Mucin-type glycopeptide structure in solution: past, present, and future.

Mucins are very high molecular weight glycoproteins that form a "mucus" barrier at the surface of epithelial cells. They are heavily glycosylated with O-linked glycans that are involved in myriad cellular functions, including protection from external changes in pH, ion flux and reactive oxygen species. Aberrations in mucin expression and their glycan constitution have been associated with many disease states including gastritis, pulmonary disorders and cancer. High resolution structural information on mucins is lacking due to their complexity, in particular their large size and the many variants of O-linked glycans produced in their biosynthesis. This review discusses the structures of glycopeptides that contain "mucin-type" glycosylation, and concentrates primarily on data obtained by NMR spectroscopy. The effect of the glycan on the peptide backbone, the features that have shown to be common to this type of glycosylation and the differences of glycosylation at serine and threonine residues are the major topics of discussion.