CALORIMETRIC STUDIES OF THE N-TERMINAL HALF-MOLECULE OF TRANSFERRIN AND MUTANT FORMS MODIFIED NEAR THE FE3+-BINDING SITE

The effects of single amino acid substitution on the thermal stability of the N-terminal half-molecule of human transferrin and its iron-binding affinity have been studied by high-sensitivity scanning calorimetry. All site-directed mutations are located on the surface of the binding cleft, and they are D63 --> S, D63 --> C, G65 --> R, H207 --> E and K206 --> Q. Differential scanning calorimetry results show that the mutations do not significantly alter the conformational stability of the apo-forms of the proteins. The changes in free energy of unfolding relative to the wild-type protein range from 0.83 to - 2.4 kJ/mol. The D63 --> S, G65 --> R and H207 --> E mutations slightly destabilize the apo-protein, while the D63 --> C and K206 --> Q mutations increase its stability by a small amount. However, there are large compensating enthalpy-entropy changes caused by all mutations. All mutants bind ferric ion, but with different affinities. Replacement of Asp-63 by either Ser or Cys decreases the apparent binding constant by 5-6 orders of magnitude. The G65 --> R mutation also decreases the apparent binding constant by 5 orders of magnitude. The K206 --> Q mutation increases the apparent binding constant by 20-fold, while the H207 --> E mutation does not significantly change the apparent iron-binding affinity of the half-molecule.