Role of Phosphate Groups in the Calcium Sensitivity of αs2-Casein

In order to explain the higher calcium sensitivity of as2-casein than other casein constitutents, properties of as2-casein and dephosphorylated as2-casein were examined and compared with those of as l-casein.~s2-Casein was more sensitive to calcium than asl-Casein.Aggregates of calcium %1-and as2-Caseinates were both thoroughly solubilized by 4 M urea.Gel filtration of asl-casein and of as2-casein in 4 M urea showed the same elution pattern in the presence and absence of 10 mM calcium chloride.Dephosphorylated as2-casein was insoluble at neutral pH although dephosphorylated asl-casein was soluble.Dephosphorylated as2-casein was solubilized by 4 M urea and alkalization to pH 11.The net proton charge, calculated from the primary structure, of dephosphorylated as2-casein was very low at pH 7.0, although dephosphorylated as 1-casein has a considerable number of negative net charges.The calculated isoionic point of dephosphorylated &s2-casein, which contains one phosphate group, was 6.98.Ester phosphate groups of as2-casein play an important role in its solubilization at neutral pH and neutralization of ionized phosphate groups by calcium ion increases hydrophobic interaction, which leads to precipitation.