Development of a chimeric hemicellulase to enhance the xylose production and thermotolerance

Xylan is an abundant plant cell wall polysaccharide and its reduction to xylose units for subsequent biotechnological applications requires a combination of distinct hemicellulases and auxiliary enzymes, mainly endo-xylanases and ß-xylosidases. In the present work, a bifunctional enzyme consisting of a GH11 endo-1,4-β-xylanase fused to a GH43 β-xylosidase, both from Bacillus subtilis, was designed taking into account the quaternary arrangement and accessibility to the substrate. The parental enzymes and the resulting chimera were successfully expressed in Escherichia coli, purified and characterized. Interestingly, the substrate cleavage rate was altered by the molecular fusion improving at least 3-fold the xylose production using specific substrates as beechwood xylan and hemicelluloses from pretreated biomass. Moreover, the chimeric enzyme showed higher thermotolerance with a positive shift of the optimum temperature from 35 to 50 °C for xylosidase activity. This improvement in the thermal stability was also observed by circular dichroism unfolding studies, which seems to be related to a gain of stability of the β-xylosidase domain. These results demonstrate the superior functional and stability properties of the chimeric enzyme in comparison to individual parental domains, suggesting the molecular fusion as a promising strategy for enhancing enzyme cocktails aiming at lignocellulose hydrolysis.