The Histidine gate in Myoglobin revisited

MARCELO A. MARTI

Parma

Congreso; XIX International Conference on Dioxygen Binding and sensing Proteins; 2012

<!-- @page { margin: 0.79in } P { margin-bottom: 0.08in } --> Since the elucidation of the myoglobin (Mb) structure, a histidine residue on the E helix (HisE7) has been proposed to act as a gate with an open or closed conformation controlling access to the active site. Although it is believed that at low pH the HisE7 gate is in its open conformation, the full relationship between the HisE7 protonation state, its conformation, and ligand migration in Mb is hotly debated. We used molecular dynamics simulations to first address the effect of HisE7 protonation on its conformation. We observed the expected shift from the closed to the open conformation upon protonation, but more importantly noted a significant difference between the conformations of the two neutral histidine tautomers. We further computed free energy profiles for oxygen migration in each of the possible HisE7 states as well as in two instructive Mb mutants: AlaE7 and TrpE7. Our results sho that even in the closed conformation the HisE7 gate does not create a large barrier to oxygen migration, and permits oxygen entry with only a small rotation of the imidazole side chain and movement of the E helix. We identify, instead, a hydrophobic site in the E7 channel that can accommodate an apolar diatomic ligand, and enhances ligand uptake particularly in the open HisE7 conformation. This rate enhancement is diminished in the closed conformation. Taken together, our results provide a new conceptual framework for the histidine gate hypothesis