Cytochrome P450 access and egress channels calculated by MOLE 2.0. Channels were imposed on a cartoon representation of structures of cytochrome P450 BM3 (in views A and B; PDB structure 1BU7 was used) and cytochrome P450 CAM (in views C and D; PDB 1RE9 was used). Important secondary structures are colored as follows: the I helix is yellow, the F/G-loop is orange, the B/C-loop is red and the heme cofactor is shown as black balls. The images on the left (A and C) show views from the side in a plane horizontal to the plane of the heme; the images on the right (B and D) show views from above the distal side. Arrows indicate the viewpoints of the respective images. Channels are shown as connected spheres colored as follows: on the proximal side, channel W is colored in cyan; on the distal side channel S is shown in red; 2a–blue; 2ac–brown; 2b–light green; 2c–green; 2d–pink; 2f–magenta.