Fourier-transform infrared spectroscopy study of the pressure-induced changes in the structure of the bovine alpha-lactalbumin: the stabilizing role of the calcium ion.

The Fourier-transform infrared spectroscopy (FTIR) technique with a diamond anvil cell has been applied for examination of the pressure-induced changes occurring in the secondary structure of the alpha-lactalbumin. This is the first high-pressure FTIR study of a calcium-binding protein which simultaneously takes into account spectral changes in both the calcium-ion-binding carboxyl groups' band and the amide I/I' vibrational band. Spectral behavior of three kinds of the protein: the undeuterated holoform, the fully deuterated holoform, and the undeuterated apoform was compared in the pressure range from 0.1 MPa up to 740 MPa. We found that the binding of calcium remarkably stabilizes the alpha-lactalbumin against pressure as it is followed approximately by a 200-MPa increase of the value of pressure at which denaturation occurs. A quantitative analysis of the band of antisymmetrical stretching vibrations of the calcium-binding carboxyl groups revealed that the pressure-induced changes in the calcium-binding loop occur in two stages. Binding of the calcium ion seemingly increases the pressure-stability of the calcium-binding loop to a higher degree than the pressure-stability of the secondary structure of the alpha-lactalbumin. We have also discussed in detail the complex pressure-enhanced H/D exchange in the alpha-lactalbumin. Finally, we have proposed a new assignment of major peaks in the helical region of the amide I/I' spectral band of the partially deuterated alpha-lactalbumin.