The carbohydrate portions of milk glycoproteins

kappa-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 alpha 2 leads to 3 Gal beta 1 leads to GalNAc; the tetrasaccharide contains one additional sialic acid. The polysaccharide part of ovine kappa-casein resembles that of bovine kappa-casein, but contains also N-glycolyl neuraminic acid. Human kappa-casein contains 3 times more carbohydrate than bovine kappa-casein with 2 additional sugars, GlcNAc and Fuc. The various polysaccharide parts isolated from bovine colostrum kappa-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 kappa-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 kappa-casein carbohydrate moiety.