Bovine dentin phosphophoryn: calcium ion binding properties of a high molecular weight preparation

The calcium ion binding properties of a high molecular weight bovine phosphophoryn preparation have been studied at high ionic strength (0.5 M KCl as supporting electrolyte). Surprisingly, even under these conditions at which intermolecular ionic interactions are effectively screened, phosphophoryn was capable of binding calcium ions. This suggests that calcium ion binding by bovine phosphophoryn involves formation of a coordination complex or chelation. Two classes of binding sites were identified. The high affinity sites have a relatively low affinity constant of approximately 775 moles but maximally represented about 90% of the (SerP + Asp) residues along the polypeptide backbone. Binding was nonideal in that the number of high affinity sites showed a marked dependence on phosphophoryn concentration, increasing with the protein concentration. This unusual concentration dependence suggests that calcium-mediated intramolecular interactions may limit available high affinity sites at lower phosphophoryn solution concentrations by a requirement for intramolecular folding. In support of this conclusion, the reduced viscosity of dilute phosphophoryn solutions was found to fall rapidly as a result of intramolecular folding, whereas at higher phosphophoryn concentrations calcium-mediated intermolecular aggregate formation slowed the rate of viscosity decrease. At the lower physiological ionic strength the affinity constant for specific calcium ion binding is several orders of magnitude greater than under the extreme conditions described in this work.