Novel structures of N-linked high-mannose type oligosaccharides containing α- d-galactofuranosyl linkages in Aspergillus niger α- d-glucosidase

Abstract

Seven oligosaccharides were isolated from α- 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 α- 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 Man 8GlcNAc 2, and GlcMan 9GlcNAc 2, 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 α- d-galactofuranosyl residues (Gal f) linked to Man (A) via an α-(1 → 2)-linkage. OS-7 was found to consist of two oligosaccharides. The structures of these four oligosaccharides were determined to be Gal fMan 5GlcNAc 2, Gal fMan 6GlcNAc 2, Gal fMan 7-GlcNAc 2, and Gal fMan 8GlcNAc 2 by 1H NMR spectroscopy and compositional analysis. The Gal f structure of Gal fMan 5GlcNAc 2 was found to be identical to that of an oligosaccharide previously isolated from the α- d-galactosidase of the same strain 1. The structure of OS-3 remains undetermined.