Contribution of a Xylan-Binding Module to the Degradation of a Complex Cellulosic Substrate by Designer Cellulosomes

Conversion of components of the Thermobifida fusca free-enzyme system to the cellulosomal mode using the designer cellulosome approach can be employed to discover the properties and inherent advantages of the cellulosome system. In this article, we describe the conversion of the T. fusca xylanases Xyn11A and Xyn10B and their synergistic interaction in the free state or within designer cellulosome complexes in order to enhance specific degradation of hatched wheat straw as a model for a complex cellulosic substrate. Endoglucanase Cel5A from the same bacterium and its recombinant dockerin-containing chimera were also studied for their combined effect, together with the xylanases, on straw degradation. Synergism was demonstrated when Xyn11A was combined with Xyn10B and/or Cel5A, and similar to 1.5-fold activity enhancements were achieved by the designer cellulosome complexes compared to the free wild-type enzymes. These improvements in activity were due to both substrate-targeting and proximity effects among the enzymes contained in the designer cellulosome complexes. The intrinsic cellulose/xylan-binding module (XBM) of Xyn11A appeared to be essential for efficient substrate degradation. Indeed, only designer cellulosomes in which the XBM was maintained as a component of Xyn11A achieved marked enhancement in activity compared to the combination of wild-type enzymes. Moreover, integration of the XBM in designer cellulosomes via a dockerin module (separate from the Xyn11A catalytic module) failed to enhance activity, suggesting a role in orienting the parent xylanase toward its preferred polysaccharide component of the complex wheat straw substrate. The results provide novel mechanistic insight into the synergistic activity of designer cellulosome components on natural plant cell wall substrates.