Ligand migration in human myoglobin: Steric effects of isoleucine 107(G8) on O-2 and CO binding

To investigate the ligand pathway in myoglobin, some mutant myoglobins, in which one of the amino acid residues constituting a putative ligand-docking site, Ile107, is replaced by Ala, Val, Leu, or Phe, were prepared and thei r structural and ligand binding properties were characterized. The kinetic barrier for the ligand entry to protein inside was lowered by decreasing th e side-chain volume at position 107, indicating that the bulky side chain i nterferes with the formation of the activation state for the ligand migrati on and the free space near position 107 would be filled with the ligand in the activation state. Another prominent effect of the reduced side-chain vo lume at position 107 is to stabilize the ligand-binding intermediate state. Because the stabilization can be ascribed to decrease of the positive enth alpy, the enlarged free space near position 107 would relieve unfavorable s teric interactions between the ligand and nearby amino acid residues. The s ide-chain volume at position 107, therefore, is crucial for the kinetic bar rier for the ligand migration and free energy of the ligand-binding interme diate state, which allows us to propose that some photodissociated O-2 move s toward position 107 to be trapped and then expelled to the solvent.