Comparative Fourier Transform Infrared and Circular Dichroism Spectroscopic Analysis of α1-Proteinase Inhibitor and Ovalbumin in Aqueous Solution

α1-Proteinase inhibitor (α1Pi) and ovalbumin are both members of the serpin superfamily. They share about a 30% sequence identity and exhibit great similarity in their three-dimensional structures. However, no apparent functional relationship has been found between the two proteins. Unlike α1Pi, ovalbumin shows no inhibitory effect to serine proteases. To see whether or not a conformational factor(s) may contribute to the functional difference, we carried out comparative analysis of the two proteinsʹ secondary structure, thermal stability, and H–D exchange using FT-IR and CD spectroscopy. FT-IR analysis reveals significant differences in the amide I spectral patterns of the two proteins. Upon thermal denaturation, both proteins exhibit a strong low-wavenumber β-sheet band at 1624 cm−1 and a weak high-wavenumber β-sheet band at 1694 cm−1, indicative of intermolecular aggregate formation. However, the midpoint of the thermal-induced transition of α1Pi (∼55°C) is 18°C lower than that of ovalbumin (∼73°C). The thermal stability analysis provides new insight into the structural changes associated with denaturation. The result of H–D exchange explains some puzzling spectral differences between the two proteins in D2O reported previously.