The N-glycan acceptor specificity of a glucuronyltransferase, GlcAT-P, associated with biosynthesis of the HNK-1 epitope

The acceptor specificity of a rat brain glucuronyltransferase, GlcAT-P, associated with biosynthesis of the HNK-1 epitope on glycoproteins, was investigated using asialoorosomucoid as a model acceptor substrate. Structural analysis of N-linked oligosaccharides, to which glucuronic acid was transferred by GlcAT-P, by means of two-dimensional mapping of pyridylamino-oligosaccharides and MS spectrometry, demonstrated that the enzyme transferred glucuronic acid to bi-, tri-, and tetra-antennary complex type sugar chains, with almost equal efficiency, indicating that the enzyme has no preference as to the number of acceptor sugar branches. Next, we studied the branch specificity of this enzyme by means of the selective branch scission method involving two step exoglycosidase digestion using authentic pyridylamino-oligosaccharides. The GlcAT-P is highly specific for the terminal N-acetyllactosamine structure and no glucuronic acid was incorporated into a Galβ1-3GlcNAc moiety. The GlcAT-P transferred glucuronic acid to the galactose residues in the N-acetyllactosamine branches of bi-, tri-, and tetra-antennary oligosaccharide chains, with different efficiencies and most preferentially to those in the Galβ1-4GlcNAcβ1-4Manα1-3 branch.