[Electron structure of the heme of reduced cytochrome P450 and P420 from the data of low-temperature magnetic circular dichroism].

Magnetic circular dichroism spectra (MCD) of reduced cytochromes P450 and P420 from rabbit liver microsomes have been recorded and analyzed for the 350-600 nm spectral region in the temperature interval from 2 to 290 K. The shape, intensity and temperature dependence of the MCD of reduced P450 in the Soret region are quite different from that of other high-spin ferrous hemoproteins, whose heme iron is coordinated to the imidazole of histidine (deoxymyoglobin, deoxyhemoglobin, reduced peroxidase and cytochrome c oxidase). Assuming that in the reduced P450 as well as in its CO-complex the protein-derived ligand is mercaptide (RS-) the differences can be explained by the existence of two electronic transitions in the Soret region: the common for hemoproteins pi----pi porphyrin transition and sulfur to porphyrin charge-transfer transition, p+(Sp)----eg (pi). The unusual spectral characteristics of the CO-complex of P450 have been ascribed earlier to strong configurational interaction of these two transitions. From the similarities of the Soret MCD and their temperature dependences for the reduced P420 and for other high-spin ferrous hemoproteins one can conclude that heme iron of the reduced P420 is high-spin and is coordinated to the imidazole of histidine. The zero-field splitting parameter D of the spin Hamiltonian has been estimated from the MCD temperature dependences. The obtained splitting of approximately 30 cm-1 for P450 and of approximately 10 cm-1 for P420 exceeds that for myoglobin and hemoglobin (approximately 5 cm-1).