The Heme Protein P450 from Adrenal Cortex: Its Reactivities in Ferric and Ferrous Forms

In the course of this reaction cycle the heme protein undergoes a series of changes of its coordination around the Fe (on reduction to its »unliganded~ Fe2+ state, on binding of the 0 2 molecule, and upon introduction of the second reducing equivalent, as the heme protein returns to its original Fe3+ state}. Steroid substrates, inhibitors, and a variety of agents bind to the heme protein P450(Fe3+), isolated in a S = 1/2 form from bovine adrenal glands, with high affinities at sites near the heme group, but do not as a rule enter the coordination sphere of the Fe. This leads to perturbations of the electronic structure which can be followed by spectroscopic techniques (optical absorption, EPR spectroscopy) in combination with suitable chemical methods. Direct replacement of a ligand can also be achieved. The ferrous heme protein P450(Fe2+), in its unliganded and liganded forms, was also investigated. P450(Fe2•) combines readily with small ligands such as 0 2 (418 nm), CO (448 nm), and nitric oxide. Larger lipophilic molecules (e. g. pyridine derivatives, other heterocyclic compounds, haloalkanes, or hydroperoxides) also bind readily to P450(Fe2+), often with high affinities. They tend to enter the coordination sphere of the Fe and form stable complexes often with distinct optical absorption (440-470 nm); additional unspecific binding is frequently observed. Representative examples of the results for the various cases of interactions are presented and are discussed in relation to a hypothetical structure of P450(Fe3• ), S = 1/2, as a protoheme IX complex, with a thiol-group and a N-containing group as axial ligands; the observations are further related to the required changes in coordination in the course of the hydroxylation reaction