Electron paramagnetic resonance measurement of the distance between the metal binding sites of transferrin.

The distance between the two metal binding sites of human serum transferrin is calculated from measurements made of the paramagnetic broadening of the EPR spectrum of Cu2+ bound at the NH2-terminal (B) site from Fe3+ bound at the COOH-terminal (A) site. Complexes of monoferric(A)-monocupric(B) transferrin, monocupric(B) transferrin, and monogallium(A)-monocupric(B) transferrin were prepared under identical conditions. Computer simulation and direct measurement of the EPR spectra from these complexes demonstrate a paramagnetic broadening of 0.6 +/- 0.1 G in the line width of the Cu2+ spectrum due to the paramagnetic Fe3+ bound at the other site. The distance between the two metal binding sites is calculated to be 41.6 +/- 2.8 A.