Basic pancreatic trypsin inhibitor has unusual thermodynamic stability parameters.

Area of Science:

• Protein stability and thermodynamics
• Biophysical chemistry
• Structural biology

Background:

• Basic pancreatic trypsin inhibitor (BPTI) is a well-studied globular protein.
• Understanding protein stability is crucial for various biological and pharmaceutical applications.
• Previous studies have explored BPTI's structure and function, but detailed thermodynamic characterization under varying conditions is ongoing.

Purpose of the Study:

• To characterize the thermodynamic stability parameters (delta Gst, delta Hst, delta Sst) of native BPTI.
• To investigate the influence of pH, buffer solutions, and guanidine hydrochloride on BPTI unfolding.
• To elucidate the reasons behind BPTI's high stability and slow amide proton exchange rates.

Main Methods:

• Microcalorimetric unfolding studies were employed to measure heat absorption during protein denaturation.
• Experiments were conducted in various buffer solutions and at different pH values.
• The effect of guanidine hydrochloride concentration on BPTI unfolding was assessed.

Main Results:

• BPTI exhibits a very small dependence of unfolding enthalpy on temperature, differing from other globular proteins.
• BPTI possesses a high specific Gibbs energy of stabilization, correlating with slow amide proton exchange.
• Unfolding entropy of BPTI is lower than other proteins by 2.9 J/(K·residue), aligning with theoretical predictions regarding cross-link influence.
• An interaction enthalpy per site of -5.6 kJ/mol was calculated from unfolding in 6 M guanidine hydrochloride.

Conclusions:

• BPTI's unique thermodynamic properties, particularly its low unfolding entropy, contribute to its exceptional stability.
• The findings support theoretical models on the impact of cross-links on protein configurational entropy.
• The study provides a comprehensive thermodynamic characterization of BPTI, enhancing our understanding of protein stability mechanisms.