[Comparison of the heme electron state of reduced cytochrome P450 and P420 in equilibrium and non-equilibrium protein conformations. The nature of the protein heme ligand].

Magnetic circular dichroism (MCD) spectra of reduced cytochromes P450 and P420 in equilibrium and non-equilibrium protein conformations are compared at 4.2 K for the 350-800 spectral region. Non-equilibrium forms have been produced by photolysis of CO-complexes at 4.2 K. The differences between MCD spectra of proteins in equilibrium and non-equilibrium conformations, in particular for the visible region, show clearly the structural changes in the heme iron coordination sphere to occur on ligand binding. The comparison of the Soret MCD spectra of reduced proteins in their equilibrium and non-equilibrium forms with those of other high-spin ferrous hemoproteins suggest that mercaptide (RS-) is the protein ligand of the heme iron in reduced P450, as well as in its CO-complex, and that imidazole of histidine is the fifth ligand of the iron both in reduced P420 and its CO-complex. The thermal recombination of the photoproducts with CO have been studied. When temperature rises from 4.2 to 77 K for two hours both proteins have similar temperature characteristics during the recombination processes. The recombination begins at T approximately equal to 10 K and is completed at approximately equal to 50 K. The temperature at which half of the total photolyzed molecules are restored to the CO-form is equal to 25 K. For products of photolysis of CO-complexes of myoglobin and hemoglobin under the same heating conditions these temperatures are equal to 35 and 23 K respectively. Thus, the photoproducts of P450, P420 and hemoglobin have similar parameters of low-temperature recombination and the kinetics of this process is faster than for photodissociated myoglobin.