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Probing heme protein conformational equilibration rates with kinetic selection
W D Tian1, J T Sage, P M Champion
1Department of Physics, Center for Interdisciplinary Research on Complex Systems, Northeastern University, Boston, Massachusetts 02115, USA.
Biochemistry
|March 19, 1996
Summary
Protein conformational changes in carbonmonoxymyoglobin (MbCO) occur on a timescale of 10^-6 to 10^-4 seconds. These dynamics influence ligand rebinding kinetics, revealing distinct pathways for ligand escape.
Area of Science:
- Biophysics
- Protein dynamics
- Myoglobin kinetics
Background:
- Carbonmonoxymyoglobin (MbCO) exhibits complex conformational substates.
- Understanding protein conformational averaging is crucial for ligand binding dynamics.
- Previous models did not fully account for protein relaxation timescales.
Purpose of the Study:
- To determine the time scale of protein conformational averaging in MbCO.
- To investigate the influence of conformational substates on ligand rebinding kinetics.
- To analyze the role of solvent composition and temperature on MbCO dynamics.
Main Methods:
- Double-pulse flash photolysis experiments on MbCO solutions.
- Kinetic analysis of geminate and bimolecular rebinding phases.
- Spectroscopic determination of open and closed state populations.
Main Results:
- Interconversion between MbCO 'open' and 'closed' states occurs in 10^-6–10^-4 s.
- Geminate rebinding kinetics are a superposition of contributions from open and closed states.
- Solvent viscosity significantly alters interconversion rates and ligand escape pathways.
Conclusions:
- Protein conformational averaging is a key factor in MbCO ligand dynamics.
- The 'open' state facilitates ligand escape from the heme pocket.
- Solvent composition fundamentally modulates protein-ligand interactions and dynamics.