Dissecting the Thermodynamics and Cooperativity of Ligand Binding in Cytochrome P450eryF

Conformational flexibility and cooperativity in ligand recognition are two key aspects of the catalytic diversity of cytochrome P450 enzymes. In this study, we dissect the ligand binding stoichiometry and energetics of the soluble bacterial P450eryF by isothermal titration calorimetry (ITC) using three allosteric and two non-allosteric ligands of diverse chemistry. Complementary spectral binding studies and sequential, two-ligand docking simulations were performed to help assign the binding sites. Binding of 4-phenylpyridine (4-PP) or 4-(4-chlorophenyl)imidazole (4-CPI) showed 1:1 stoichiometry in ITC, consistent with the lack of cooperativity observed in spectral binding studies. The larger ligands 9-aminophenanthrene (9-AP), 1-pyrenebutanol (1-PB), and α-naphthoflavone (ANF) show cooperative spectral binding and yielded 2:1 stoichiometry. The associated thermodynamic parameters for the sites were calculated using a sequential binding mechanism. The binding constant (K_D) for the first site was almost two times lower than that of the second site for all three compounds. Ligand binding at site 1 was entropically favored, whereas binding at site 2 was weak and entropically unfavorable. Simulations showed that two molecules of 9-AP, ANF or 1-PB can be adequately docked to two individual sub-sites within a large binding pocket. The absence of hydrophobic tethering and ligand stacking are consistent with the single low affinity binding site observed for 4-CPI and 4-PP. Competitive binding studies with P450eryF preloaded with either 1-PB or ANF showed a decrease in the affinities for 9-AP at both the sites, with large entropy−enthalpy compensation, indicating the ability of the binding pocket to accommodate two ligands of diverse chemistry and enable cooperativity.