Ligand binding and conformational motions in myoglobin

Area of Science:

• Biochemistry
• Structural Biology
• Protein Dynamics

Background:

• Myoglobin (Mb) is a model protein for studying gaseous ligand binding dynamics.
• Ligand photodissociation in Mb reveals complex kinetic intermediates and protein relaxation.

Purpose of the Study:

• To investigate structural changes in myoglobin upon ligand dissociation using X-ray crystallography.
• To elucidate the pathways of ligand migration and rebinding within myoglobin at different temperatures.

Main Methods:

• X-ray crystallography of carbon monoxy-myoglobin (MbCO mutant L29W) crystals.
• Time-resolved infrared spectroscopy of carbon monoxide (CO) rebinding.
• Illumination of crystals below and above 180 K.

Main Results:

• Below 180 K, photodissociated ligands migrate to and rebind from specific sites within the distal pocket of immobilized Mb.
• Above 180 K, protein fluctuations open transient exit channels, allowing ligands to escape the distal pocket.
• Ligands were recovered in a cavity on the opposite side of the haem group at higher temperatures.

Conclusions:

• Ligand escape from myoglobin is temperature-dependent, facilitated by protein dynamics and transient channel openings.
• Protein structural fluctuations play a critical role in controlling ligand egress and ingress.
• Myoglobin's internal cavities act as crucial reservoirs and pathways for ligand migration.