Chemical Protein Unfolding - A Simple Cooperative Model.

Area of Science:

• Biochemistry
• Chemical Physics
• Molecular Biology

Background:

• Chemical denaturation using guanidine hydrochloride (GdnHCl) or urea is standard for studying protein conformational stability.
• The linear extrapolation method (LEM) is commonly used for analyzing unfolding isotherms but yields counterintuitive positive free energy for native proteins.

Purpose of the Study:

• To develop a multistate cooperative model for protein unfolding that accounts for denaturant binding and cooperativity.
• To provide a more accurate thermodynamic description of protein chemical denaturation.

Main Methods:

• Developed a molecular statistical-mechanical partition function-based model for protein unfolding.
• Calculated binding isotherms and free energies using simple analytical solutions.
• Applied the model to 23 published unfolding isotherms of various proteins.

Main Results:

• The model predicts zero free energy for native proteins, aligning with expectations.
• Denaturant binding constants show temperature and pH dependence but minimal protein specificity.
• Chemical unfolding is less cooperative than thermal unfolding, with a significantly larger cooperativity parameter (σ).

Conclusions:

• The multistate cooperative model offers molecular insight into protein chemical denaturation.
• It provides a conceptually sound and easily applicable alternative to the LEM.
• The model accurately predicts unfolding thermodynamics, including entropy and enthalpy.