The Bacillus subtilis expansin BsEXLX1 enhances cellobiohydrolase I activity on sugar beet pulp as shown by QCM-D

Microbial expansins are proteins that are thought to mediate loosening of plant cell wall networks by non-catalytic disruption of non-covalent bonds between polysaccharides, particularly between cellulose chains. As a result, the expansins may facilitate the enzymatic hydrolysis of these polysaccharides. In this work, we used quartz-crystal microbalance with dissipation monitoring (QCM-D) to follow the influence of the Bacillus subtilis expansin BsEXLX1 and an inactive BsEXLX1 mutant (BsEXLX1_D81N) on the enzymatic degradation of never dried sugar beet pulp by a fungal cellobiohydrolase I (Cel7A) EC 3.2.1.176. The presence of the wild type BsEXLX1 enhanced the enzymatic degradation rate on the substrate in a dose dependent manner achieving a doubling of the rate, while the inactive BsEXLX1_D81N mutant exhibited only very limited increase in the degradation rate. This result suggests that the ability of BsEXLX1 to boost the action of Cel7A is linked to the ability of BsEXLX1 to bind to the glucan chains that in turn may increase the distance between two glucan chains enabling increased enzymatic attack.