Role of carbohydrate-binding module of fungal GH10 xylanase in synergistic hydrolysis of lignocellulosic biomass

Talaromyces cellulolyticus is a promising fungus for cellulase production and produces a wide range of glycoside hydrolases (GH). GH family 10 thermostable end-β-1,4-xylanase (Xyl10A) has been identified as one of core enzymes to hydrolysis lignocellulosic biomass in the T. cellulolyticus cellulase system. Xyl10A consists of an N-terminal catalytic domain and a family 1 carbohydrate-binding module (CBM1) separated by a linker region.  In this study, we investigated the impact of Xyl10A CBM1 on the hydrolysis of lignocellulose. Two mutants lacking CBM1 (Xyl10AdC) and both CBM1 and linker (Xyl10AdLC) were constructed and overexpressed in T. cellulolyticus under the glaA promoter. The mutant enzymes had approximately 1.6-fold higher specific activities on soluble substrates than wild type (Xyl10Awt). The presence of CBM1 was increased in the xylan hydrolysis on wet-disc milling treated rice straw (WDM-RS) compared with Xyl10AdC and Xyl10AdLC by 1.5- to 1.2-fold, respectively. Only Xyl10Awt was found to adsorb on cellulose by quartz crystal microbalance with dissipation monitoring (QCM-D).  These results indicate that the binding of CBM1 to cellulose promotes the hydrolysis of WDM-RS xylan by GH10 catalytic domain.  Hydrolysis of WDM-RS glucan by mixtures of purified enzymes showed that the three Xyl10As each had synergistic effects with a mixture of cellobiohydrolase I and II. Furthermore, the addition of a small amount of Xyl10Awt to the cellulase mixture resulted in significant synergistic hydrolysis distinct from that of mutant enzymes. These results suggest that Xyl10A CBM1 plays important roles in the specific removal of trace xylan on the cellulose surface.