Product-identification and substrate-specificity studies of the GDP-L-fucose:2-acetamido-2-deoxy-beta-D-glucoside (FUC goes to Asn-linked GlcNAc) 6-alpha-L-fucosyltransferase in a Golgi-rich fraction from porcine liver

Abstract

Golgi-rich membranes from porcine liver have been shown to contain an enzyme that transfers L-fucose in alpha-(1 goes to 6) linkage from GDP-L-fucose to the asparagine linked 2-acetamido-2-deoxy-D-glucose residue of a glycopeptide derived from human alpha 1-acid glycoprotein. Product identification was performed by high resolution, 1H-n.m.r. spectroscopy at 360 MHz and by permethylation analysis. The enzyme has been named GDP-L-fucose: 2-acetamido-2-deoxy-beta-D-glucoside (Fuc goes to Asn-linked GlcNAc) 6-alpha-L-fucosyltransferase, because the substrate requires a terminal beta-(1 goes to 2)-linked GlcNAc residue on the alpha-Man (1 goes to 3) arm of the core. Glycopeptides with this residue were shown to be acceptors whether they contain 3 or 5 Man residues. Substrate-specificity studies have shown that diantennary glycopeptides with two terminal beta-(1 goes to 2)-linked GlcNAc residues and glycopeptides with more than two terminal GlcNAc residues are also excellent acceptors for the fucosyltransferase. An examination of four pairs of glycopeptides differing only by the absence or presence of a bisecting GlcNAc residue in beta-(1 goes to 4) linkage to the beta-linked Man residue of the core showed that the bisecting GlcNAc prevented 6-alpha-L-fucosyltransferase action. These findings probably explain why the oligosaccharides with a high content of mannose and the hybrid oligosaccharides with a bisecting GlCNAc residue that have been isolated to date do not contain a core L-fucosyl residue.