Possible Identity of β-Xylosidase and β-Glucosidase ofChaetomium trilaterale

Abstract

The nature of the active site of Chaetomium trilaterale β-xylosidase catalyzing the hydrolysis of β-d-glucopyranoside and β-d-xylopyranoside was investigated by kinetic methods. On experiments with mixed substrates, such as phenyl β-d-xylopyranoside and phenyl β-d-glucopyranoside, the kinetic features agreed very closely with those features theoretically predicted for a single active site of the same enzyme catalyzing the hydrolysis of these two kinds of substrates. Both the β-glucosidase and β-xylosidase activities were strongly inhibited by glucono-1,5-lactone and nojirimycin (5-amino-5-deoxy-d-glucopyranose). β-Xylosidase activity was inhibited non-competitively by the two inhibitors, but β-glucosidase activity was competitive. Methyl β-d-xylopyranoside, methyl β-d-glucopyranoside, 1-thiophenyl β-d-xylopyranoside, and 1-thiophenyl β-d-glucopyranoside poorly inhibited both activities. Methyl β-d-xylopyranoside inhibited the β-xylosidase activity competitively but the β-glucosidase activity was non-competitive, whereas methyl β-d-glucopyranoside inhibited the β-xylosidase activity non-competitively but the β-glucosidase activity was competitive. 1-Thiophenyl β-d-xylopyranoside and 1-thiophenyl β-d-glucopyranoside behaved as competitive inhibitors. From these results, it was concluded that the β-xylosidase and β-glucosidase activities reside in one catalytic site, and this suggests that there might be two kinetically distinct binding sites in the active center of the same enzyme.