The carbohydrate portions of milk glycoproteins

• Published in: Journal of dairy research 1979-04, Vol.46 (2), p.187-191
• Main Authors: JOLLÈS, P, FIAT, A.-M.
• Format: Article
• Language: English
• Subjects: Amino Acid Sequence; Animals; Carbohydrates - analysis; Cattle; Female; Glycoproteins; Milk Proteins; Molecular Conformation; Oligosaccharides - analysis; Section A. Biological Aspects of Milk Proteins; Sheep; Sialic Acids - analysis; Species Specificity
• ISSN: 0022-0299; 1469-7629
• DOI: 10.1017/S0022029900017027

k-Casein is the main glycoprotein of cow's milk. Its polysaccharide part is O-glycosidically linked to threonine residue 133. It contains only 3 different sugars (Gal, GalNAc, NeuNAc), but a microheterogeneity has been detected at the sugar level. Two main polysaccharides have so far been characterized. The structure of the trisaccharide is NeuNAc α → 3 Gal β1 →3 GalNAc; the tetrasaccharide contains one additional sialic acid. The polysaccharide part of ovine k-casein resembles that of bovine k-casein, but contains also N-glycolyl neuraminic acid. Human k-casein contains 3 times more carbohydrate than bovine k-casein with 2 additional sugars, GlcNAc and Fuc. The various polysaccharide parts isolated from bovine colostrum k-caseinoglycopeptide are much more complex than those obtained from the normal glycopeptide, indicating an evolution of the sugar part as a function of time after parturition. Some aspects of the secondary structure of k-casein and the role of the sugar part are discussed. The carbohydrate moiety of another milk protein, human lactotransferrin, is also discussed briefly. It is comprised of 2 identical glycan groups, N-glycosidically linked to the protein, and quite different from the k-casein carbohydrate moiety.