Effect of dephosphorylation on calcium binding to αs1-casein-enriched protein

The calcium binding to enriched α_s1-casein (71 % α_s1-casein on a protein basis) that was native or dephosphorylated to different extents was studied using a calcium ion selective electrode and at an ionic strength comparable to that found in milk. The calcium binding to α_s1-casein decreased as the level of dephosphorylation was increased. Langmuir adsorption modelling of the calcium binding gave the maximum calcium binding and calcium binding affinity for the native α_s1-casein and progressively dephosphorylated α_s1-casein. This evaluation indicated that the number of calcium binding sites on α_s1-casein decreased as dephosphorylation increased; however, the affinity for any remaining calcium binding sites was unchanged. Correlations were observed between calcium binding of native and dephosphorylated α_s1-casein with the calculated charge at pH 7.0, or the calculated or experimentally determined isoelectric pH of the α_s1-casein. Understanding the calcium binding to native and dephosphorylated α_s1-casein is important in understanding casein association in systems such as natural and artificial casein micelles.