Proximity of the substrate binding site and the heme-iron catalytic site in cytochrome P-450scc.

As an approach to "mapping" the active site of the cytochrome P-450 that catalyzes cholesterol side-chain cleavage, designated cytochrome P-450scc, we have synthesized steroid derivatives with the potential to interact with both the substrate binding site and the heme-iron catalytic site of the enzyme. The effects of these substrate analogs were studied with cytochrome P-450scc purified from bovine adrenal cortex. One derivative, 22-amino-23,24-bisnor-5-cholen-3 beta-ol, was found to be a potent inhibitor of pregnenolone formation in a reconstituted enzyme system, and a kinetic analysis of the inhibition showed that binding of the derivative is competitive with respect to cholesterol. The spectral properties of a stable complex formed between the steroidal amine and the purified cytochrome suggest that the 22-amine group coordinates directly to the heme-iron. A model for the structure of this inhibitor-enzyme complex is proposed in which the 5-androstene ring system of the steroid occupies the substrate binding site, and the amine group of the side chain occupies an axial coordination position of the Fe(III) center. This places limits on the distance between these two domains in the enzyme and offers support for proposed mechanisms of cytochrome P-450-catalyzed oxygen-insertion reactions in which an iron-bound oxidant directly attacks the substrate.