Involvement of polyphosphorylserine blocks in the Fe(III) binding by phosvitin

It has been suggested that the binding of iron(III) by phosvitin involves the phosphoric radicals of phosphorylserine residues, many of which are arranged in rows of several consecutive phosphoamino acids. In this paper we present evident that, unlike free phosphorylserine which does not interact with Fe(III), polyphosphorylserine blocks--(Ser-P)n, with n greater than or equal to 4 -- bind Fe(III) like phosvitin, though less actively, to give complexes stable at very acidic pHs. The binding of iron does not cause any polymerization of the phosphopeptides, and the (Ser-P)n-Fe(III) complexes display an Fe/P ratio significantly lower than 0.5, found in Fe-saturated phosvitin. These findings indicate that polyphosphorylserine blocks play an important role in the binding of iron by phosvitin, and that in the intact protein their binding capacity is optimized by the conformation of the polypeptide chain.