Fourier transform infrared spectroscopy and differential scanning calorimetry of transferrins: human serum transferrin, rabbit serum transferrin and human lactoferrin

Abstract

Fourier transform infrared spectroscopy (FTIR) and differential scanning calorimetry (DSC) have been used to investigate the solution structure and thermal stability of human serum transferrin (HST), human lactoferrin (HLF) and rabbit serum transferrin (RST) in their diferric and apo forms. Our study shows that: (A) The secondary structure of all the proteins studied (estimated in H 2O) was in the range 43–53% α-helix and 23–28% β-sheet. These values differ markedly from previously reported circular dichroism (CD) data. This is attributed to the fact that FTIR and CD measure different aspects of secondary structure (hydrogen bonding and dihedral angles, respectively). (B) The secondary structural content of the proteins is not altered by iron binding or release. However, the iron-free proteins undergo a greater extent of 1H- 2H exchange than the diferric proteins indicating that significant structural changes do occur upon iron binding/release. (C) The removal of iron leads to thermal de stabilization of HST, HLF and RST. Structural variation in the apo transferrins is indicated by the observation of a single irreversible DSC transition for apo human lactoferrin, a double DSC transition for apo human serum transferrin (one reversible) and a broad irreversible asymmetric DSC transition for apo rabbit serum transferrin. FTIR spectroscopy shows that a distinct loss of protein secondary structure occurs at the transition temperatures shown by DSC.