Exploration of glycoprotein structures: Sequences and consequences

The vast expansion in our knowledge of the detailed molecular structure of many glycoproteins has been achieved by purpose-designed methodologies. In this review, principal experimental methods for gaining fundamental evidence are assessed with respect to well-known iV-glycoproteins (e.g. ribonucleases), 0,N-glycoproteins (e.g. glycophorin A) and O-glycoproteins (e.g. submaxiilary gland mucins and some sulfated forms). Relevant properties of N-carbohydrate-peptide links (N-acetylglucosaminyl-asparagine) and O-linkages (N-acetylgalac-tosaminyl-Thr (or -Ser)) are reviewed and the classification and structure of more elaborate glycoproteins is outlined. Key reactions, both chemical and enzymic, effect the cleavage of N- and O-oligosaccharide side-chains from core protein, and subsequent calibrated chromatographic analysis can permit the identification of certain oligosaccharide groupings characteristic of glycoforms (families of glycoproteins different only in the oligosaccharide chain lengths and bond structures). Basic sequencing of individual oligosaccharides is generally achieved by chemical methods (e.g. methylation analysis, degradation with periodate) and by the action of well-characterised specific exo- and endoglycosidases. Degradation of a glycoprotein chemically (e.g. by cyanogen bromide) or enzymically by endopeptidases to glycopeptides provides means for locating oligosaccharide position(s) on the core protein. The presence of O-sulfate ester groups on oligosaccharide side-chains presents further challenging problems for sequencing investigations. Some correlations between oligosaccharide sequences and biological function are considered. While, in the main, the best-known glycoprotein structures are for those of mammalian origin (e.g. Igs), evidence points to other areas of interest in plants (especially yeasts), invertebrates and micro-organisms. Allusion is made to current comparative and fundamental physico-chemical techniques, especially gel permeation chromatography, membrane-transfer, FAB-MS and NMR (NOE, COSY, etc.).