Differential flexibilities in three branches of an N-linked triantennary glycopeptide

Abstract

The solution conformation behavior of complex oligosaccharides was studied by resonance energy transfer, as measured by the time-resolved fluorescence method, to determine the conformational heterogeneity of a triantennary glycopeptide at various temperatures. Groups that acted as a fluorescence donor (naphthyl-2-acetyl, Nap) or acceptor (dansylethylenediamine, Dan) were selectively attached to the N terminus of the peptide and a Gal residue [either 6′ (shown below), 6, or 8] of the oligosaccharide, respectively. (6′) Dan-Galβ4GlcNAcβ2Manα6↘ Manβ4GlcNAcβ4GlcNAcβ-Asn-Ala-NH-Nap (6) Galβ4GlcNAcβ2Manα3↗ (8) Galβ4GlcNAcβ4↗ Time-resolved fluorescence energy-transfer measurements revealed two populations of conformers when Dan was attached to either Gal-6′ or Gal-6. One conformer contained the antenna folded back toward the core region, and a second was in an extended conformation. The two conformations differed in donor-acceptor distance by about 10 Å. Systematically increasing the temperature from 0°C to 40°C increased the ratio of extended to folded forms 2-fold for the Gal-6 isomer and 4-fold for the Gal-6′ isomer, whereas the Gal-8 isomer showed only a single distance population throughout this temperature range. From these data, ΔH and ΔS for the reversible conformational change were calculated to be 3.1 kcal/mol and 10.8 cal/(mol·K) for the Gal-6 isomer and 7.1 kcal/mol and 25.8 cal/(mol·K) for the Gal-6′ isomer. In addition to the structural microheterogeneity commonly associated with glycoproteins, the differential flexibilities of the different branches in the oligosaccharides contribute conformational heterogeneity and should be considered in conformational analysis. The data are discussed in terms of the most probable linkages that contribute to the observed flexibility of the individual triantennary branches, and the biological significance of flexible linkages in complex carbohydrates is considered.