Structure of cytochrome b5 in solution by Fourier-transform infrared spectroscopy

Fourier-transform infrared spectroscopy was used to examine the secondary structure of rabbit liver cytochrome b5 and the polar and nonpolar domains of the protein. The data for both the polar and nonpolar domains agree well with those previously obtained by other physical techniques. In particular it was found that the nonpolar membrane-binding domain was predominantly alpha helix and that the polar domain was also highly helical, but not all alpha helix. The independence of the two domains in the whole molecule was, in general, confirmed by the additivity of the spectra of the two domains. The small differences that were seen indicate that there is a loss of alpha helix when the protein is cut into the two domains. In addition, there appeared to be a slight difference in the exposure to solvent of the amide NH groups in the alpha-helical portion of the nonpolar domain when it was examined in isolation.