(511a) Computational Study of the Stability of the Miniprotein Trp-Cage in Extreme Environments

The native state of a protein is an ensemble of compactly folded conformations in which the protein is stable and functional. While perturbations, such as changes in temperature, pressure or cosolvent concentration, often destabilize the protein leading to the formation of highly populated denatured states, it is known that these perturbations can have stabilizing effects as well. Fundamental understanding of the relationship between protein stability and deviations from physiological conditions is important due to its evolutionary implications and relevance to pharmaceutical applications, but the molecular mechanism of stabilization/destabilization by these external conditions is yet to be fully unveiled. Heat denaturation of globular proteins is widely known to be manifested by loss of secondary and tertiary structures due to the gain of configurational entropy. Cold-denaturation at ambient pressure has recently been found to be accompanied by compact, partially-folded denatured states in which the secondary structure is mainly preserved. The application of high pressure is known to shift the equilibrium of conformational states promoting denaturation near room temperature. We use extensive replica-exchange molecular dynamics simulations and thermodynamic analysis to investigate the effects of high pressure in the low temperature range and ionic liquid on the folding/unfolding thermodynamics of the Trp-cage miniprotein. We find that Trp-cage is stabilized when the pressure is increased at subzero temperatures and conformational states in the low temperature range show strong resemblance to room temperature states. We also present that mechanisms of cold- and ionic-liquid-induced denaturations exhibit similarities where ionic liquids show signs of stabilizing secondary structures.