Oligomannose Fc Glycans Reprogram the Energetic and Conformational Basis of CD16a Recognition

IgG1 Fc recognition by Fc{gamma}RIIIa/CD16a is a central determinant of antibody-dependent cellular cytotoxicity and is strongly regulated by Fc N297 glycosylation. While afucosylation and galactosylation have been extensively studied, the structural basis by which oligomannosidic Fc glycans modulate CD16a binding remains less clear, despite their prevalence in therapeutic antibodies and association with accelerated serum clearance. Here, we use all-atom molecular dynamics simulations to investigate how mannose-5 (M5) Fc glycosylation alters IgG1 Fc-CD16a recognition across Paired Biantennary (complex glycans on both Fc), asymmetric Unpaired (complex glycan on one Fc arm and M5 on the other), and Paired M5 glycoforms. Computed interaction energies reproduce the experimental trend that Paired M5 glycoforms bind CD16a less favorably than complex-type paired glycans, supporting the use of the simulations to interrogate the structural origin of this energetic hierarchy. Residue-wise energetic decomposition and contact analyses show that Paired M5 glycosylation redistributes energetic contributions away from the productive Fc-CD16a interface and reduces both protein-mediated and glycan-mediated physical contacts. Free energy surface analyses further reveal that Paired M5 systems sample broader, less stable receptor-bound conformational ensembles, while dynamic cross-correlation analysis shows reduced intra-domain and inter-domain coupling across the complex. Importantly, a single M5 glycan is sufficient to perturb productive recognition by increasing Fc-arm separation heterogeneity, reducing high-frequency protein contacts, and weakening long-range dynamic communication. Glycan identity on the receptor-proximal Fc arm emerges as a decisive determinant of binding, indicating that Fc glycan composition, pairing, and receptor-bound placement jointly encode CD16a recognition. Together, these findings provide a mechanistic framework for understanding how oligomannose Fc glycans remodel antibody-receptor engagement and suggest that asymmetric Fc glycosylation, combined with residue-level interface engineering, may offer new strategies for tuning therapeutic antibody effector function.