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Conformational substates in azurin
1Department of Physics, University of Illinois, Urbana-Champaign 61801.
Summary
Azurin protein forms a complex with nitric oxide (NO). Studying its temperature-dependent binding reveals internal recombination and bimolecular rebinding, suggesting similar conformational dynamics across diverse proteins.
Area of Science:
- Biochemistry
- Biophysics
- Protein Dynamics
Background:
- Azurin is a blue copper protein central to electron transfer in denitrifying bacteria.
- It forms a temperature-sensitive, photolabile complex with nitric oxide (NO).
Purpose of the Study:
- To investigate the temperature dependence of azurin-nitric oxide (NO) binding kinetics.
- To explore the mechanisms of ligand association and dissociation in azurin across a broad temperature range.
Main Methods:
- Studied temperature dependence of ligand binding equilibrium and photodissociation kinetics from 80-280 K.
- Analyzed NO rebinding kinetics over timescales from 10^-6 to 10^2 seconds.
- Modeled rebinding using Arrhenius law and analyzed concentration-dependent kinetics.
Main Results:
- Observed nonexponential, NO concentration-independent internal recombination below 200 K.
- Identified a slower, exponential, bimolecular rebinding process above 200 K.
- Found Arrhenius parameters for internal recombination: preexponential factor 6.3 x 10^8 s^-1, Gaussian enthalpy barriers centered at 23 kJ/mol (width 11 kJ/mol).
Conclusions:
- Azurin exhibits distinct internal recombination and bimolecular rebinding mechanisms for NO.
- The binding kinetics of NO to azurin show striking parallels with carbon monoxide binding to myoglobin.
- Conformational substates are a general feature of proteins, extending beyond heme proteins to those with different active sites and structures.
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