.

synthesized higher amounts of propyl β--glucoside at pH 3.5 (250 µg\ml) and pH 5.0 (150 µg\ml). The amount of glucose released from cellobiose was only marginally higher than the amount of propyl β--glucoside formed at both pH 3.5 and pH 5.0. The formation of other transglycosylated products was noted at pH 3.5 and pH 5.0. At pH 6.5 and in the presence of 80 % (v\v) propan-2-ol, the enzyme was inactive. Thus conditions of pH 3.5 and 80 % (v\v) propan-2-ol were found to be ideal for the synthesis of relatively high levels of propyl β--glucoside from cellobiose.

Stereochemical course of cellobiose hydrolysis 1

H and 13C NMR analysis

The time course of the hydrolysis of cellobiose by β-glucosidase in the presence of methanol over 36 h was analysed by "H NMR ; the data are presented in Figure 6. Assignment of the proton signals to specific hydroxy groups and the glucosidic bond was performed according to the standard analysis, with cross-referencing to published Aldrich standards. At 0 h, the proton signals corresponding to α and β β conformations at # # "

Figure 8

N-terminal sequencing The N-terminal sequence of 20 amino acid residues of T. aurantiacus β-glucosidase showed a high degree of identity with the N-terminal sequences of five other β-glucosidases belonging to family 3 of glycoside hydrolases (Figure 8).

DISCUSSION An extracellular β-glucosidase purified from T. aurantiacus was a homotrimer, with a monomeric molecular mass of 120 kDa. The high molecular mass of this β-glucosidase is in agreement with those of many extracellular β-glucosidases characterized from other fungal sources [8,20–22]. Under non-reduced conditions, the enzyme retained the high-molecular-mass conformation even after boiling, but the protein was no longer active. Only the native protein was fully active in its non-reduced form. This suggested that the enzyme was structurally conferred to some degree by cysteine residues, and thus exhibited a low molecular mass under reduced conditions. Nevertheless, under such conditions and without boiling, the enzyme was present as a high-molecular-mass protein, but showed low activity. Hence both boiling and the presence of a reducing agent were essential to denature and separate the monomers of β-glucosidase, which implied the presence of heat-labile cysteines. The temperature optimum (80 mC) of this β-glucosidase was higher than that reported for low-molecular-mass β-glucosidases

N-terminal sequence alignment of T. aurantiacus β-glucosidase with β-glucosidases of glycoside hydrolase family 3

Amino acid residues common to all β-glucosidases are highlighted in bold. Database numbers and microbial sources of β-glucosidases are given on the right and left sides respectively. # 2001 Biochemical Society

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from T. aurantiacus [23,24], as well as for other fungal cellulases [25,26]. Also, this β-glucosidase showed greater thermostability than other β-glucosidases from mesophilic and thermophilic fungi that have been studied [8,11,27,28]. Based on substrate specificity, β-glucosidases can be classified as aryl β--glucosidases (which hydrolyse exclusively aryl β--glucosides), cellobiases (which hydrolyse only cellooligosaccharides) or broad-specificity β-glucosidases (which hydrolyse both aryl β--glucosides and cello-oligosaccharides). The β-glucosidase from T. aurantiacus was active on aryl β--glucosides as well as on cello-oligosaccharides, and was found to have a broad substrate specificity. Nevertheless, this enzyme exhibited highest activity towards pNPGlc. Plant et al. [29] suggested that the preference of β-glucosidases for aryl glycosides is due to the high electrophilicity of the aglycone moiety, which enhances the stability of the ortho or para nitrophenoxide anion generated during the first step of catalysis. A low activity obtained with the nitro group in the ortho position suggests steric hindrance with a hydroxy group at C-6. The importance of this hydroxy group was demonstrated by the very low activity of β-glucosidase towards pNPXyl. Furthermore, the enzyme was more active on oNPXyl than on pNPXyl, indicating that the nitro group of oNPXyl was interacting favourably at the active site, since the steric hindrance with a hydroxy group at C6 was then no longer a problem. The importance for catalysis of hydroxy groups at the C-2 and C-4 positions of glucose, as well as the specificity of this β-glucosidase for a β1-4-glucosidic bond, was evident from its negligible activity on pNP β--galactoside, pNPmannose, pNPgalactosamine, pNPgentiobiose, pNP β--lactoside, pNP 1-thio-β--glucoside and pNP 1-thio-β-–galactoside. The specificity of T. aurantiacus β-glucosidase towards βlinked diglycosides was, however, broad. The enzyme hydrolysed disaccharides with β1-1, β1-6 and β1-2 glucosidic bonds. The glucose derivatives arbutin, esculin and salicin were hydrolysed to a limited extent, indicating the occurrence of steric hindrance with these compounds, as observed by Fadda et al. [30]. A high activity on n-octyl β--glucoside is probably due to favourable interactions between the alkyl group and a hydrophobic residue in the active site of the enzyme. The kinetics of the hydrolysis of preferred substrates showed that the enzyme was more specific for pNP glycosides than for normal disaccharides such as cellobiose and gentiobiose. Although the studies on mode of action suggested that this β-glucosidase attacks the terminal glucosidic bond of cellooligosaccharides, it was not possible to demonstrate the precise mode of action using these substrates. However, using pNP cellooligosaccharides, it was demonstrated that this enzyme cleaved one glucose unit at a time from the non-reducing end. In contrast, the β-glucosidase from Fusarium oxysporum hydrolysed MeUmb cello-oligosaccharides from both ends [28]. The rate of hydrolysis of cello-oligosaccharide by β-glucosidase from T. aurantiacus decreased with increasing chain length, indicating its β-glucosidase character. Also, a large decrease in hydrolysis rates from pNPGlc and pNPGlc to pNPGlc and pNPGlc demon# $ % strates the specificity of this enzyme for short-chain cellooligosaccharides. Determination of the transglycosylation activity of βglucosidase in the presence of alcohols and with cellobiose as a donor provided some interesting results. With 25 % (v\v) methanol, the yields of glucose and methyl β--glucoside were equimolar, indicating simultaneous hydrolysis of the glucosidic bond and transfer of glucose to the alkyl group. In the presence of 50 % (v\v) methanol, the partitioning of products occurred, with more methyl β--glycoside formed than glucose. Methanol, # 2001 Biochemical Society

