Cytochrome P-450scc-substrate interactions. Role of the 3 beta- and side chain hydroxyls in binding to oxidized and reduced forms of the enzyme.

The steroid binding specificity of cytochrome P-450scc has been investigated for different oxidation/reduction and ligand-binding states of the enzyme (oxidized, reduced, oxygen-bound, and carbon monoxide-bound forms). The oxygen of the 3 beta-hydroxyl of cholesterol is important for the initial enzyme-substrate interaction. Significant binding requires the correct stereochemistry and appears to be controlled by the electron density on the 3 beta-oxygen. Interactions at this position (located at least 13 A from the heme iron) can modulate the heme midpoint potential. The binding site in this region contains a cleft which can accommodate up to two carbons joined in an ether linkage to the 3 beta-oxygen. The steroid intermediates of side chain cleavage (22R-hydroxycholesterol and 20 alpha,22R-dihydroxycholesterol) bind more tightly to the ferric enzyme than does cholesterol and utilize specific interactions of these side chain hydroxyls with a grouping(s) on the polypeptide chain (i.e. not with the heme iron). The interaction requires the correct stereochemistry; a 22S-hydroxyl cannot be readily accommodated in the binding site. The specificity of the interaction for hydroxyls at the 22R- versus the 20 alpha-position is altered upon reduction of the enzyme, indicating a reduction-induced conformational change in the active site. The specific interference of binding of 22R-hydroxy steroids by heme-bound carbon monoxide (but not oxygen), together with the known bond angles and distances for Fe-C-O and Fe-O-O, allows localization of the 22R-hydroxyl group on a line perpendicular to the heme plane, between 2 and 3 A from the iron.