title: Magnetic circular dichroism studies of cytochrome P-450cam. Characterization of axial ligands of ferric and ferrous low-spin complexes.
abstract: MCD was applied to ferric and ferrous low-spin complexes of cytochrome P-450 cam to elucidate the electronic states and the nature of the axial ligands of the heme in cytochrome P-450cam. (1) Low-spin complexes of ferric cytochrome P-450cam, produced either by ligation of external ligands such as pyridine and imidazole derivatives or by being freed of (-)-camphor, showed sinusoidal Soret and alpha-MCD bands. The magnitude ratio of the Soret vs. alpha-MCD bands was quite sensitive to the nature of axial ligands of the ferric low-spin complexes. The ratio (2.7) for the camphor-free form of cytochrome P-450cam, thus, was the smallest among those (2.7-9.0) for low-spin forms of cytochrome P-450cam and other corresponding low-spin hemoproteins (ratio 7.8-13.9). The ratio (4.2) for the alpha-picoline-bound form of cytochrome P-450cam, however, was the closest to that (2.7) for the camphor-free form of cytochrome P-450cam among those (4.2-9.0) for the external ligand-bound form of cytochrome P-450cam. The ratio for the 2-methylimidazole-bound form of cytochrome P-450cam was the smallest among those of cytochrome P-450cam bound with imidizole derivatives. Thus, among the nitrogen-bound low-spin forms, the low-spin form with a sterically hindered nitrogen ligand trans to the thiolate anion (-S-) most reproduced spectral characteristics of the native low-spin ferric form. Low-temperature absorption studies offered the same results. (2) It was found that MCD magnitudes of alpha-bands of ferrous low-spin complexes are intimately related to the electronic character of axial ligands. Thus, the CO, O2 and NO-bound forms of cytochrome P-450cam, which have two pi-type axial ligands, showed the smallest alpha-MCD bands ([theta]M = 5.2-7.5) among complexes, while ferrous cytochrome b5 and cytochrome c, which have two sigma-electron-donating axial ligands, showed the largest magnitude ([theta]M = 120-176). The data for the ferrous low-spin complexes of other hemoproteins so far available were well rationalized in consideration of the property of the axial ligands.
mesh_terms: D000818:Animals; D002942:Circular Dichroism; D003577:Cytochrome P-450 Enzyme System / Q000096:biosynthesis; D003580:Cytochromes; D015786:Cytochromes b5; D006418:Heme; D008024:Ligands; D008748:Methylcholanthrene / Q000494:pharmacology; D008862:Microsomes, Liver / Q000187:drug effects / Q000378:metabolism; D010634:Phenobarbital / Q000494:pharmacology; D011485:Protein Binding; D011487:Protein Conformation; D011549:Pseudomonas / Q000378:metabolism; D011817:Rabbits
chemical_list: D003580:Cytochromes; D008024:Ligands; D006418:Heme; D008748:Methylcholanthrene; D015786:Cytochromes b5; D003577:Cytochrome P-450 Enzyme System; D010634:Phenobarbital