Nonnative Electrostatic Interactions Can Modulate Protein Folding: Molecular Dynamics with a Grain of Salt

In recent years, a growing number of protein folding studies have focused on the unfolded state, which is now recognized as playing a major role in the folding process. Some of these studies show that interactions occurring in the unfolded state can significantly affect the stability and kinetics of the protein folding reaction. In this study, we modeled the effect of electrostatic interactions, both native and nonnative, on the folding of three protein systems that underwent selective charge neutralization or reversal or complete charge suppression. In the case of the N-terminal L9 protein domain, our results directly attribute the increase in thermodynamic stability to destabilization of the unfolded ensemble, reaffirming the experimental observations. These results provide a deeper structural insight into the ensemble of the unfolded state and predict a new mutation site for increased protein stability. In the second case, charge reversal mutations of RNase Sa affected protein stability, with the destabilizing mutations being less destabilizing at higher salt concentrations, indicating the formation of charge-charge interactions in the unfolded state. In the N-terminal L9 and RNase Sa systems, changes in electrostatic interactions in the unfolded state that cause an increase in free energy had an overall compaction effect that suggests a decrease in entropy. In the third case, in which we compared the β-lactalbumin and hen egg-white lysozyme protein homologues, we successfully eliminated differences between the folding kinetics of the two systems by suppressing electrostatic interactions, supporting previously reported findings. Our coarse-grained molecular dynamics study not only reproduces experimentally reported findings but also provides a detailed molecular understanding of the elusive unfolded-state ensemble and how charge-charge interactions can modulate the biophysical characteristics of folding.