Characterization of CBM36-containing GH11 endoxylanase NtSymX11 from the hindgut metagenome of higher termite Nasutitermes takasagoensis displaying prominent catalytic activity

Abstract

Symbionts in the gut of termites are expected to be large sources of enzymes involved in lignocellulose degradation, but their biotechnological potential has not been fully explored. In this study, we expressed, purified, and biochemically characterized a glycoside hydrolase family 11 xylanase, NtSymX11, from a symbiotic bacterium of the higher termite, Nasutitermes takasagoensis. NtSymX11 is a multimodular enzyme consisting of a catalytic domain and two tandem carbohydrate-binding modules (CBM36). The pH and temperature optima of NtSymX11 were pH 6.0 and 40 °C, respectively. By comparing the properties of full-length and truncated variants of NtSymX11, it was shown that CBM36 decreases the enzyme stability at acidic pH and high temperature. The main products from xylohexaose and various xylan substrates were X1-X3 xylooligosaccharides. Analysis of kinetic parameters indicated that NtSymX11 displays an outstanding catalytic performance when compared to other reported xylanases, and CBM36 enhances the activity by increasing the affinity to the substrate. Addition of Ca2+ boosted the activity of full-length enzyme, but not the truncated variant lacking the CBM, against the insoluble substrate, suggesting that CBM36 plays a role in the Ca2+-dependent increase of catalytic efficiency.