Calcium-induced associations of the caseins: Thermodynamic linkage of calcium binding to colloidal stability of casein micelles

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Abstract
The caseins occur in milk as colloidal complexes of protein aggregates, calcium, and inorganic phosphate. As determined by electron microscopy, these particles are spherical and have approximately a 650 Å radius (casein micelles). In the absence of calcium, the protein aggregates themselves (submicelles) have been shown to result from mainly hydrophobic interactions. The fractional concentration of stable colloidal casein micelles can be obtained in a calcium caseinate solution by centrifugation at 1500g. Thus, the amount of stable colloid present with varying Ca2+ concentrations can be determined and then analyzed by application of equations derived from Wyman's Thermodynamic Linkage Theory. Ca2+-induced colloid stability profiles were obtained experimentally for model micelles consisting of only α s1- (a calcium insoluble casein) and the stabilizing protein κ-casein, eliminating the complications arising from β- and minor casein forms. Two distinct genetic variants α s1-A andB were used. Analysis of α s1-A colloid stability profiles yielded a precipitation (salting-out) constantk 1, as well as colloid stability (salting-in) parameterk 2. No variations ofk 1 ork 2 were found with increasing amounts of κ-casein. From the variation of the amount of colloidal casein capable of being stabilized vs. amount of added κ-casein an association constant of 4 L/g could be calculated for the complexation of α s1-A and κ-casein. For the α s1-B and κ-casein micelles, an additional Ca2+-dependent colloidal destabilization parameter,k 3, was added to the existingk 1 andk 2 parameters in order to fully describe this more complex system. Furthermore, the value ofk 3 decreased with increasing concentration of κ-casein. These results were analyzed with respect to the specific deletion which occurs in α s1-caseinA in order to determine the sites responsible for these Ca2+-induced quaternary structural effects.