The beta-D-xylosidase of Trichoderma reesei is a multifunctional beta-D-xylan xylohydrolase

An extracellular multifunctional beta-D-xylan xylohydrolase, previously described as beta-xylosidase, was purified from Trichoderma reesei RUT C-30 to physical homogeneity. The active enzyme was a 100 (+/-5) kDa glycosylated monomer that exhibited a pI of 4.7. Its activity was optimal at pH 4 and it was stable between pH 3 and 6. Its temperature-stability was moderate (70% of activity remaining after 60 min at 50 degrees C) and optimal activity was observed at 60 degrees C. It is capable of hydrolysing beta-1,4-xylooligosaccharides [degree of polymerization (DP) 2-7], the apparent V-max increasing with increasing chain length. The enzyme also attacked debranched beech-wood (Lenzing) xylan and 4-O-methylglucuronoxylan, forming xylose as the only end product. The K-m for xylan was 0.7 g/l. For this reason we consider the enzyme to be a beta-D-xylan xylohydrolase. The enzyme also exhibits alpha-L-arabinofuranosidase activity on 4-nitrophenyl alpha-L-arabinofuranoside, and evidence is presented that this is not caused by an impurity in the enzyme preparation. The beta-D-xylan xylohydrolase exhibits glycosyltransferase activity with xylooligosaccharides and at high concentrations of 4-nitrophenyl beta-D-xylopyranoside (4-Nph-beta-Xyl). The enzyme hydrolyses beta-1, 4-linkages preferentially to beta-1,3-linkages, and beta-1,2-linked xylo-oligosaccharides are not hydrolysed at all. The enzyme liberates terminal beta-1,4-xylopyranose residues linked to a 2-O-substituted xylopyranose residue, but not that linked to a 3-O-substituted xylopyranose residue. The enzyme does not attack methyl, methyl 1-thio-, benzyl or butyl 1-thio-beta-D-xylopyranosides and 4-naphthyl, 2-naphthyl and phenyl beta-D-xylopyranosides.