Determination of ligand pathways in globins. Apolar tunnels versus polar gates

• Published in: Journal of Biological Chemistry
• Main Authors: Salter, M., Blouin, G., Soman, J., Singleton, E., Dewilde, S., Moens, L., Pesce, A., Nardini, M., Bolognesi, M., Olson, J.
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2012
• Subjects: Sperm whale
• Online Access: https://www.vliz.be/imisdocs/publications/346610.pdf

Although molecular dynamics simulations suggest multiple interior pathways for O 2 entry into and exit from globins, most experiments indicate well defined single pathways. In 2001, we highlighted the effects of large-to-small amino acid replacements on rates for ligand entry and exit onto the three-dimensional structure of sperm whale myoglobin. The resultant map argued strongly for ligand movement through a short channel from the heme iron to solvent that is gated by the distal histidine (His-64(E7)) near the solvent edge of the porphyrin ring. In this work, we have applied the same mutagenesis mapping strategy to the neuronal mini-hemoglobin from Cerebratulus lacteus (CerHb), which has a large internal tunnel from the heme iron to the C-terminal ends of the E and H helices, a direction that is 180° opposite to the E7 channel. Detailed comparisons of the new CerHb map with expanded results for Mb show unambiguously that the dominant (>90%) ligand pathway in CerHb is through the internal tunnel, and the major (>75%) ligand pathway in Mb is through the E7 gate. These results demonstrate that: 1) mutagenesis mapping can identify internal pathways when they exist; 2) molecular dynamics simulations need to be refined to address discrepancies with experimental observations; and 3) alternative pathways have evolved in globins to meet specific physiological demands.