Function of the Family-9 and Family-22 Carbohydrate-Binding Modules in a Modular β-1,3-1,4-Glucanase/Xylanase Derived from Clostridium stercorariumXyn10B

Clostridium stercorariumXyn10B having hydrolytic activities on xylan and β-1,3-1,4-gllucan is a modular enzyme composed of two family-22 carbohydrate-binding modules (CBMs), a family-10 catalytic module of the glycoside hydrolases, a family-9 CBM, and two S-layer homologous modules, consecutively from the N-terminus. We investigated the function of family-9 and family-22 CBMs in a modular enzyme by comparing the enzymatic properties of a truncated enzyme composed of two family-22 CBMs and the catalytic module (rCBM22-CM), an enzyme composed of the catalytic module and family-9 CBM (rCM-CBM9), an enzyme composed of two family-22 CBMs, the catalytic module, and family-9 CBM (rCBM22-CM-CBM9), and the catalytic module polypeptide (rCM). Although the addition of family-9 CBM to rCM and rCBM22-CM did not significantly change catalytic activity toward xylan and β-1,3-1,4-glucan, the addition of family-22 CBM to rCM and rCM-CBM9 drastically enhanced catalytic activity toward xylan and especially β-1,3-1,4-glucan. Furthermore, the addition of family-22 CBM to rCM and rCM-CBM9 shifted the optimum temperature from 65 °C to 75 °C, but that of family-9 CBM to rCM and rCBM22-CM did not affect the optimum temperature. These facts suggest that the enzyme properties of Xyn10B were mainly dependent on the presence of the family-22 CBMs but not family-9 CBM.