The β-d-xylosidase of Trichoderma reesei is a multifunctional β-d-xylan xylohydrolase

Abstract

An extracellular multifunctional α-d-xylan xylohydrolase, previously described as α-xylosidase, was purified fromTrichoderma reesei RUT C-30 to physical homogeneity. The active enzyme was a 100 (ŷ5) 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 ŶC) and optimal activity was observed at 60 ŶC. It is capable of hydrolysing α-1,4-xylo-oligosaccharides [degree of polymerization (DP) 2Ő7], the apparent Vmax 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 Km for xylan was 0.7 g/l. For this reason we consider the enzyme to be a α-d-xylan xylohydrolase. The enzyme also exhibits α-l-arabinofuranosidase activity on 4-nitrophenyl α-l-arabinofuranoside, and evidence is presented that this is not caused by an impurity in the enzyme preparation. The α-d-xylan xylohydrolase exhibits glycosyltransferase activity with xylo-oligosaccharides and at high concentrations of 4-nitrophenyl α-d-xylopyranoside (4-Nph-α-Xyl). The enzyme hydrolyses α-1, 4-linkages preferentially to α-1,3-linkages, and α-1,2-linked xylo-oligosaccharides are not hydrolysed at all. The enzyme liberates terminal α-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-α-d-xylopyranosides and 4-naphthyl, 2-naphthyl and phenyl α-d-xylopyranosides.