Purification and biochemical properties of a thermostable xylose-tolerant β-D-xylosidase from Scytalidium thermophilum

Abstract

The thermophilic fungus Scytalidium thermophilum produced large amounts of periplasmic β-D-xylosidase activity when grown on xylan as carbon source. The presence of glucose in the fresh culture medium drastically reduced the level of β-D-xylosidase activity, while cycloheximide prevented induction of the enzyme by xylan. The mycelial β-xylosidase induced by xylan was purified using a procedure that included heating at 50°C, ammonium sulfate fractioning (30–75%), and chromatography on Sephadex G-100 and DEAE-Sephadex A-50. The purified β-D-xylosidase is a monomer with an estimated molecular mass of 45 kDa (SDS-PAGE) or 38 kDa (gel filtration). The enzyme is a neutral protein (pI 7.1), with a carbohydrate content of 12% and optima of temperature and pH of 60°C and 5.0, respectively. β-D-Xylosidase activity is strongly stimulated and protected against heat inactivation by calcium ions. In the absence of substrate, the enzyme is stable for 1 h at 60°C and has half-lives of 11 and 30 min at 65°C in the absence or presence of calcium, respectively. The purified β-D-xylosidase hydrolyzed p-nitrophenol-β-D-xylopyranoside and p-nitrophenol-β-D-glucopyranoside, exhibiting apparent K _m and V _max values of 1.3 mM, 88 μmol min⁻¹ protein⁻¹ and 0.5 mM, 20 μmol min⁻¹ protein⁻¹, respectively. The purified enzyme hydrolyzed xylobiose, xylotriose, and xylotetraose, and is therefore a true β-D-xylosidase. Enzyme activity was completely insensitive to xylose, which inhibits most β-xylosidases, at concentrations up to 200 mM. Its thermal stability and high xylose tolerance qualify this enzyme for industrial applications. The high tolerance of S. thermophilum β-xylosidase to xylose inhibition is a positive characteristic that distinguishes this enzyme from all others described in the literature.