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Mapping the pathways for O2 entry into and exit from myoglobin
E E Scott1, Q H Gibson, J S Olson
1Department of Biochemistry and Cell Biology, Rice University, Houston, TX 77005, USA.
Abstract:
The effects of mutagenesis on geminate and bimolecular O2 rebinding to 90 mutants at 27 different positions were used to map pathways for ligand movement into and out of sperm whale myoglobin. By analogy to a baseball glove, the protein "catches" and then "holds" incoming ligand molecules long enough to allow bond formation with the iron atom. Opening of the glove occurs by outward movements of the distal histidine (His(64)), and the ligands are trapped in the interior "webbing" of the distal pocket, in the space surrounded by Ile(28), Leu(29), Leu(32), Val(68), and Ile(107). The size of this pocket is a major determinant of the rate of ligand entry into the protein. Immediately after photo- or thermal dissociation, O2 moves away from the iron into this interior pocket. The majority of the dissociated ligands return to the active site and either rebind to the iron atom or escape through the His(64) gate. A fraction of the ligands migrate further away from the heme group into cavities that have been defined as Xe binding sites 4 and 1; however, most of these ligands also return to the distal pocket, and net escape through the interior of wild-type myoglobin is <20-25%.
Insights
Mutagenesis studies reveal how sperm whale myoglobin
Area of Science:
- Biochemistry
- Protein Dynamics
- Molecular Biology
Background:
- Sperm whale myoglobin facilitates oxygen transport.
- Understanding ligand binding and release is crucial for protein function.
Purpose of the Study:
- To map ligand pathways into and out of myoglobin using mutagenesis.
- To investigate the role of the distal pocket and His64 in ligand dynamics.
Main Methods:
- Site-directed mutagenesis of 90 myoglobin variants at 27 positions.
- Analysis of geminate and bimolecular O2 rebinding kinetics.
Main Results:
- The distal pocket size significantly influences ligand entry rates.
- Distal histidine (His64) movements control ligand "glove" opening and closing.
- Most dissociated O2 molecules return to the active site, with <20-25% net escape.
Conclusions:
- Myoglobin's "glove-like" mechanism involves catching and holding ligands.
- Ligand "trapping" occurs within the distal pocket's internal "webbing."
- His64 acts as a gate for ligand entry and exit.