Effects of Pressure and Temperature on the Thermodynamics of α-Helices: Implications for Piezophilic Adaptation

In light of the recent realization that a significant fraction of microbial biomass lives under high hydrostatic pressure, there is a renewed interest in understanding the molecular details by which proteins in these organisms modulate their functional native state. The effects of pressure on protein stability are defined by the volume changes between the native and denatured states. The conformational ensemble of the denatured state can depend on several extrinsic variables, such as pH and ionic strength of the solvent, temperature, and pressure. The effect of the latter on the elements of the secondary structures, including α-helical structures, has been inconclusive. This has been mainly due to the inherent difficulties associated with high-pressure experiments. Here, we adapted the method of choice, circular dichroism spectroscopy, on a well-established series of model peptides to study helical structure formation while focusing on the pressure and temperature dependencies of the helix-coil transition. We find that at low temperatures, pressure stabilizes the helical structure, suggesting that the volume of the helix-coil transition is positive. However, at higher temperatures (>40 °C), the volume changes become negative, and pressure destabilizes the helical structure. The stabilizing effect of pressure on the helical structure at low temperature correlates with the increase in the fraction of high-helix-forming amino acid residues at the expense of low-helix forming residues in psychrophilic bacterial strains adapted to live at the bottom of the ocean.