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Abstract
The casein form that has four organic phosphoryl groups, β-casein (CN)-4P, is the major constituent (∼35%) of the β-CN fraction of human milk and should play an important role in micelle structure and formation. In 3.3 M urea, the monomer is present with a molecular mass of 24,500 Da and a sedimentation coefficient of 1.3 S (Svedberg units, 10–13s). In 0.02 M NaCl and 0.01 M imidazole (low salt buffer) at pH 7, the sedimentation coefficient was 1.5 S, which increased to 14 S at 37°C. Laser light scattering in low salt buffer and 9 mg/ml of protein indicated monomers with a radius of about 4 nm at 4°C. The size of the radius increased as temperature increased, and, at 37°C, the radius was about 12 nm. The molecular mass suggested the presence of about 47 monomers per polymer. In 0.25 M NaCl and with 10 mM Ca2+ prior to precipitation, the polymer attained a maximum radius of about 15 nm, which perhaps is the size of the smallest human milk micelles. The low value for reduced viscosity of 8.2 ml/g for the calcium-induced polymer was independent of protein concentration, suggesting a spherical shape and fixed size. Calcium apparently binds strongly to the phosphates; the dissociation constant was 8.1 × 10–4 M. Other constituents of milk, such as inorganic orthophosphate, may contribute to differences in the manner by which β-CN, with various phosphorylation levels, participate in micelle formation.
Key words
Abbreviation Key:
D20,w (diffusion coefficient corrected to 20°C and water solution), LSB (low salt buffer), Mr (relative molecular mass), ηred (reduced viscosity), s20,w (sedimentation coefficient corrected to 20°C and water solution)References
- The least number of phosphate groups for crosslinking of casein by colloidal calcium phosphate.J. Dairy Sci. 1992; 75: 971
- Effect of bound phosphate on the calcium-binding ability and calcium dependent precipitability of human β-casein.Agric. Biol. Chem. 1989; 53: 1037
- Hydrodynamic studies of structure of biological macromolecules. Experimental and theoretical advances yield structural details of proteins, DNA, viruses, and polysomes.Science. 1968; 161: 1212
- Binding of diffusible molecules by macromolecules: rapid measurement by rate of dialysis.J. Biol. Chem. 1969; 244: 774
- Human β-casein.J. Dairy Res. 1979; 46: 235
- Human bcasein: amino acid sequence and identification of phosphorylation sites.J. Biol. Chem. 1984; 259: 5132
- The major component of human casein: a protein phosphorylated at different levels.Arch. Biochem. Biophys. 1970; 140: 47
- Interaction of triply phosphorylated human β-casein: fluorescence spectroscopy and light-scattering studies of conformation and self association.Arch. Biochem. Biophys. 1991; 289: 39
- Phospholipid in cultured guinea pig skin.J. Invest. Dermatol. 1970; 54: 174
- Comparison of calcium induced association of bovine and caprine caseins and the relationship of αs1-casein content to colloidal stabilization: thermodynamic linkage analysis.J. Dairy Sci. 1993; 76: 3690
- Human αs1-casein: purification and characterization.Comp. Biochem. Physiol. 1995; 111B: 75
- The attraction of proteins for small molecules and ions.Ann. New York Acad. Sci. 1949; 51: 660
- Hydrophobic interactions in human casein micelle formation: β-casein aggregation.J. Dairy Res. 1989; 56: 427
- Interactions in human casein systems: self-association of fully phosphorylated human β-casein.Arch. Biochem. Biophys. 1985; 242: 355
- Interactions in human casein systems: self-association of nonphosphorylated human β-casein.Arch. Biochem. Biophys. 1988; 264: 574
- Interactions of triply phosphorylated human β-casein: monomer characterization and hydrodynamic studies of self-association.Arch. Biochem. Biophys. 1990; 277: 415
- Interaction properties of doubly phosphorylated β-casein: a major component of the human milk caseins.J. Dairy Sci. 1992; 75: 2937
- Interaction properties of singly phosphorylated human β-casein: similarities with other phosphorylation levels.J. Dairy Sci. 1993; 77: 405
- Physical Chemistry of Macromolecules.John Wiley & Sons, New York, NY1961 (Page 390)
- Solubility, solvation, and stabilization of as1- and β-caseins.J. Dairy Sci. 1969; 52: 1166
- Core polymers of casein micelles.Biochemistry. 1970; 9: 786
- Effect of chemical phosphorylation of bovine casein components on the properties related to casein micelle formation.Agric. Biol. Chem. 1981; 45: 909
- Effect of dephosphorylation on self-association and the precipitation of β-casein.Agric. Biol. Chem. 1974; 38: 2051
Article info
Publication history
Accepted:
January 17,
1997
Received:
May 7,
1996
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© 1997 American Dairy Science Association. Published by Elsevier Inc.
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