Purification and properties of a thermostable extracellular ?- D -xylosidase produced by a thermotolerant Aspergillus phoenicis

A beta-D-xylosidase was purified from cultures of a thermotolerant strain of Aspergillus phoenicis grown on xylan at 45 degrees C. The enzyme was purified to homogeneity by chromatography on DEAE-cellulose and Sephadex G-100. The purified enzyme was a monomer of molecular mass 132 kDa by gel filtration and SDS-PAGE. Treatment with endoglycosidase H resulted in a protein with a molecular mass of 104 kDa. The enzyme was a glycoprotein with 43.5% carbohydrate content and exhibited a pl of 3.7. Optima of temperature and pH were 75 degrees C and 4.0-4.5, respectively. The activity was stable at 60 degrees C and had a Km of 2.36 mM for p-nitrophenyl-beta-D-xylopiranoside. The enzyme did not exhibit xylanase, cellulase, galactosidase or arabinosidase activities. The purified enzyme was active against natural substrates, such as xylobiose and xylotriose.