Novel structures of N-linked high-mannose type oligosaccharides containing alpha-D-galactofuranosyl linkages in Aspergillus niger alpha-D-glucosidase.

Seven oligosaccharides were isolated from alpha-D-glucosidase (EC 3.2.1.20) from Aspergillus niger, and the structures of these oligosaccharides were studied by 1H NMR spectroscopy. After treatment of the alpha-D-glucosidase with N-glycosidase F, seven major oligosaccharide peaks were detected by Dionex anion-exchange HPLC. The structures corresponding to the three peaks OS-1, OS-2, and OS-4 were determined to be Man8GlcNAc2, Man9GlcNAc2, and GlcMan9GlcNAc2, respectively, from 1H NMR spectra of the isolated fractions. Each of the four oligosaccharides OS-5, OS-6, OS-7-1, and OS-7-2 contained an alpha-D-galactofuranosyl residue (Galf) linked to Man(A) via an alpha-(1-->2)-linkage. OS-7 was found to consist of two oligosaccharides. The structures of these four oligosaccharides were determined to be GalfMan5GlcNAc2, GalfMan6GlcNAc2, GalfMan7GlcNAc2, and GalfMan8GlcNAc2 by 1H NMR spectroscopy and compositional analysis. The Galf structure of GalfMan5GlcNAc2 was found to be identical to that of an oligosaccharide previously isolated from the alpha-D-galactosidase of the same strain. The structure of OS-3 remains undetermined.