ethanol and propan-2-ol, at different concentrations, activated and inhibited the hydrolytic activity of β-glucosidase. Interestingly, at or below 40 % (v\v) alcohol, the production of alkyl glycoside was greatest with methanol, and decreased as the alcohol chain length increased. Above 40 % (v\v), the production of alkyl glycosides was promoted with increased alcohol chain length, as has been demonstrated with other β-glucosidases [7–9,28]. Although transglycosylation activity has been reported with other glycosidases [31,32], the importance of pH for transglycosylation activity has not been demonstrated previously. In the present study, varying the pH above and below the pI (4.1–4.5) of T. aurantiacus β-glucosidase was used to demonstrate the pH effects. The most interesting phenomenon was the effect of pH 3.5 and pH 5.0 on the formation of transglycosylated products other than methyl, ethyl or propyl β-glucosides in the presence of the respective alcohols. Synthesis of a series of nitrophenyl oligosaccharides by β-xylosidase from Thermoanaerobacterium saccharolyticum strain B6A-RI has been reported [33]. Interestingly, at pH 6.5, T. aurantiacus β-glucosidase catalysed the synthesis of one of each transglycosylated product in the presence of the different alcohols tested. This will be of industrial significance, owing to the fact that, by selecting the right pH, high concentrations of desired transglycosylated products can be produced. Enzymic hydrolysis of the glycosidic bond takes place by general acid catalysis involving a proton donor and a nucleophile, with overall retention or inversion of the anomeric configuration [34–36]. The stereochemical course of cellobiose hydrolysis in the presence of methanol by β-glucosidase from T. aurantiacus showed the formation of methyl β--glucoside, with retention of the anomeric configuration. The enzyme is presumed to follow a double-displacement mechanism, as proposed in case of retaining glycosidases, involving initial binding of the enzyme to cellobiose, followed by a general acid catalysis of an enzyme nucleophile at the anomeric centre, forming a glycosyl-enzyme intermediate [34–36]. This intermediate is then hydrolysed by methanol, assisted by the conjugate base of the general acid at the anomeric centre, forming methyl β--glucoside, and thereby returning the enzyme to its original protonation state. Thus the β-glucosidase from T. aurantiacus was found to be a retaining glycosidase. Based on amino acid sequence similarities, glycosidases have been classified into several families, with most β-glucosidases belonging to either family 1 or family 3 [37–39]. A high degree of N-terminal amino acid sequence identity between T. aurantiacus β-glucosidase and other β-glucosidases of family 3 suggested that this enzyme could be a member of glycoside hydrolase family 3. So far, only one enzyme (barley β--glucan exohydrolase) belonging to this family has been crystallized and its threedimensional structure determined [40]. It is a two-domain, globular protein comprising 605 amino acid residues. The first domain has an (α\β) TIM (triose phosphate isomerase) barrel ) structure, while the second domain is arranged in a six-stranded β-sandwich, which contains a β-sheet of five parallel β-strands and one anti-parallel β-strand. Also, the active site of this enzyme contains several conserved residues, including a nucleophile (Asp-285) and an acid\base catalyst (Glu-495). Sequence alignment of the region containing the proposed catalytic nucleophile of over 15 β-glucosidases of family 3 shows the presence of Asp as a fully conserved catalytic nucleophile [41]. Such absolute conservation is consistent with the key role played by the catalytic nucleophile in (a) forming the covalent glycosyl-enzyme intermediate, (b) stabilizing the oxocarbenium ion-like transition states, (c) modulating the ionization state of the acid\base catalytic residue, and (d) forming a strong hydrogen bond to the sugar 2-hydroxy group at the transition state [41]. The importance

β-Glucosidase from Thermoascus aurantiacus of this catalytic nucleophile has also been demonstrated by different substitutions at the catalytic centre [42]. The data presented here show that the β-glucosidase from T. aurantiacus follows a retention mechanism, and most probably belongs to family 3 of the glycoside hydrolase, as well as possessing an Asp or a Glu residue as a catalytic nucleophile. The authors gratefully acknowledge financial support from the CEC under the AIR II contract CT93-1272, FWO-Flanders through contract G.0068.96, and the BBSRC. We also acknowledge the help of Dr S. Andrews (BBSRC IT Services) in sequence alignment, and Dr J. Khan (IFR, Norwich) in NMR data analysis.

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Received 2 June 2000/28 September 2000 ; accepted 23 October 2000

# 2001 Biochemical Society