Abstract

The active site topology of heterologously expressed CYP3A4 purified from an Escherichia coli expression system was examined using phenyldiazene. Incubation of CYP3A4 with phenyldiazene and subsequent oxidation yielded all four potential N-phenylprotoporphyrin IX regioisomers derived from attack on an available nitrogen atom in pyrrole rings B, A, C, or D (N_B:N_A:N_C:N_D = 6:73:7: 13). Further study using 28 active site mutants showed that substitution of residues closer to the heme, Ala-305, Thr-309, or Ala-370, with a larger residue caused the most drastic changes in regioisomer formation, which reflected the location of each amino acid residue replaced in a CYP3A4 homology model. Previous studies have suggested a conformational change in CYP3A4 upon binding of NADPH-cytochrome P450 reductase (CPR) or cytochrome b₅ (b₅). Therefore, regioisomer formation was also compared in the absence of redox partners and in the presence of CPR, b₅, or both. Formation of all four regioisomers in CYP3A4 wild type, particularly the minor ones, was reduced in the presence of b₅. CPR also greatly decreased the three minor isomers but increased the major isomer significantly. The presence of b₅ and CPR restored minor isomer formation and suppressed the enhancement of N_A formation caused by CPR alone. Interestingly, the effects of the redox partners differed among representative active site mutants. In particular, the increase in N_C upon substitution of Ala-370 with Phe was significantly reversed in the presence of redox partners, strongly suggesting that a conformational change occurs around pyrrole ring C due to protein-protein interactions between CYP3A4 and CPR or b₅.