Exploring the synergy between fungal CE15 glucuronoyl esterases and xylanases for lignocellulose saccharification

Lignin-carbohydrate complexes in lignocellulosic biomass act as a barrier to its biodegradation and biotechnological exploitation. Enzymatic dissociation between lignin and hemicellulose is a key process that allows the efficient bioconversion of both polymers. Glucuronoyl esterases of the Carbohydrate Esterase 15 family target the ester linkages between the glucuronic acid of xylan and lignin moieties, assisting enzymatic biodegradation of lignocellulose.

In this study, two CE15 glucuronoyl esterases from the white-rot fungi Artolenzites elegans and Trametes ljubarskyi were heterologously expressed in Pichia pastoris and biochemically characterized on the model substrate D-glucuronic acid ester with cinnamyl alcohol and a variety of pretreated lignocellulosic biomasses. The pretreatment method was shown to be a determining factor in revealing both the activity of the esterases on lignocellulose and their synergistic relationships with other hemicellulases. AeGE15 and TlGE15 demonstrated activity on pretreated biomass with high hemicellulose and lignin content, increasing saccharification by 57 ± 1 μM and 61 ± 3 μM of xylose equivalents, respectively. Furthermore, the synergy between these CE15 esterases and three xylanases from distinct glycoside hydrolase families (GH10, GH11 and GH30) was investigated on pretreated lignocellulosic samples, highlighting beneficial enzymatic interplays. Pretreated birchwood degradation by AnXyn11 was increased from 6% to approximately 10% by the esterases, based on xylose equivalents of unsubstituted xylooligomers. The GEs also promoted the glucuronoxylanase specificity of TtXyn30A, leading up to three-times higher release in aldouronic acids. Finally, a synergistic effect between AeGE15 and TmXyn10 was observed on pretreated corn bran, increasing xylose and xylotriose release by 27 ± 8% and 55 ± 15%, respectively.

Both CE15 esterases promoted biomass saccharification by the xylanases, while there was a prominent effect on the GH30 glucuronoxylanase regarding the release of aldouronic acids. Overall, this study shed some light on the role of CE15 glucuronoyl esterases in the enzymatic biodegradation of plant biomass, particularly its (arabino)glucuronoxylan component, during cooperative activity with xylanases.