Fe3+ coordination and redox properties of a bacterial transferrin

The Fe3+ 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 Fe3+ binding constant in the presence of 350 muM phosphate at pH 6.5 and 25 degreesC was determined as 2.4 x 10(18) M-1. EPR spectra for the recombinant Fe3+ nFbp gave g' = 4.3 and 9 signals characteristic of high spin Fe3+ in a strong ligand field of low (orthorhombic) symmetry. P-31 NMR experiments demonstrated the presence of bound phosphate in the hole form of nFbp and showed that phosphate can be dialyzed away in the absence of Fe3+ in apo-nFbp, Finally, an uncorrected Fe3+/2+ redox potential for Fe-nFbp was determined to be -290 mV (NHE) at pH 6.5, 20 degreesC, Whereas our findings show that nFbp and mammalian transferrin have similar Fe3+ 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.