Role of Acidic Residues in the Interaction of NADPH-Cytochrome P450 Oxidoreductase with Cytochrome P450 and Cytochrome c

Abstract

Site-directed mutagenesis of the acidic clusters 207Asp-Asp-Asp209 and 213Glu-Glu-Asp215 of NADPH-cytochrome P450 oxidoreductase demonstrates that both cytochrome c and cytochrome P450 interact with this region; however, the sites and mechanisms of interaction of the two substrates are clearly distinct. Substitutions in the first acidic cluster did not affect cytochrome c or ferricyanide reductase activity, but substitution of asparagine for aspartate at position 208 reduced cytochrome P450-dependent benzphetamine N-demethylase activity by 63% with no effect on KP450m or KNADPHm. Substitutions in the second acidic cluster affected cytochrome c reduction but not benzphetamine N-demethylase or ferricyanide reductase activity. The E213Q enzyme exhibited a 59% reduction in cytochrome c reductase activity and a 47% reduction in KCyt cm under standard conditions (x0.27 M potassium phosphate, pH 7.7), as well as a decreased KCyt cm at every ionic strength and a shift of the salt dependence of cytochrome c reductase activity toward lower ionic strengths. The E214Q substitution did not affect cytochrome c reductase activity under standard conditions, but shifted the salt dependence of cytochrome c reductase activity toward higher ionic strengths. Measurements of the effect of ionic strength on steady-state kinetic properties indicated that increasing ionic strength destabilized the reductase-cytochrome c3+ ground state and reductase-cytochrome c transition state complexes for the wild-type, E213Q, and E214Q enzymes, suggesting the presence of electrostatic interactions involving Glu213 and Glu214 as well as additional residues outside this region. The ionic strength dependence of kcat/KCyt cm for the wild-type and E214Q enzymes is consistent with the presence of charge-pairing interactions in the transition state and removal of a weak ionic interaction in the reductase-cytochrome c transition-state complex by the E214Q substitution. The ionic strength dependence of the E213Q enzyme, however, is not consistent with a simple electrostatic model. Effects of ionic strength on kinetic properties of E213Q suggest that substitution of glutamine stabilizes the reductase-cytochrome c3+ ground-state complex, leading to a net increase in activation energy and decrease in kcat. Glu213 is also involved in a repulsive interaction with cytochrome c3+. Cytochrome c2+ Ki for the wild-type enzyme was 82.4 microM at 118 mM ionic strength and 10.8 microM at 749 mM ionic strength; similar values were observed for the E214Q enzyme. Cytochrome c Ki for the E213Q enzyme was 17.6 microM at 118 mM and 15.7 microM at 749 mM ionic strength, consistent with removal of an electrostatic repulsion between the reductase and cytochrome c2+.