The carbohydrate portions of milk glycoproteins

Abstract

SUMMARYk-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.

Characterization of caseinomacropeptides released from renneted raw and UHT treated milks

“…Such an observation has already been made by numerous authors: Sinkinson and Wheelock, 1970;Doi et al, 1979;Fournet et al, 1979;Jollès and Fiat, 1979;Addeo et al, 1984;Van Hooydonk et al, 1984. It has been attributed to a reduced accessibility to chymosin of the glycosylated sites of x-casein which would have higher electronegativity than the carbohydrate-free forms (Van Hooydonk et al, 1984).…”

Casein phosphoproteome: Identification of phosphoproteins by combined mass spectrometry and two‐dimensional gel electrophoresis

“…MALDI analysis of the tryptic peptides of spots [23][24][25][26][27] confirmed that they all belong to b-CN. In fact, each generated peptide 1-28, 2P to 5P /molecule, and peptide 33-48, not phosphorylated or 1P, as shown in Table 5.…”

“…Ovine k-casein is O-glycosylated and six Thr residues have been proposed as putative glycosylation sites [22,23]. Ovine k-CN contains both N-acetyl and N-glycolyl-neuraminic acids at all stages of lactation [24][25][26]. The disaccharide, Among the mass signals relating to k-CN tryptic peptides, only those referring to the glycosylated and phosphorylated peptides k-CN (113-171), 113-143, and 144-171 have been reported in Table 4.…”

Interactions between casein layers adsorbed on hydrophobic surfaces from self consistent field theory: κ-casein versus para-κ-casein

“…A schematic diagram showing the primary sequence of -casein (Jolles & Fiat, 1979), classified according to our five groups is illustrated in Fig. 1.…”