Do Macromolecular Crowding Agents Exert Only an Excluded Volume Effect? A Protein Solvation Study

Area of Science:

• Biophysics
• Protein Dynamics
• Macromolecular Crowding

Background:

• Traditionally, macromolecular crowding's effects on protein structure and dynamics were attributed to excluded volume effects, an entropic phenomenon.
• Recent research indicates a shift towards considering soft, enthalpic interactions as significant contributors.
• Understanding these interactions is crucial for comprehending protein behavior in crowded cellular environments.

Purpose of the Study:

• To investigate the role of soft interactions in macromolecular crowding on protein solvation and dynamics.
• To differentiate between hard-sphere (excluded volume) and soft-sphere (enthalpic) interactions.
• To explore the implications of these interactions on protein function under varying crowding conditions.

Main Methods:

• Utilized low concentrations (1-10 g/L) of synthetic (dextran, poly(ethylene glycol)) and protein (α-synuclein, myoglobin) crowders.
• Studied the solvation of 1-anilinonaphthalene-8-sulfonate (ANS) bound to serum proteins (bovine serum albumin, human serum albumin).
• Analyzed protein solvation as a spectral signature to capture local dynamics.

Main Results:

• Protein solvation was significantly modulated in a protein- and crowder-dependent manner.
• Observed effects at low crowder concentrations suggest direct evidence of soft interactions.
• Protein solvation serves as a sensitive indicator for local dynamics, distinguishing hard from soft interactions.

Conclusions:

• Soft, enthalpic interactions play a crucial role in modulating protein solvation and dynamics, even at low macromolecular crowding levels.
• Protein solvation dynamics can capture subtle local changes, decoupling hard-sphere from soft-sphere effects.
• These findings suggest that functionally important protein motions are modulated by soft interactions before significant excluded volume effects become apparent in vivo.