Comparison of a GH74 xyloglucanase and its CBM-deleted variant from Thielavia terrestris and their roles in xyloglucan-rich biomass degradation

Abstract

Background The hydrolysis of lignocellulose was greatly hindered by the associated hemicellulose. Xyloglucan (XG) is a major complex hemicellulose in the primary plant cell wall, however, the blocking effect of xyloglucan on lignocellulose biodegradation was still less understood compared with xylan and mannan. Glycoside hydrolase family 74 (GH74) enzymes are specific xyloglucanases widely distributed in bacteria and fungi. Fungal GH74 xyloglucanases generally contains family 1 carbohydrate binding module (CBM1), but its effect on the enzymatic properties of GH74 xyloglucanases and its roles in xyloglucan-rich biomass degradation has not been investigated in depth. Results TtGH74 and CBM-deleted variant (TtGH74ΔCBM) from Thielavia terrestris had the same optimum temperature and pH, but TtGH74ΔCBM had higher thermostability. TtGH74 displayed high binding affinity on xyloglucan and cellulose, while TtGH74ΔCBM almost completely lost the adsorption capacity on cellulose. Their hydrolysis action alone or combined with other glycoside hydrolases on the free XG, xyloglucan-coated phosphoric acid swollen cellulose or pretreated corn bran and apple pomace was compared in this study. CBM1 might not be essential for hydrolysis of free XG, but effective for the associated XG to some extent. TtGH74 was more effective in hydrolysis of xyloglucan in corn bran, while TtGH74ΔCBM showed relatively higher catalytic activity on apple pomace. Similar phenomenon was also observed when TtGH74 or TtGH74ΔCBM synergistically acting with CBH1/EG1 mixture. The addition of TtGH74 or TtGH74ΔCBM in CBH1/EG1 mixture significantly facilitated the overall hydrolysis of pretreated corn bran and apple pomace by 1.22–2.02 folds in terms of the degree of synergy. The extra addition of GH10 xylanase in TtGH74 or TtGH74ΔCBM /CBH1/EG1 mixture further improved the overall hydrolysis efficiency, and the degree of synergy was up to 1.50–2.16 because of co-existence of xyloglucan and xylan in pretreated corn bran and apple pomace. Conclusions TtGH74 can remove the bound xyloglucan on cellulose, therefore efficiently boost the hydrolysis of the pretreated lignocellulosic biomass by synergistic action with cellulase and xylanase. The presence of CBM1 in TtGH74 is conducive to enzymatic hydrolysis, but its role and significance is substrate-specific due to the differences of xyloglucan contents and structure in various biomasses.