The Structural Role of Antibody N-Glycosylation in Receptor Interactions

Asparagine(N)297-linked glycosylation of immunoglobulin G(IgG) Fc is required for binding to Fc gamma RIIa, IIb, and IIIa, although it is unclear how it contributes. We found the quaternary structure of glycosylated Fc was indistinguishable from aglycosylated Fc, indicating that N-glycosylation does not maintain relative Fc C gamma 2/C gamma 3 domain orientation. However, the conformation of the C'E loop, which contains N297, was significantly perturbed in the aglycosylated Fc variant. The conformation of the C'E loop as measured with a range of Fc variants shows a strong correlation with Fc gamma RIIIa affinity. These results indicate that the primary role of the IgG1 Fc N-glycan is to stabilize the C'E loop through intramolecular interactions between carbohydrate and amino acid residues, and preorganize the FcgRIIIa interface for optimal binding affinity. The features that contribute to the capacity of the IgG1 Fc N-glycan to restrict protein conformation and tune binding affinity are conserved in other antibodies including IgG2-IgG4, IgD, IgE, and IgM.