Possible role for peptide-oligosaccharide interactions in differential oligosaccharide processing at asparagine-107 of the light chain and asparagine-297 of the heavy chain in a monoclonal IgG1.kappa.

Abstract

The carbohydrate attached at Asn-107 of the L chain of a human myeloma IgG1.kappa. (Hom) was isolated and the structure determined by 1H NMR. Two oligosaccharides were found corresponding to mono- and disialylated forms of the bisected biantennary class of glycopeptides. Both structures had Fuc.alpha.1-6 linked to the GlcNAc residue attached to Asn and NeuNAc residues linked .alpha.2-6. Because of the unusual nature of these structures, the Asn-297 oligosaccharides of the same IgG were prepared from Fc fragments and H chains. Comparison of the structures of the latter glycopeptides with structures from the same site on a 2nd human myeloma IgG1.kappa. (Tem) showed them to be quite similar in that the majority of the structures were biantennary but not bisected. The completely bisected nature of the L-chain oligosaccharides comes from a high level of activity of GlcNAc-T-III (the enzyme responsible for the attachment of the bisecting GlcNAc) in the cells producing the IgG. A mechanism is suggested for differential glycosylation between the Asn-107 and Asn-297 sites based on the stabilization of the Asn-297 oligosaccharide in a conformation with the torsional angle .omega. about the C5.sbd.C6 bond of the Man.alpha.1-6 linkage equal to -60.degree.. It was previously postulated that this conformation is not a substrate for GlcNAc-T-III. In support of this model are the X-ray crystallographic studies of the Fc fragment of human IgG from pooled serum. In these crystals the conformation for the oligosaccharide at the Asn-297 site was in the .omega. = -60.degree. form.