Solution Conformations of a Biantennary Glycopeptide and a Series of Its Exoglycosidase Products from Sequential Trimming of Sugar Residues

Abstract Linkages between sugar residues in branched oligosaccharides exhibit various degrees of flexibility. This flexibility, together with other forces, determines the overall solution conformation of oligosaccharides. We used the method of time-resolved resonance energy transfer to study the solution conformations of a biantennary glycopeptide and its partially trimmed products by exoglycosidases. The N-terminal of the glycopeptide was labeled with 2-naphthyl acetic acid as a fluorescent donor. Either terminal sugar residue, Gal6′, on the branch bearing 6-linked Man (antenna 6′), or Neu5Ac on the branch bearing 3-linked Man (antenna 6) was labeled with 5-dimethylaminonaphthalene-1-sulfonyl as an acceptor. The distance and distance distributions between the terminals were measured. In the intact biantennary glycopeptide, the donor-acceptor distance distribution of antenna 6′ is bimodal with a majority of the population in the extended conformation and that of antenna 6 in one very broad population. The Neu5Ac on antenna 6 is oriented toward the N-terminal at low temperature and adopts a more extended form at high temperature. The removal of individual sugar residues along one of the two antenna in the biantennary oligosaccharide has a small effect on the distance distribution of the remaining antenna for both antennae 6 and 6′. Together with previous studies of the triantennary glycopeptides (Rice, K. G., Wu, P. G., Brand, L., and Lee, Y. C.(1993) Biochemistry 32, 7264-7270), our results suggest that both steric hindrance and inter-residue hydrogen bonding are very important in the folding pattern in oligosaccharide structures.