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.

The carbohydrate attached at Asn-107 of the light 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 heavy chains. Comparison of the structures of the latter glycopeptides with structures from the same site on a second human myeloma IgG1 kappa (Tem) showed them to be quite similar in that the majority of the structures were biantennary but not bisected. We suggest that the completely bisected nature of the light-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. We suggest a mechanism 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-C6 bond of the Man alpha 1-6 linkage equal to -60 degrees. It has previously been postulated that this conformation is not a substrate for GlcNAc-T-III [Brisson, J.-R., & Carver, J. P. (1983) Can. J. Biochem. 61, 1067-1078].(ABSTRACT TRUNCATED AT 250 WORDS)