Fourier Transform Infrared Study of Lipoxygenase Conformation in Organic Solvent Media

Abstract

The secondary structure of commercially purified soybean lipoxygenase (EC 1.13.11.12) was investigated in selected monophasic organic solvents, including chloroform, methanol, acetonitrile, hexane, and octane. The Fourier transform infrared (FT-IR) spectra of the enzyme obtained in chloroform, methanol, and acetonitrile showed an absorption band at 1617 cm⁻¹ indicative of significant protein aggregation, whereas spectra of lipoxygenase in hexane and octane exhibited substantially less aggregate formation. Variable-temperature infrared studies of lipoxygenase in D₂O show that the predominately α-helical structure of the protein undergoes an irreversible transition to intermolecular β-sheet at and above 65 °C. Chemical imaging technology employing an FT-IR spectrometer equipped with an infrared microscope and a focal-plane array detector was used to examine the changes in the secondary structure of lipoxygenase at the water–hexane interface in the presence and absence of substrate. The secondary structure of lipoxygenase at the hexane–water interface was comparable to that of the structure of lipoxygenase in D₂O after exposure of lipoxygenase solution to hexane.