Iron binding by phosvitin: variation of rate of iron release as a function of the degree of saturation of iron binding sites

Abstract

The rate of iron release from Fe(III)-phosvitin complexes, at varied degrees of saturation, was studied. Iron release was induced by reduction in the presence of the ferrous ion chelator, o-phenanthroline. If iron release was induced photochemically (without a chemical reductant), the reactions proceeded in zero order fashion, independently of the degree of saturation but with a strong dependence on the concentration of phenanthroline. When hydroquinone was added and the reactions were conducted in the dark, iron release followed first-order kinetics and the rate constants showed a clear dependence on the degree of saturation of the protein, which was most marked at lower levels of saturation. The results imply control of iron release by binding site differences produced by different intramolecular environments as the protein provides different combinations of its phosphoserine groups as ligands depending on the number of iron atoms to be accommodated per protein molecule.