Characterization of an extracellular α-xylosidase involved in xyloglucan degradation in Aspergillus oryzae.

α-Xylosidases release the α-D-xylopyranosyl side chain from di- and oligosaccharides derived from xyloglucans and are involved in xyloglucan degradation. In this study, an extracellular α-xylosidase, named AxyB, is identified and characterized in Aspergillus oryzae. AxyB belongs to the glycoside hydrolase family 31 and releases D-xylose from isoprimeverose (α-D-xylopyranosyl-(1 → 6)-D-glucopyranose) and xyloglucan oligosaccharides. In the hydrolysis of xyloglucan oligosaccharides (XLLG, Glc4Xyl3Gal2 nonasaccharide; XLXG/XXLG, Glc4Xyl3Gal1 octasaccharide; and XXXG, Glc4Xyl3 heptasaccharide), AxyB releases one molecule of the xylopyranosyl side chain attached to the non-reducing end of the β-1,4-glucan main chain of these xyloglucan oligosaccharides to yield GLLG (Glc4Xyl2Gal2), GLXG/GXLG (Glc4Xyl2Gal1), and GXXG (Glc4Xyl2). A. oryzae has both extracellular and intracellular α-xylosidase, suggesting that xyloglucan oligosaccharides are degraded by a combination of isoprimeverose-producing oligoxyloglucan hydrolase and intracellular α-xylosidase and a combination of extracellular α-xylosidase and β-glucosidase(s) in A. oryzae. KEY POINTS: • An extracellular α-xylosidase, AxyB, is identified in Aspergillus oryzae. • AxyB releases the xylopyranosyl side chain from xyloglucan oligosaccharides. • Different sets of glycosidases degrade xyloglucan oligosaccharides in A. oryzae.