Linkage and sequence analysis of mannose-rich glycoprotein core oligosaccharides by proton nuclear magnetic resonance...

The anomeric proton (H-1) chemical shifts of D-mannopyranosides in aqueous solution are affected both by the aglycon and by substitution of the ring [Lee, Y. C., & Ballou, C. E. (1965) Biochemistry 4, 257]. We have examined the 1H NMR spectra for a variety of linear and branched mannooligosaccharides and have assigned the H-1 resonances to the component sugars. The chemical shifts, which range from delta 4.76 to 5.36, provide information regarding the linkages, sequences, and anomeric configurations of mannose residues in an oligomer. Thus, 1H NMR spectroscopy can complement enzymatic hydrolysis, methylation analysis, and acetolysis for the structural characterization of oligosaccharides. Furthermore, small structural differences between otherwise identical oligosaccharides are often accompanied by long-range chemical shift changes for the anomeric protons. Because sugars three or more residues away from the structural alteration can be affected, the changes must reflect conformational differences. We have placed emphasis on the mannose-rich oligosaccharides from glycoproteins, particularly those produced by endo-beta-N-acetylglucosaminidase digestion. Two mannose-rich glycopeptides were isolated from a monoclonal human IgM and their positions of origin on the polypeptide chain were determined. The oligosaccharides were released with endo-beta-N-acetylglucosaminidase and fractionated into several size classes. Our structural studies show that each glycopeptide possessed a unique set of oligosaccharides, in agreement with a recent report [Chapman, A. & Kornfeld, R. (1979) J. Biol. Chem. 254, 816]. The NMR spectra were particularly valuable in detecting and quantitating isomeric fragments not observed previously, and our results suggest a modification of the scheme presented by Chapman and Kornfeld for the processing of mannose-rich IgM oligosaccharides.