Modification of oligosaccharide antenna flexibility induced by exoglycosidase trimming.

We have investigated the solution conformation of a triantennary glycopeptide using resonance energy transfer [Rice et al. (1991) Biochemistry 30, 6646-6655]. Triantennary glycopeptide was derivatized with a donor fluorophore on the N-terminus and with an acceptor fluorophore attached individually to each terminal galactose residue, resulting in three isomeric donor-acceptor pairs. Time-resolved energy-transfer experiments revealed two distinct donor-acceptor distance populations for two of the three antennae of the oligosaccharide. An extended conformation and a folded conformation were detected for the two flexible antennae whereas the third antenna on the oligosaccharide was rigid, containing only an extended conformer. The ratios of the extended to folded conformers of the two flexible antennae were reversibly modulated by temperature, allowing the calculation of delta H and delta S for the conformational change [Wu et al. (1991) Proc. Natl. Acad. Sci. U.S.A. 88, 9355-9359]. In the present study, we have trimmed the fluorescent glycopeptides with exoglycosidases which specifically removed the unmodified antennae of the oligosaccharide. The resulting single-chain isomeric glycopeptides each contained identical core sugar residues and a terminally located donor and acceptor, but differed only in the linkage configuration of the sugar residues. Analysis of these glycopeptides by time-resolved energy transfer indicated that each antenna of the oligosaccharide is, by itself, maintained exclusively in the extended conformation. Temperature modulation failed to induce antenna folding as was previously observed for the complete triantennary structure. These data suggest that interantenna interactions modulate the conformation of individual antenna of complex oligosaccharides.