ROLE OF THE O-PHOSPHOSERINE CLUSTERS IN THE INTERACTION OF THE BOVINE-MILK ALPHA(S1)-CASEIN, BETA-CASEIN, KAPPA-CASEIN AND THE PP3 COMPONENT WITH IMMOBILIZED IRON(III) IONS

alpha(sl)- and beta-Caseins have a sequence cluster -Ser(P)-Ser(P)-Ser (P)-Glu-Glu- which is not present in kappa-casein and the whey PP3 com ponent. The affinity of these phosphoproteins for the iron(III)-iminod iacetic acid (IDA) complex immobilized on Sepharose was studied as a f unction of pH, urea concentration, calcium ion concentration, enzymati c dephosphorylation and temperature. The affinity of the three polypho sphorylated proteins (alpha(sl)- and beta-caseins, PP3) was similar. T he sequence cluster was not a specific recognition pattern of the iron (III) ion. These three proteins presented a site of high affinity and a site of weak affinity. kappa-Casein, which had only one Ser(P) resid ue, presented only the site of weak affinity. Their primary site which was absent after dephosphorylation or calcium ion addition required t he presence of at least two Ser(P) residues close in space. Their seco ndary site was sensitive to the presence of urea. It was sensitive to pH variation for PP3 and kappa-casein. The study of the affinity of a few free amino acids towards iron(III)-IDA showed that the secondary s ite involved tryptophan and tyrosine residues for alpha(sl)- beta-case ins, histidine residues for PP3 and cysteine residues for kappa-casein .