Biochemical and catalytic properties of endo-1,4-β-xylanases from Thermomyces lanuginosus (wild and mutant strains)

Abstract

Endoxylanases from the thermophilic fungus, Thermomyces lanuginosus ATCC 44008 (cellulase free wild and mutant strains), were purified to homogeneity by anion-exchange and molecular-sieve chromatographic methods. The purified enzymes were monomers with molecular masses of 22 kDa (wild type) and 24 kDa (mutant), estimated by SDS-PAGE and gel filtration. As glycoproteins, the purified enzymes had 0.74% (wild type) and 11.8% (mutant) carbohydrate contents, and pI values of 5.8 and 6, respectively. The optimal pH and temperature values of wild type xylanase were determined to be pH 7 and 60 °C, whereas pH 6.7 and 70 °C, were optimal for the purified mutant enzyme (Km and Vmax values of 3.7 mg ml−1 and 670 μmol min−1 xylose compared to the kinetic values of the purified wild type xylanase −5.1 mg ml−1 and 385 μmol min−1 xylose). Inhibition studies suggested the possible involvement of histidine, tryptophan residues and carboxylic groups in the binding or catalysis.