Phosphate accelerates displacement of Fe(III) by Fe(II) in the ferroxidase center of Pyrococcus furiosus ferritin

The iron-storage protein, ferritin, is widely found in all Domains of life. A conserved diiron center in ferritin catalyzes oxidation of Fe(II) and regulates storage of the resultant Fe(III) oxidation product. When this center is filled with Fe(III), in bacterial or archaeal ferritin the presence of phosphate accelerates the rate of Fe(II) oxidation. The molecular mechanism underlying this stimulatory effect of phosphate is unknown. Using site directed mutagenesis of the residues in the diiron center of the archaeal ferritin from Pyrococcus furiosus we show that phosphate facilitates displacement of Fe(III) by Fe(II) from this site. Therefore, the rate of Fe(II) oxidation increases only when the ferroxidase center is filled with Fe(III).