Switch in the Aggregation Pathway of Bovine Serum Albumin Mediated by Electrostatic Interactions

A strong denaturant, guanidinium hydrochloride (GdnHCI), is shown to delay and alter the inherent aggregation pathway of bovine serum albumin (BSA) from a downhill polymerization to a nucleated polymerization. We hypothesize that such an alteration is closely connected to the conformational population of the protein, and ion-binding to such an ensemble. Hindered molecular collisions due to electrostatic repulsions in an ion-bound denatured ensemble increase the activation barrier for aggregation to such an extent that the growth, which was spontaneous in the absence of any cosolute, goes through an unfavorable nucleation phase. Our study shows that the behavior in GdnHCI is not unique to it, but occurs in a certain class of cosolutes-those which are charged and bind to BSA. Variation in pH of the medium, which gives rise to extra charges on the protein backbone, also showed such repulsive effects, further confirming the involvement of electrostatic interactions. We have further shown that the coexistence of both an appropriate population and an appropriate cosolute is necessary. An absence of either of these prevents a switch in the pathway.