A new synergistic relationship between xylan-active LPMO and xylobiohydrolase to tackle recalcitrant xylan

Background Hemicellulose accounts for a significant part of plant biomass, and still poses a barrier to the efficient saccharification of lignocellulose. The recalcitrant part of hemicellulose is a serious impediment to the action of cellulases, despite the use of xylanases in the cellulolytic cocktail mixtures. However, the complexity and variety of hemicelluloses in different plant materials require the use of highly specific enzymes for a complete breakdown. Over the last few years, new fungal enzymes with novel activities on hemicelluloses have emerged. In the present study, we explored the synergistic relationships of the xylan-active AA14 lytic polysaccharide monooxygenase (LPMO), Pc AA14B, with the recently discovered glucuronoxylan-specific xylanase Tt Xyn30A, of the (sub)family GH30_7, displaying xylobiohydrolase activity, and with commercial cellobiohydrolases, on pretreated natural lignocellulosic substrates. Results Pc AA14B and Tt Xyn30A showed a strong synergistic interaction on the degradation of the recalcitrant part of xylan. Pc AA14B was able to increase the release of xylobiose from Tt Xyn30A, showing a degree of synergism (DS) of 3.8 on birchwood cellulosic fibers, and up to 5.7 on pretreated beechwood substrates. The increase in activity was dose- and time- dependent. A screening study on beechwood materials pretreated with different methods showed that the effect of the Pc AA14B– Tt Xyn30A synergism was more prominent on substrates with low hemicellulose content, indicating that Pc AA14B is mainly active on the recalcitrant part of xylan, which is in close proximity to the underlying cellulose fibers. Simultaneous addition of both enzymes resulted in higher DS than sequential addition. Moreover, Pc AA14B was found to enhance cellobiose release from cellobiohydrolases during hydrolysis of pretreated lignocellulosic substrates, as well as microcrystalline cellulose. Conclusions The results of the present study revealed a new synergistic relationship not only among two recently discovered xylan-active enzymes, the LPMO Pc AA14B, and the GH30_7 glucuronoxylan-active xylobiohydrolase Tt Xyn30A, but also among Pc AA14B and cellobiohydrolases. We hypothesize that Pc AA14B creates free ends in the xylan polymer, which can be used as targets for the action of Tt Xyn30A. The results are of special importance for the design of next-generation enzymatic cocktails, able to efficiently remove hemicelluloses, allowing complete saccharification of cellulose in plant biomass.