Non-Catalytic Domains of Glycoside Hydrolase Family 5 from Paenibacillus curdlanolyticus are Important for Promoting Multifunctional Enzyme Activities and Degradation of Agricultural Residues

Abstract

PcGH5 from Paenibacillus curdlanolyticus strain B-6 is a modular protein consisting of a catalytic domain of glycoside hydrolase family 5 (GH5), and three non-catalytic domains (a family 11 carbohydrate-binding module (CBM11), a fibronectin type 3 (Fn3), and a family 3 carbohydratebinding module (CBM3). In this study, the recombinants full-length PcGH5 and the catalytic domain (PcGH5_CD) were expressed in Escherichia coli and purified. Most GH5 members exhibit endocellulase activity. However, the catalytic domain enzyme of strain B-6 exhibited unique properties, showing multifunctional enzyme activities of endo-cellulase, endo-xylanase, endo-mannanase, and endo-1,3-1,4-β-glucanase. The sequence alignment of PcGH5_CD compared to other characterized GH5 enzymes suggests that the two catalytic residues and the six substrate-binding subsites of endo-cellulases were conserved with other different GH5 enzyme properties. Whereas a few conserved amino acid residues and/or short peptides located outside the active site of the GH5 endo-cellulases may be involved in broad substrate specificity of PcGH5_CD enzyme on xylan, mannan and 1,3-1,4-β-glucan. Moreover, the non-catalytic domains (CBM11-Fn3-CBM3) linked to the GH5 catalytic domain are important for promoting the multifunctional enzyme activities of PcGH5 on the β-1,4 glycosidic linkages of crystalline cellulose, highly branched polysaccharides, and β-1,4-1,6 and β-1,3-1,4 glycosidic linkages of polysaccharides, especially for the polysaccharides complex contained in agricultural residues. The full-length PcGH5 is effective in producing oligosaccharides from agricultural residues without pretreatment. Therefore, it is interesting to use it as a source of prebiotics producer for use in various food products.