Energy transfer between terbium and iron bound to transferrin: Reinvestigation of the distance between metal-binding sites

The addition of trivalent iron, gallium, and terbium ions to the metal binding sites of human transferrin has been investigated by fluorescence and spectrophotometric measurements. Results are consistent with the possibility that the addition of ferric nitrilotriacetate to apotransferrin does not lead to a random distribution of iron bound to the two metal-binding sites on the protein, but rather an asymmetric distribution with iron bound mainly to one of the sites. The subsequent addition of terbium leads to the binding of terbium ions to the vacant sites on monoferric transferrin molecules, and observations of the intensity of terbium fluorescence from such samples provides clear evidence of transfer of excitation energy from the terbium site to the iron site. These results lead to the estimate that the two metal-binding sites of human transferrin are separated by a distance of 25 ± 2 Å, in disagreement with an earlier report (Luk, C. K. (1971), Biochemistry 10, 2838) that the sites were separated by more than 43 Å. Consideration of the dimensions of transferrin indicates that the two sites lie relatively close to each other on the macromolecule.