title: Iron(III)--phosphoprotein chelates: stoichiometric equilibrium constant for interation of iron(III) and phosphorylserine residues of phosvitin and casein.
abstract: Estimates of the strength of iron binding to model phosphoproteins were obtained from equilibrium dialysis experiments. Iron-free phosvitin (chicken and frog) or alpha sl-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 10(18.0) was calculated for the formation of the Fe3+(SerP.SerP) chelate. When comared 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 (10(17.5) 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 greater than phosphoprotein di-O-phosphorylserine greater than citrate greater than NTA.
mesh_terms: D000818:Animals; D001001:Anura; D001665:Binding Sites; D002364:Caseins; D002645:Chickens; D004527:Egg Proteins; D007501:Iron; D007700:Kinetics; D008433:Mathematics; D010750:Phosphoproteins; D010768:Phosphoserine; D010774:Phosvitin; D011485:Protein Binding; D011894:Rana pipiens; D012694:Serine / Q000031:analogs & derivatives
chemical_list: D002364:Caseins; D004527:Egg Proteins; D010750:Phosphoproteins; D010768:Phosphoserine; D012694:Serine; D010774:Phosvitin; D007501:Iron