Abstract
In this study, the thermodynamic activation parameters of cold-adapted proteins from Archaea are described for the first time for the irreversible protein unfolding and ribosome-dependent GTPase activity of elongation factor 2 (EF-2) from the psychrotolerant Methanococcoides burtonii and the thermophilic Methanosarcina thermophila. Thermolability of Methanococcoides burtonii EF-2 was demonstrated by a low activation free-energy of unfolding as a result of low activation-enthalpy. Although structural data for EF-2 are presently limited to protein homology modeling, the observed thermodynamic properties are consistent with a low number of noncovalent bonds or an altered solvent interaction, causing a loss of entropy during the unfolding process. A physiological concentration of potassium aspartate or potassium glutamate was shown to stabilize both proteins against irreversible denaturation by strengthening noncovalent interactions, as indicated by increased activation enthalpies. The transition state of GTPase activity for Methanococcoides burtonii EF-2 was characterized by a lower activation enthalpy than for Methanosarcina thermophila EF-2. The relative entropy changes could be explained by differential displacement of water molecules during catalysis, resulting in similar activation free energies for both proteins. The presence of solutes was shown to facilitate the breaking of enthalpy-driven interactions and structuring of more water molecules during the reaction. By studying the thermodynamic activation parameters of both GTPase activity and unfolding and examining the effects of intracellular solutes and partner proteins (ribosomes), we were able to identify enthalpic and entropic properties that have evolved in the archaeal EF-2 proteins to enable Methanococcoides burtonii and Methanosarcina thermophila to adapt to their respective thermal environments.