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.