The binding of beta-casein to hydroxyapatite: the effect of phosphate content and location

The elution behaviour of beta-casein from columns of hydroxyapatite has been studied and the effect examined of the enzymic addition of an extra phosphate residue. When the additional phosphate is located near pre-existing phosphate residues stronger binding to hydroxyapatite is observed. When the additional phosphate is remote from the pre-existing phosphates no increase in strength of binding is observed. It is suggested that the clustering of phosphate residues which characterizes alphas- and beta-caseins can be rationalized on the basis that it facilitates co-operative interactions between these phosphates and the Ca phosphate of the casein micelle and/or basic residues within casein polypeptide chains.