Calculation of the electronic structure and spectra of model cytochrome P450 compound I

The electronic structure acid spectra of the oxyferryl (Fe=O) compound I P4 50 heme species. the transient putative active intermediate of cytochrome P 450s, have been calculated employing a full protoporphyrin IX heme model re presentation. The principal aim of this work was to compare the computed sp ectra of this species with the observed transient spectra attributed to it. Computations were made using both nonlocal density functional theory (DFT) and semiempirical INDO/CI methods to characterize the electronic structure of the compound I P450 species. Both methods resulted in a similar antifer romagnetic doublet as the ground state with a ferromagnetic quartet excited state partner, slightly higher in energy. The INDO/ROHF/CI semiempirical m ethod was used to calculate the spectrum of the protoporphyrin IX P450 comp ound I heme species in its lowest energy antiferromagnetic doublet state at the DFT optimized geometry. As a reference, the spectrum of the ferric res ting form of the protoporphyrin IX P450 heme species was also calculated. T he computed shifts in the Soret and Q bands of compound I relative to the r esting state were both in good agreement with the corresponding experimenta lly observed shifts in the transient spectra of cytochrome P450cam (Biochem . Biophys. Res. Commun. 201 (1994) 1464) and chloroperoxidase (Biochem. Bio phys. Res. Commun. 94 (1980) 1123) both ascribed to their common compound 1 heme site. This consistency provides additional, independent support fur t he assignment of compound I as the origin of the reported observed transien t spectra. (C) 2001 Elsevier Science B.V. All rights reserved.