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

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Abstract
The addition of trivalent Fe, Ga, and Tb ions to the metal binding sites of human transferrin was 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 Fe bound to the 2 metal-binding sites on the protein, but rather an asymmetric distribution with Fe bound mainly to 1 of the sites. The subsequent addition of Tb leads to the binding of Tb ions to the vacant sites on monoferric transferrin molecules, and observations of the intensity of Tb fluorescence from such samples provides clear evidence of transfer of excitation energy from the Tb site to the Fe site. There results lead to the estimate that the 2 metal-binding sites of human transferrin are separated by a distance of 25 .+-. 2 .ANG., in disagreement with an earlier report that the sites were separated by more than 43 .ANG.. Consideration of the dimensions of transferrin indicates that the 2 sites lie relatively close to each other on the macromolecule.

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