Studies on carbohydrate moieties of the glycoprotein, glucoamylase II of Aspergillus niger: nature of carbohydrate-peptide linkage and structure of oligosaccharides

Abstract

Electrophoretically homogeneous type 1 (GP-C₁ and GP-C₂), type 2 (GP-C_3a and GP-C_3b,) and type 3 (GP-D₁, and GP-D₂) glycopeptides from Aspergillus niger glucoamylase II (Manjunath and Raghavendra Rao, preceding paper) were separately treated with alkaline borohydride. The (β-eliminated oligosaccharides were subjected to single and sequential digestion with specific glycosidases and the products analysed by gas liquid chromatography. The studies revealed that carbohydrate moieties were present as mannose, Man-Man-, and trisaccharide structures, namely, (a) GIc-Man-Man-, (b) Gal-Man-Man, (c) Man-Man-Man-, (d) GlcNAc-Man-Man-, and (e) Xyl-Man-Man. None of the glycopeptides contained all the trisaccharide structures (a) to (e). Type 1 glycopeptide contained structures (a), (b) and (c); type 2, (a) and (d) and type 3, (a), (b) and (e). The number of carbohydrate units (mono-, di-and trisaccharides) present in the major glycopeptides was determined and tentative structures for the glycopeptides proposed. Carbohydrate units appeared to occur in clusters of 4 to 7 in each glycopeptide, a structure unique to the carbohydrate moiety in Aspergillus niger glucoamylase. Based on carbohydrate analysis and yields of glycopeptide, the number of units of each type of glycopeptide present in glucoamylase II was tentatively calculated to give two of type Man:Glc:Gal = 12-15:l:l, one of type Man:Glc:GlcN = 10-l1:1:2 and one of type Man :GIc :Gal:Xyl = 4-8:0.1:0.5-0.8:0.3-1 glycopeptides.