Monitoring the iron status of the ferroxidase center of Escherichia coli bacterioferritin using fluorescence spectroscopy.

Ferritins solubilize and detoxify the essential metal iron through formation of a ferric mineral within the protein's central cavity. Key to this activity is an intrasubunit catalytic dinuclear iron center called the ferroxidase center. Here we show that the fluorescence intensity of Escherichia coli bacterioferritin (BFR), due to the presence of two tryptophan residues (Trp35 and Trp133) in each of the 24 subunits, is highly sensitive to the iron status of the ferroxidase center and is quenched to different extents by Fe2+ and Fe3+. Recovery of the quench following oxidation of Fe2+ to Fe3+ at the ferroxidase center was not observed, indicating that the di-Fe3+ form of the center is stable. Studies of the single-tryptophan variants W35F and W133F showed that Trp133, which lies approximately 10 A from the ferroxidase center, is primarily responsible for the observed fluorescence sensitivity to iron, while studies of a stable E. coli BFR subunit dimer demonstrated that the observed quench properties are principally derived from the interaction of iron with tryptophan residues within the subunit dimer. A double-tryptophan variant (W35F/W133F) was found to exhibit fluorescence from the seven tyrosine residues present in each subunit, which was also sensitive to the iron status of the ferroxidase center. Finally, we demonstrate using Zn2+, a potent competitive inhibitor of Fe2+ binding and oxidation, that the fluorescence response can be used to monitor the loss of iron from the ferroxidase center.