Structural dynamics of myoglobin: an infrared kinetic study of ligand migration in mutants YQR and YQRF

Recombination of carbon monoxide to myoglobin mutants YQR and YQRF was studied using transient infrared absorption spectroscopy and Fourier transform infrared–temperature derivative spectroscopy (FTIR–TDS). Photoproduct states B, C′, C″ and D associated with ligands residing in different protein cavities have been identified. After photolysis, ligands migrate to primary docking site B and subsequently rebind or escape to a secondary site (C) within the Xe4 cavity. For YQR, a global analysis of the isothermal rebinding kinetics below 160 K and the TDS data reveal a correlation between the enthalpy barriers governing the two processes. Above 120 K, a protein conformational change in both YQR and YQRF converts photoproduct C′ into C″ with markedly slowed kinetics. Above ∼180 K, ligands migrate to the proximal Xe1 site (D) and also exit into the solvent, from where they rebind in a bimolecular reaction.