The dinuclear iron-oxo ferroxidase center of Pyrococcus furiosus ferritin is a stable prosthetic group with unexpectedly high reduction potentials.

Recombinant ferritin from Pyrococcus furiosus expressed in Escherichia coli exhibits in EPR monitored redox titrations a mixed valence (Fe(3+)-Fe2+) S=1/2 signal at intermediate potentials that is a high-resolution homolog of the ferroxidase signal previously described for reconstituted horse spleen apo-ferritin. P. furiosus reconstituted apo-ferritin reduced to intermediate potentials exhibits the same mixed-valence signal, which integrates to close to one spin per subunit. The reduction potentials of +210 and +50 mV imply that the iron dimer is a stable prosthetic group with three redox states.