Distance between metal-binding sites in transferrin: energy transfer from bound terbium (III) to iron (III) or manganese (III).

The distance between the two metal-binding sites of human serum transferrin has been studied by observing energy transfer between an excited terbium ion bound at one site and a ferric (or manganic) ion bound at the other site of the same transferrin molecule. From the observed reduction in terbium lifetime (relative to that of terbium transferrin), it is concluded that the intersite distance is 3.55 +/ 0.45 nm. This distance is reconciled with two conflicting earlier reports that the separation between sites is greater than 4.3 nm [Luk, C.K. (1971) Biochemistry 10,2838-2844] or is equal to 2.5 +/ 0.2 nm [Meares, C.F., & Ledbetter, J.E. (1977) Biochemistry 16, 5178-5180]. The difficulty of accurately measuring the quantum yield of protein-bound terbium provides the principal source of uncertainty in these measurements.