Ion pairs and the thermotolerance of proteins from hyperthermophiles: a "traffic rule" for hot roads

The proteins from hyperthermophilic organisms maintain their biologically active structure at temperatures that are significantly higher than the denaturation temperatures of their mesophilic counterparts. The fact that there is usually a high degree of sequence and structural homology between these two classes of proteins suggests that the source of this extreme thermal tolerance is hidden in the delicate balance of the non-covalent interactions. Among the large number of factors identified in the literature as being responsible for the thermostability of these proteins, this article focuses on electrostatic interactions. It demonstrates that the optimization of electrostatic interactions by increasing of the number of salt bridges is a driving force for enhancement of the thermotolerance of proteins from hyperthermophilic microorganisms. This feature is less evident in proteins from thermophilic organisms and is absent from mesophile-derived proteins.