Functions of family-22 carbohydrate-binding modules in Clostridium josui Xyn10A.

Clostridium josui xylanase Xyn10A is a modular enzyme comprising two family-22 carbohydrate-binding modules (CBMs), a family-10 catalytic module (CM), a family-9 CBM, and two S-layer homologous modules, consecutively from the N-terminus. To study the functions of the family-22 CBMs, truncated derivatives of Xyn10A were constructed: a recombinant CM polypeptide (rCM), a family-22 CBM polypeptide (rCBM), and a polypeptide composed of the family-22 CBMs and CM (rCBM-CM). Recombinant proteins were characterized by enzyme and binding assays. rCBM-CM showed the highest activity toward xylan and weak activity toward some polysaccharides such as barley beta-glucan and carboxymethyl-cellulose. Although rCBM showed an affinity for insoluble and soluble xylan as well as barley beta-glucan and Avicel in qualitative binding assays, removal of the CBMs negligibly affected the catalytic activity and thermostability of the CM.