Calorimetric studies of serum transferrin and ovotransferrin. Estimates of domain interactions, and study of the kinetic complexities of ferric ion binding.

Human serum transferrin and hen ovotransferrin have been studied by differential scanning calorimetry (DSC), in an effort to quantitatively estimate the free energy of interaction of the N- and C-domains in each protein and to further understand their interaction with chelated ferric ions. In the case of serum transferrin, separate DSC transitions are observed for the two domains while only a single, coupled transition is seen for ovotransferrin. Although domain interactions are somewhat larger for ovotransferrin (-4100 cal/mol) than for serum transferrin (-3100 cal/mol), the major cause of separated transitions for serum transferrin is that the difference in intrinsic folding stability of the N- and C-domains is about 4-fold larger than for ovotransferrin. Chelated ferric ions bind strongly to each site in both proteins and produce changes in Tm by as much as 30 degrees C. When apparent binding constants are estimated from DSC results, these appear to be substantially larger than those estimated previously from equilibrium methods at low temperatures, where very long equilibrium times must be used because of slow ligand release. Although second DSC upscans on each protein show good "reversibility", downscans on serum transferrin revealed that liganded forms of the protein are in fact not in true equilibrium during upscanning, which causes Tm values during upscans to be higher than the true reversible Tm values. The likely reason for this kinetic control over unfolding is the slow release of bound ferric ions and those effects, for technical reasons, cannot be totally eliminated by lowering the scan rate.