<p>(Panel A) The secondary structure elements are labeled A–H’. The 20 <i>N</i>-terminal residues and the extended CE and FG loops that seal the heme pocket and prevent the access of small ligands to the heme distal cavity are in orange. The pre-A Z-helix is in green. The heme (red) is displayed edge on. The proximal HisF8 residue is shown on the left hand side of the heme. The picture includes the mutated SerE20 residue, located at the <i>C</i>-terminus of the E-helix. The HisF8 and SerE20 side chains and residues building up the heme distal pocket are drawn as skeletal models (C atoms yellow, N atoms blue, and O atoms red) and labeled. (Panel B) Mono views of “tunnel 1” (top) and “tunnel 2” (bottom) access sites in Ma-Pgb*. Helices flanking the tunnel entries are labelled. The heme group (seen through the tunnel apertures) is shown in red. The protein is correctly oriented in both images, to bring each tunnel in the direction of sight. The images are rotated by 90°. The pictures have been drawn by UCSF - Chimera <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0095391#pone.0095391-Pettersen1" target="_blank">[55]</a>. For details, see ref. <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0095391#pone.0095391-Nardini1" target="_blank">[11]</a>.</p
