The Phosphate on the Iron Core of Horse Spleen Ferritin as Studied by a 1H NMR Probe Method

Abstract

Abstract Glycylglycylglycylglycine and tetramethylammonium chloride have been used as 1H NMR probes to study the state and role of the phosphate on the surface of the iron core of ferritin. 1H NMR signals of the probe molecules in a ferritin solution do not show any observable paramagnetic shift, and the linewidths are very small compared to an aqueous iron(III) solution which contains the same amount of iron. Iron of ferritin is markedly different from that of aqueous iron(III), but quite resembles that of synthetic iron polymer. It has been shown that the parameter k, which is the linewidth for the NMR probe divided by the concentration of iron, is dependent on the content of the phosphorus in ferritin. The k value for glycylglycylglycylglycine has a maximum in the physiological pH range 6–7, whereas that for tertramethylammonium chloride in which the charge does not change in the whole pH range gives a sigmoidal curve. From the pH dependence of the k values for tetramethylammonium chloride, the pKa value of the phosphate group on the surface of the iron core has been estimated at 7.0±0.2.