Osmolyte-driven contraction of a random coil protein

Area of Science:

• Biochemistry
• Chemical Physics
• Protein Science

Background:

• Protein stability is crucial for biological function.
• Osmolytes are small molecules that can modulate protein structure and stability.
• Understanding osmolyte-protein interactions informs protein folding and stabilization mechanisms.

Purpose of the Study:

• To investigate the effect of protecting osmolytes and urea on the conformational changes of reduced and carboxamidated ribonuclease A (RCAM RNase).
• To elucidate the thermodynamic principles governing protein dimensional changes in response to different osmolytes.
• To compare the osmolyte-specific interactions with protein backbone and side chains.

Main Methods:

• Determining Stokes radius of RCAM RNase in water and 1 M osmolyte solutions.
• Measuring transfer Gibbs energy of RCAM RNase from water to various osmolytes.
• Analyzing contributions of peptide backbone and side chain interactions to overall transfer energy.

Main Results:

• RCAM RNase expands in urea but contracts in protecting osmolytes (trimethylamine N-oxide, sarcosine, sucrose, proline).
• Protein dimensional changes are proportional to the transfer Gibbs energy.
• Unfavorable osmolyte-backbone interactions dominate in protecting osmolytes, while favorable urea-backbone interactions dominate with urea.

Conclusions:

• Protecting osmolytes stabilize proteins by increasing the chemical potential of the denatured ensemble, leading to contraction.
• Hydrophobic side chains favor transfer to osmolytes, challenging the unique role of urea in hydrophobic solubilization.
• Osmolyte-induced protein contraction decreases conformational entropy and increases hydrophobic group density, promoting protein folding.