Abstract
The N-glycan at Asn297 of the immunoglobulin G Fc fragment modulates cellular responses of the adaptive immune system. However, the underlying mechanism remains undefined, as existing structural data suggest the glycan does not directly engage cell surface receptors. Here we characterize the dynamics of the glycan termini using solution NMR spectroscopy. Contrary to previous conclusions based on X-ray crystallography and limited NMR data, our spin relaxation studies indicate that the termini of both glycan branches are highly dynamic and experience considerable motion in addition to tumbling of the Fc molecule. Relaxation dispersion and temperature-dependent chemical shift perturbations demonstrate exchange of the α1-6Man-linked branch between a protein-bound and a previously unobserved unbound state, suggesting the glycan samples conformational states that can be accessed by glycan-modifying enzymes and possibly glycan recognition domains. These findings suggest a role for Fc-glycan dynamics in Fc-receptor interactions and enzymatic glycan remodeling.