NMR analysis of human salivary mucin (MUC7) derived O-linked model glycopeptides:: comparison of structural features and carbohydrate-peptide interactions

Two series of glycopeptides with mono- and disaccharides, [GalNAc and Gal beta (1-3)GalNAc] O-linked to serine and threonine at one, two or three contiguous sites were synthesized and characterized by H-1 NMR. The conformational effects governed by O-glycosylation were studied and compared with the corresponding non-glycosylated counterparts using NMR, CD and molecular modelling. These model peptides encompassing the aa sequence, PAPPSSSAPPE (series I) and APPETTAAPPT (series II) were essentially derived from a 23-aa tandem repeat sequence of low molecular weight human salivary mucin (MUC7). NOEs, chemical shift perturbations and temperature coefficients of amide protons in aqueous and nonaqueous media suggest that carbohydrate moiety in threonine glycosylated peptides (series II) is in close proximity to the peptide backbone. An intramolecular hydrogen bonding between the amide proton of GalNAc or Gal beta (1-3)GalNAc and the carbonyl oxygen of the O-linked threonine residue is found to be the key structure stabilizing element. The carbohydrates in serine glycosylated peptides (series I), on the other hand, lack such intramolecular hydrogen bonding and assume a more apical position, thus allowing more rotational freedom around the O-glycosidic bond. The effect of O-glycosylation on peptide backbone is clearly reflected from the observed overall differences in sequential NOEs and CD band intensities among the various glycosylated and non-glycosylated analogues. Delineation of solution structure of these (glyco)peptides by NMR and CD revealed largely a poly L-proline type II and/or random coil conformation for the peptide core. Typical peptide fragments of tandem repeat sequence of mucin (MUC7) showing profound glycosylation effects and distinct differences between serine and threonine glycosylation as observed in the present investigation could serve as template for further studies to understand the multifunctional role played by mucin glycoproteins.