Comparison of human cytochrome P450 enzyme active sites. A, CYP2E1 (PDB 3E6I); B, CYP2A6 (PDB 1Z10); C, CYP2A13 (PDB 2P85); D, CYP1A2 (PDB 2HI4); E, CYP2C9 (1OG2); F, CYP2C8 (PDB 1PQ2); G, CYP2D5 (PDB 2FQ9); H, CYP3A4 (PDB 1TQN).