Thermal-induced Changes in the Secondary Conformation of Superoxide Dismutase Containing Different Metal Ions

Abstract

The thermal stability of three superoxide dismutases (SODs) with different metal ions (Mn, Cu/Zn, Fe) in the solid state was studied by a Fourier transform infrared (FT-IR) microspec-troscopy combined with thermal analyzer. The IR spectra showed a maximum peak at 1652 cm⁻¹ for all the native SODs in the amide I band, suggesting a predominant random coil with less α-helix structures. By heating each sample, a shoulder at 1631 cm⁻¹ in the amide I band gradually appeared from 45° C for Fe SOD and from 50° C for Mn SOD but another shoulder at 1639 cm⁻¹ appeared from 50° C for Cu/Zn SOD. The peak at 1631 cm⁻¹ is due to the intermolecular β-sheet structure, but the peak at 1639 cm⁻¹ corresponds to the major intramolecular β-sheet with less random coil structure. This reveals that in the first heating process the transformation from random coil/a-helix structure to β-sheet structure initiated from around 45–50° C. There was about 16–22% compositional change resulting from that transformation. However, both additional shoulders stood there and did not restore to their original spectra even with cooling to room temperature, suggesting the denaturation and irreversible properties of the solid SODs after heating. The thermal-dependent denaturation and irreversibility of Mn SOD, Cu/Zn SOD and Fe SOD were clearly evidenced by the increase in intramolecular and intermolecular β-sheet structure.