Evidence of conformational changes in the non-equivalent binding sites of human serum transferrin.

Samples of monoferric human serum transferrin have been prepared in which the iron occupies predominantly the N-site (sample A) and the C-site (sample B). 111In was then added in concentrations small enough to ensure that there was always an excess of specific binding sites. Because of the presence of apo-transferrin in both the samples, the occupancy by 111In in the two sites was only 75-78% C-site in sample A and only 61-65% N-site in sample B. Time differential PAC spectra showed a transition in the quadrupole frequency which took place at different temperatures, approximately 275 K in sample A and between 290 and 305 K in sample B. Debye and Arrhenius plots of the temperature dependence of the correlation time associated with molecular reorientation indicated an effective molecular volume about 50% larger than that of the hydrated diferric molecule determined by "biochemical" methods, and an activation energy for re-orientation of approximately 0.065 eV.