A novel acid-tolerant β-xylanase from Scytalidium candidum 3C for the synthesis of o-nitrophenyl xylooligosaccharides

Endo-β-xylanases are hemicellulases involved in the conversion of xylans in plant biomass. Here, we report a novel acidophilic β-xylanase (ScXynA) with high transglycosylation abilities that was isolated from the filamentous fungus Scytalidium candidum 3C. ScXynA was identified as a glycoside hydrolase family 10 (GH10) dimeric protein, with a molecular weight of 38 ± 5 kDa per subunit. The enzyme catalyzed the hydrolysis of different xylans under acidic conditions and was stable in the pH range 2.6–4.5. The kinetic parameters of ScXynA were determined in hydrolysis reactions with p-nitrophenyl-β-d-cellobioside (pNP-β-Cel) and p-nitrophenyl-β-d-xylobioside (pNP-β-Xyl₂), and k_cat/K_m was found to be 0.43 ± 0.02 (s·mM)⁻¹ and 57 ± 3 (s·mM)⁻¹, respectively. In the catalysis of the transglycosylation o-nitrophenyl-β-d-xylobioside (oNP-β-Xyl₂) acted both as a donor and an acceptor, resulting in the efficient production of o-nitrophenyl xylooligosaccharides, with a degree of polymerization of 3–10 and o-nitrophenyl-β-d-xylotetraose (oNP-β-Xyl₄) as the major product (18.5% yield). The modeled ScXynA structure showed a favorable position for ligand entry and o-nitrophenyl group accommodation in the relatively open −3 subsite, while the cleavage site was covered with an extended loop. These structural features provide favorable conditions for transglycosylation with oNP-β-Xyl₂. The acidophilic properties and high transglycosylation activity make ScXynA a suitable choice for various biotechnological applications, including the synthesis of valuable xylooligosaccharides.