Role of the O-phosphoserine clusters in the interaction of the bovine milk αs1-, β-, κ-caseins and the PP3 component with immobilized iron (III) ions

Abstract

α s1- and β-Caseins have a sequence cluster -Ser( P)-Ser( P)-Ser( P)-Glu-Glu- which is not present in κ-casein and the whey PP3 component. The affinity of these phosphoproteins for the iron(III)-iminodiacetic acid (IDA) complex immobilized on Sepharose was studied a a function of pH, urea concetnration, calcium ion concentration, enzymatic dephosphorylation and temperature. The affinity of the three polyphosphorylated proteins ( α s1- and β-caseins, PP3) was similar. The sequence cluster was not a specific recognition pattern for the iron(III) ion. These three proteins presented a site of high affinity and a site of weak affinity. κ-Casein, which had only one Ser( P) residue, presented only the site of weak affinity. Their primary site which was absent after dephosphorylation or calcium ion addition required the presence of at least two Ser( P) residues close in space. Their secondary site was sensitive to the presence of urea. It was sensitive to pH variation for PP3 and κ-casein. The study of the affinity of a few free amino acids towards iron(III)-IDA showed that the secondary site involved tryptophan and tyrosine residues for α s1- and β-caseins, histidine residues for PP3 and cysteine residues for κ-casein.