Fe3+ coordination and redox properties of a bacterial transferrin

The Fe(3+) binding site of recombinant nFbp, a ferric-binding protein found in the periplasmic space of pathogenic Neisseria, has been characterized by physicochemical techniques. An effective Fe(3+) binding constant in the presence of 350 microm phosphate at pH 6.5 and 25 degrees C was determined as 2.4 x 10(18) m(-1). EPR spectra for the recombinant Fe(3+)nFbp gave g' = 4.3 and 9 signals characteristic of high spin Fe(3+) in a strong ligand field of low (orthorhombic) symmetry. (31)P NMR experiments demonstrated the presence of bound phosphate in the holo form of nFbp and showed that phosphate can be dialyzed away in the absence of Fe(3+) in apo-nFbp. Finally, an uncorrected Fe(3+/2+) redox potential for Fe-nFbp was determined to be -290 mV (NHE) at pH 6.5, 20 degrees C. Whereas our findings show that nFbp and mammalian transferrin have similar Fe(3+) binding constants and EPR spectra, they differ greatly in their redox potentials. This has implications for the mechanism of Fe transport across the periplasmic space of Gram-negative bacteria.