Comparative Fourier Transform Infrared Studies of the Secondary Structure and the CO Heme Ligand Environment in Cytochrome P-450cam and Cytochrome P-420cam

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Abstract
For the first time, Fourier transform infrared spectroscopy has been applied to cytochrome P-450 to analyze the protein secondary structure. From Fourier self-deconvolution and fitting the infrared spectra in the amide I‘ region (1600−1700 cm-1), we estimate 44% α-helix, 31% β-sheet, and 18% turns for substrate-free cytochrome P-450cam. In the presence of camphor, 54% α-helix and 310-helix, 21% β-sheet, and 21% turns are obtained which agree with the crystallographic data of 53% α-helix, 19% β-sheet, and 16% turns [Poulos, T. L., Finzel, B. C., & Howard, A. J. (1987) J. Mol. Biol.195, 687−700]. Cytochrome P-420cam is produced from substrate-free cytochrome P-450cam in two ways: (i) by temperature elevation up to 60 °C and (ii) by exposure to KSCN up to 1.5 M. The secondary structure composition is determined for each temperature and KSCN concentration and compared with the changes observed in the iron ligand CO stretch vibration bands appearing between 1900 and 2000 cm-1. Thermally induced cytochrome P-420 has an α-helix content of 19%, a β-sheet content of 53%, 14% turns, and 5% antiparallel β-sheets from intermolecular hydrogen bonds within protein aggregates. The formation of cytochrome P-420 as a function of the KSCN concentration indicates two types of cytochrome P-420. Up to 1 M KSCN, the induced cytochrome P-420 displays only little modification of the secondary structure, whereas at 1.5 M KSCN, larger changes are observed, resulting in 85% cytochrome P-420 without protein precipitation and containing 30% α-helix, 48% β-sheet, and 17% turns. Infrared spectra in the iron ligand CO stretch region show several subconformers for cytochrome P-420. During the cytochrome P-420 formation, the CO stretch modes are shifted to higher frequencies by 3−11 cm-1, with a main feature at about 1964 cm-1, compared to those of substrate-free cytochrome P-450cam−CO.