Cytochrome P-450scc-adrenodoxin interactions. Ionic effects on binding, and regulation of cytochrome reduction by bound steroid substrates.

The binding of adrenodoxin to cytochrome P-450scc and the intracomplex electron transfer from the iron-sulfur center to the heme have been studied. Salt sensitivity of the protein complex suggests the participation of electrostatic forces, as is also seen for the complex of adrenodoxin with NADPH-adrenodoxin reductase. Differences in ion specificities for the complexes of adrenodoxin with the other two proteins suggest some differences in binding requirements. Insensitivity of the heme reduction to solution conditions (salt, detergent) and kinetic analysis indicate that the protein complex is formed rapidly and that intracomplex electron transfer then occurs more slowly. Factors governing the rate of this electron transfer were investigated; binding of a series of cholesterol derivatives was used to perturb the spin state, midpoint potential, and reduction rate of the heme, and thus to test for relationships among these parameters. A linear free energy relationship between the substrate-induced midpoint potential and reduction rate is seen, but none of the other parameters (including the strength of substrate binding) are correlated. Data indicate that factors other than spin state (i.e. steric requirements and bonding groups within the steroid-binding site) regulate the strength of steroid binding. The bound steroid then modulates both midpoint potential/reduction rate and spin state but by independent mechanisms.