The two Rasamsonia emersonii α-glucuronidases, ReGH67 and ReGH115, show a different mode-of-action towards glucuronoxylan and glucuronoxylo-oligosaccharides

Background: The production of biofuels and biochemicals from grass-type plant biomass requires a complete utilisation of the plant cellulose and hemicellulosic xylan via enzymatic degradation to their constituent monosaccharides. Generally, physical and/or thermochemical pretreatments are performed to enable access for the subsequent added carbohydrate-degrading enzymes. Nevertheless, partly substituted xylan structures remain after pretreatment, in particular the ones substituted with (4-O-methyl-)glucuronic acids (UA_me). Hence, α-glucuronidases play an important role in the degradation of UA_mexylan structures facilitating the complete utilisation of plant biomass. The characterisation of α-glucuronidases is a necessity to find the right enzymes to improve degradation of recalcitrant UA_mexylan structures. Results: The mode-of-action of two α-glucuronidases was demonstrated, both obtained from the fungus Rasamsonia emersonii; one belonging to the glycoside hydrolase (GH) family 67 (ReGH67) and the other to GH115 (ReGH115). Both enzymes functioned optimal at around pH 4 and 70 °C. ReGH67 was able to release UA_me from UA_me-substituted xylo-oligosaccharides (UA_meXOS), but only the UA_me linked to the non-reducing end xylosyl residue was cleaved. In particular, in a mixture of oligosaccharides, UA_meXOS having a degree of polymerisation (DP) of two were hydrolysed to a further extent than longer UA_meXOS (DP 3-4). On the contrary, ReGH115 was able to release UA_me from both polymeric UA_mexylan and UA_meXOS. ReGH115 cleaved UA_me from both internal and non-reducing end xylosyl residues, with the exception of UA_me attached to the non-reducing end of a xylotriose oligosaccharide. Conclusion: In this research, and for the first time, we define the mode-of-action of two α-glucuronidases from two different GH families both from the ascomycete R. emersonii. To date, only four α-glucuronidases classified in GH115 are characterised. ReGH67 showed limited substrate specificity towards only UA_meXOS, cleaving UA_me only when attached to the non-reducing end xylosyl residue. ReGH115 was much less substrate specific compared to ReGH67, because UA_me was released from both polymeric UA_mexylan and UA_meXOS, from both internal and non-reducing end xylosyl residues. The characterisation of the mode-of-action of these two α-glucuronidases helps understand how R. emersonii attacks UA_mexylan in plant biomass and the knowledge presented is valuable to improve enzyme cocktails for biorefinery applications.