Biochemical characterization and mechanism of action of a thermostable β-glucosidase purified from Thermoascus aurantiacus

Abstract

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 120kDa; only the trimer was optimally active at 80°C and at pH 4.5. At 90°C, the enzyme showed 70% of its optimal activity. It was stable at pH 5.2 and at temperatures up to 70°C for 48h, but stability decreased above 70°C and at pH values above and below 5.0. The enzyme hydrolysed aryl and alkyl β-d-glucosides and cello-oligosaccharides, and was specific for substrates with a β-glycosidic 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-d-glucoside (0.1137mM) and the highest kcat towards cellobiose and β,β-trehalose (17052min-1). 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 d-δ-gluconolactone inhibited the β-glucosidase competitively, with Ki values of 0.29mM and 8.3nM respectively, while methanol, ethanol and propan-2-ol activated the enzyme. The enzyme catalysed the synthesis of methyl, ethyl and propyl β-d-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 β-glucosidase 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.