Dual Role of Human Cytochrome P450 3A4 Residue Phe-304 in Substrate Specificity and Cooperativity

The structural basis of cooperativity of progesterone hydroxylation catalyzed by human cytochrome P450 3A4 has been investigated. A recent study suggested that substitution of larger side chains at positions Leu-211 and Asp-214 partially mimics the action of effector by reducing the size of the active site. Based on predictions from molecular modeling that Phe-304 in the highly conserved I helix is involved in both effector and substrate binding, a tryptophan residue was substituted at this position. The purified F304W mutant displayed hyperbolic progesterone hydroxylase kinetics, indicating a lack of homotropic cooperativity. However, the mutant remained responsive to stimulation by α-naphthoflavone, exhibiting a 2-fold decrease in the K m value for progesterone 6β-hydroxylation in the presence of 25 μM effector. Combining substitutions to yield the triple mutant L211F/D214E/F304W maintained the V max and decreased the K m for progesterone 6β-hydroxylation, minimized stimulation by α-naphthoflavone, and decreased the rate of α-naphthoflavone oxidation to one-eighth of the wild type. Interestingly, the Δ A max for spectral binding of α-naphthoflavone was unaltered in L211F/D214E/F304W. Overall, the results suggest that progesterone and α-naphthoflavone are oxidized at separate locations within the P450 3A4 binding pocket, although both substrates appear to have equal access to the reactive oxygen.