Nature of Mesoscopic Aggregates in Solutions of Lysozyme

Abstract

An anomalous class of mesoscopic aggregates have previously been observed in solutions of lysozyme. These aggregates are thought to play an important role in nucleation of protein crystals and ordered protein aggregates, like amyloid fibers. Mesoscopic aggregates are currently thought to be in thermodynamic equilibrium with the protein solution, where transient oligomers of partially unfolded lysozyme monomers are thought to be the formation source of these aggregates. However, there is little experimental evidence to back up this proposed formation mechanism and thermodynamic behavior. Specifically, the effects of temperature on these aggregates and their thermodynamic reversibility have not been systematically tested. In this thesis, we investigate the equilibrium nature and the formation source of mesoscopic aggregates in solutions of model protein, lysozyme. We tested the effects of temperature on aggregate size and concentration and the aggregate reversibility after removal by systematic filtration. We used light and x-ray scattering and chromatography to experimentally characterize the aggregates during this study. Our findings indicate that mesoscopic aggregates are minimally sensitive to temperature changes and do not reform after removal by filtration. Together, these results indicate that mesoscopic aggregates are not in thermodynamic equilibrium with protein monomers or oligomers in solution. Overall, our experimental results contrast the current accepted formation mechanism of these mesoscopic aggregates and suggest they instead form due to contaminants present in solution or a sub-population of partially unfolded proteins.