A novel GH12 xyloglucanase from the white rot fungus Abortiporus biennis, synergistically enhances lignocellulose saccharification by commercial cellulases

Xyloglucan is a complex, highly substituted plant biomass polysaccharide, which is largely overlooked in the design of enzyme cocktails for lignocellulose saccharification, due to its presence in specific plant tissues only, and its low content. Thus, the microbial mechanisms for its degradation have not been thoroughly studied. However, in the frame of the biorefinery concept, xyloglucan monomers also have to be utilized for the design of efficient bioprocesses. Moreover, in plant tissues, xyloglucan often covers cellulose fibrils, impeding the access of cellulases. In order to shed light on the enzymatic degradation of xyloglucan, a novel GH12 family xyloglucanase was studied, from the basidiomycete Abortiporus biennis. The enzyme was heterologously produced in Pichia pastoris, purified and characterized. AbiXeg12a is a 28 kDa glycoprotein, with relatively strict substrate specificity, since it is only active in xyloglucan and β-glucan. The main hydrolysis products are the oligomers XXXG, XLXG/XXLG, XLLG and the optimum activity conditions are pH 4.5 and 55 °C. The enzyme contributes to the saccharification of corn bran and apple pulp by a commercial cellulase preparation, increasing the release of reducing sugars by up to 39 % and 18 %, respectively, while the addition of AbiXeg12a can minimize the enzyme load of the reaction, at least for apple pulp, without loss in reducing sugar yield. Overall, the importance of xyloglucanases on the saccharification of xyloglucan-containing substrates was demonstrated in this study. The results could contribute to the design of more efficient, tailor-made enzyme cocktails for the saccharification and subsequent valorization of lignocellulose.