Thermodynamic versus Conformational Metastability in Fibril-Forming Lysozyme Solutions

The role of intermolecular interaction in fibril-forming protein solutions and its relation with molecular conformation is a crucial aspect for the control and inhibition of amyloid structures. Here, we study the fibril formation and the protein-protein interactions of lysozyme at acidic pH and low ionic strength. The amyloid formation occurs after a long lag time and is preceded by the formation of oligomers, which seems to be off-pathway with respect to fibrillation. By measuring the osmotic isothermal compressibility and the collective diffusion coefficient of lysozyme in solution, we observe that the monomeric solution is kept in a thermodynamically metastable state by strong electrostatic repulsion, even in denaturing conditions. The measured repulsive interaction between monomers is satisfactorily accounted for by classical polyelectrolyte theory. Further, we observe a slow conformational change involving both secondary and tertiary structure, which drives the proteins toward a more hydrophobic conformation. Denatured proteins are driven out of metastability through conformational substates, which are kinetically populated and experience a lower activation energy for fibril formation. Thus, our results highlight the role of electrostatic repulsion, which hinders the aggregation of partially denatured proteins and operates as a gatekeeper favoring the association of those monomers whose conformation is capable of forming amyloid structure.