The galactosylation of xyloglucan blocks many of the enzymatic processes targeting this oligosaccharide. We found that the expression of a gene encoding Aspergillus oryzae β-galactosidase (LacA) is induced in the presence of xyloglucan oligosaccharides. With detailed analyses of the substrate specificity of purified recombinant LacA, we show that LacA cleaves galactopyranosyl residues from xyloglucan oligosaccharides, but not from xyloglucan polysaccharide, and plays a vital role in xyloglucan degradation. LacA acts cooperatively with the isoprimeverose-producing oligoxyloglucan hydrolase IpeA to hydrolyze xyloglucan oligosaccharides. Galactosylation of the xylopyranosyl side chain at the nonreducing end of oligoxyloglucan saccharides completely abolishes IpeA activity while LacA efficiently removes the galactopyranosyl residue. Conversely, an isoprimeverose unit at the nonreducing end of the main chain of xyloglucan oligosaccharides blocks LacA activity, while IpeA can still remove the isoprimeverose moiety. This is the first study reporting the cooperative action of β-galactosidase and isoprimeverose-producing oligoxyloglucan hydrolase on xyloglucan oligosaccharide degradation. Our findings shed light on the true role of LacA and the enzymatic coordination between β-galactosidase and other hydrolases on xyloglucan degradation.
Keywords: Aspergillus oryzae; isoprimeverose-producing oligoxyloglucan hydrolase; oligosaccharide; xyloglucan; β-galactosidase.
© 2019 Federation of European Biochemical Societies.