Influence of Core Fucosylation on the Flexibility of a Biantennary N-Linked Oligosaccharide

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Abstract
Fluorescence energy transfer was used to study the conformation of each antenna of a complex biantennary oligosaccharide. A core fucosylated biantennary oligosaccharide was converted to a glycosylamine which allowed coupling of a naphthyl donor fluorophore directly to the reducing-end GlcNAc 1. After generating an aldehyde at C-6 of residue 6 or 6‘ using galactose oxidase, a dansyl ethylenediamine acceptor fluorophore was coupled to either antenna of the oligosaccharide resulting in two donor−acceptor pairs. The fluorescence properties of the naphthyl group allowed determination of the end-to-end donor−acceptor distance and antenna flexibility of each isomer by steady-state and time-resolved fluorescence energy transfer at temperatures ranging from 0 to 40 °C. Extended (20.6 Å) and folded (11.4 Å) donor−acceptor distance populations were identified for the isomer containing dansyl attached to Gal 6‘, whereas only a single extended population (19.7 Å) was determined when dansyl was attached to Gal 6. The presence of Fuc 1‘ had a dramatic effect on the conformation of the 6‘ antenna. Temperature modulation failed to alter the ratio of extended/folded populations when fucose was present. However, following the removal of fucose, the ratio of the extended/folded populations for 6‘ exhibited a temperature dependent conformational equilibrium allowing calculation of the enthalpy and entropy of unfolding. These results established a unique conformational property for the 6‘ antenna of a biantennary oligosaccharide that is influenced by core fucosylation. Comparison of the results obtained for the 6 antenna of biantennary with previous fluorescence energy transfer studies on a triantennary glycopeptide also established conformational differences in this antenna which are dependent on oligosaccharide structure.