Essential role of the family-22 carbohydrate-binding modules for β-1,3-1,4-glucanase activity of Clostridium stercorarium Xyn10B

Clostridium stercorarium Xyn10B is a modular enzyme comprising two family-22 carbohydrate-binding modules (CBMs), a family-10 catalytic module of glycoside hydrolases, a family-9 CBM, and two S-layer homologous modules consecutively from the N-terminus. To investigate the role of the family-22 CBMs, truncated proteins were constructed: a recombinant catalytic module polypeptide (rCD), a CBM polypeptide composed of two family-22 CBMs (rCBM) and a polypeptide composed of the family-22 CBMs and the catalytic module (rCBM-CD). We found that rCBM-CD was highly active toward β-1,3-1,4-glucan; however, rCD was negligibly active toward the same substrate. The V_max/K_m value of rCBM-CD for β-1,3-1,4-glucan was 7.8 times larger than that for oat-spelt xylan, indicating that rCBM-CD should be specified as a β-1,3-1,4-glucanase rather than a xylanase despite the fact that family-10 catalytic modules are well-known xylanase modules. These results indicate that the family-22 CBMs in rCBM-CD are essential for hydrolysis of β-1,3-1,4-glucan. [Copyright &y& Elsevier]