Iron(III)-phosphoprotein chelates: stoichiometric equilibrium constant for interaction of iron(III) and phosphorylserine residues of phosvitin and casein

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Abstract
Estimates of the strength of Fe binding to model phosphoproteins were obtained from equilibrium dialysis experiments. Fe-free phosvitin (chicken [egg] and frog [ovary]) or .alpha.s1-casein (cow) was dialyzed against the Fe(III) chelates of nitrilotriacetate (NTA), (ethylenedinitrilo)tetraacetate (EDTA) or citrate. Protein-bound metal was measured at equilibrium; competition of chelator and phosphoprotein for Fe(III) was determined by reference to comprehensive equilibrium equations presented in the Appendix. Analysis of the Fe-binding data for phosvitin suggested that clusters of di-O-phosphorylserine residues (SerP .cntdot. SerP) were the most probable Fe-binding sites. A stoichiometric equilibrium constant of 1018.0 was calculated for the formation of the Fe3+(SerP .cntdot. SerP) chelate. When compared on the basis of phosphate content, casein bound Fe more weakly than phosvitin. However, if the stoichiometric equilibrium constant for the formation of the casein Fe3+(SerP .cntdot. SerP) chelate (1017.5) was adjusted to account for the fact that a smaller percentage of casein phosphoserines occurs in di-O-phosphorylserine clusters, the affinity of casein and phosvitin for Fe was very similar. A theoretical comparison showed that the strengths of the ferric chelates can be ranked: EDTA > phosphoprotein di-O-phosphorylserine > citrate > NTA. [The binding of metal by phosphoproteins has great physiological and nutritional relevance.].