IRON(III)-PHOSPHOPROTEIN CHELATES - STOICHIOMETRIC EQUILIBRIUM-CONSTANT FOR INTERACTION OF IRON(III) AND PHOSPHORYLSERINE RESIDUES OF PHOSVITIN AND CASEIN

Estimates of the strength of iron binding to model phosphoproteins were obtained from equilibrium dialysis experiments. Iron-free phosvitin (chicken and frog) or αs1-casein (cow) was dialyzed against the iron(III) chelates of nitrilotriacetate (NTA), (ethylenedinitrilo)tetraacetate (EDTA), or citrate. Protein-bound metal was measured at equilibrium; competition of chelator and phosphoprotein for iron(III) was determined by reference to comprehensive equilibrium equations presented in the Appendix. Analysis of the iron-binding data for phosvitin suggested that clusters of di-O-phosphorylserine residues (SerP-SerP) were the most probable iron-binding sites. A stoichiometric equilibrium constant of 10180 was calculated for the formation of the Fe3+(SerP-SerP) chelate. When compared on the basis of phosphate content, casein bound iron more weakly than phosvitin. However, if the stoichiometric equilibrium constant for the formation of the casein Fe3+(SerP-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 iron was very similar. A theoretical comparison showed that the “strengths” of the ferric chelates can be ranked: EDTA > phosphoprotein di-O-phosphorylserine > citrate > NTA. © 1979, American Chemical Society. All rights reserved.