Critical role of electrostatic interactions in protein kinetic stability: Kinetic studies of unfolding ubiquitin using SDS at varying pH values.

Protein stability in surfactant environments is crucial for various applications including industrial enzyme formulations. The interaction between Ubiquitin (UBI) and Sodium Dodecyl Sulfate (SDS) is investigated to understand the effect of pH (2-9.5) on the unfolding of UBI in SDS. We carried out kinetic studies in which tyrosine fluorescence (Yfl), circular dichroism (CD) and Small-angle X-ray scattering (SAXS) monitored changes in UBI's tertiary and secondary structure and structural dynamics of the UBI-SDS system from initial interactions to endpoint complex formation. All approaches concurred in demonstrating that the unfolding rate of UBI in SDS increased significantly with decreasing pH, with an inflexion point around UBI's isoelectric point due to changes in its net charge. The study demonstrates that the highly stable UBI unfolds by a similar mechanism across the studied pH range, through an asymmetric core-shell structure that leads to protein decorated micelles with an increase in alpha helical structure. However, the kinetics of the process and the UBI-SDS aggregation states are highly dependent on electrostatic interactions between UBI and SDS.