Temperature Dependence of the Stability of Ion Pair Interactions, and its Implications on the Thermostability of Proteins from Thermophiles

Abstract

An understanding of the determinants of the thermal stability of thermostable proteins is expected to enable design of enzymes that can be employed in industrial biocatalytic processes carried out at high temperatures. A major factor that has been proposed to stabilize thermostable proteins is the high occurrence of salt bridges. The current study employs free energy calculations to elucidate the thermodynamics of the formation of salt bridge interactions and the temperature dependence, using acetate and methylguanidium ions as model systems. Three different orientations of the methylguanidinium approaching the carboxylate group have been considered for obtaining the free energy profiles. The association of the two ions becomes more favorable with an increase in temperature. The desolvation penalty corresponding to the association of the ion pair is the lowest at high temperatures. The occurrence of bridging water molecules between the ions ensures that the ions are not fully desolvated, and this could provide an explanation for the existence of internal water molecules in thermostable proteins reported recently. The findings provide a detailed picture of the interactions that make ion pair association at high temperatures a favorable process, and reaffirm the importance of salt bridges in the design of thermostable proteins.