Spectroscopic investigations of ferric cytochrome P-450-CAM ligand complexes. Identification of the ligand trans to cysteinate in the native enzyme.

An extensive series of ligand complexes of ferric cytochrome P-450-CAM has been examined by UV-visible absorption, magnetic circular dichroism, and electron paramagnetic resonance spectroscopy in an attempt to identify the ligand trans to cysteinate in the six-coordinate resting state of the enzyme. Thus, the ligands used have been chosen to serve as models for coordination by potential endogenous amino acids and include alcohol, amide and carboxylate oxygen donors, amine, imidazole and indole nitrogen donors and disulfide, thioether, thiol, and thiolate sulfur donors. As this investigation has been by nature an empirical one, the conclusions are strengthened by the concurrent use of three different spectroscopic techniques. All of the complexes formed except those resulting from thiolate addition display spectroscopic properties that are broadly similar to those of low spin, six-coordinate P-450. Of the sulfur donor adducts, disulfide and thioether-bound P-450 have properties that are different enough in detail to distinguish them from native P-450. While the spectral features of the thiol-bound species and of low spin ferric P-450 are alike, the former are pH dependent due to interconversion to bound thiolate, whereas the latter display essentially no spectral changes with pH. Of the oxygen donor complexes, all but carboxylate have spectra that very closely match those of the resting enzyme. Adducts formed with most nitrogenous ligands, including several imidazole derivatives, exhibit spectra that are sufficiently different from native P-450 to exclude them as candidates for the sixth ligand. Interestingly, the spectral properties of a complex formed with an imidazole derivative having a bulky electron-withdrawing substituent in the alpha position are comparable to those native P-450 except for the line shape of the EPR spectrum. Previously published theoretical work suggests that the spectral differences seen between this imidazole derivative and the other examined are electronic and not steric in origin. As no similar electronic mechanism exists for the protein to reduce the electron-donating ability or histidine, it is felt that coordination of histidine in the sixth position of P-450 can be ruled out. In conclusion, close examination of all spectral data reveals that amino acid analog adducts of P-450-CAM with amides and, in particular, alcohols, produce spectra that almost exactly duplicate those of native P-450 and suggests that the ligand trans to cysteinate in the six-coordinate ferric enzyme has an oxygen donor atom.