Interaction properties of doubly phosphorylated beta-casein, a major component of the human milk caseins.
Doubly phosphorylated beta-casein constitutes nearly 30% of the total human beta-caseins and is thus one of the major components of that fraction. The properties and mode of association of doubly phosphorylated beta-casein are therefore important determinants of the structure and function of the human casein micelle. Doubly phosphorylated beta-casein has an absorbency of 6.2 and a partial specific volume of .74. The protein precipitated at room temperature when 10 mM Ca2+ was added but produced a clear solution in 1 M NaCl. Equilibrium dialysis produced an average of 2.06 major Ca(2+)-binding sites at 37 degrees C with a dissociation constant of 12.1 x 10(-4) M. The monomer at 20 degrees C was calculated to have a solvation of 2.1 g of H2O/g of protein and an axial ratio of 6.8, suggesting a prolate ellipsoid of about 11 by 2 nm. At high ionic strength, evidence exists for a spherical structure with a molecular weight of 2.25 x 10(6). This structure would represent a polymer of about 90 monomers with a radius of 14.8 nm and a solvation of 1.93 g of H2O/g of protein. This association behavior is similar to that of other phosphorylated human beta-caseins but differs from the nonphosphorylated form. It changes when both Ca2+ and inorganic orthophosphate are present.
Sood
S M
SM
Department of Biochemistry, Loma Linda University School of Medicine, CA 92350.
Chang
P
P
Slattery
C W
CW
D002118:Calcium / Q000378:metabolism; D002364:Caseins / Q000737:chemistry*; D055598:Chemical Phenomena; D002627:Chemistry, Physical; D002853:Chromatography, Liquid; D006801:Humans; D009994:Osmolar Concentration; D010750:Phosphoproteins / Q000737:chemistry*; D011485:Protein Binding; D012995:Solubility; D013696:Temperature
D002364:Caseins; D010750:Phosphoproteins; D002118:Calcium