Thermodynamic Basis for the Increased Thermostability of CheY from the Hyperthermophile Thermotoga maritima

The CheY protein isolated from the hyperthermophile Thermotoga maritima is much more resistant to thermally induced unfolding than is its counterpart from the mesophile Bacillus subtilis. To determine the basis of this increased thermostability, the temperature dependence of the free energy of unfolding was determined for these CheY homologues using denaturant-induced unfolding experiments. This allowed comparison of T. maritima CheY with B. subtilis CheY and determination of the thermodynamic qualities responsible for the enhanced thermostability of T. maritima CheY. The stability of the thermophilic CheY protein is a direct result of the increased enthalpy contribution at the temperature of zero entropy, T(s), and the decreased heat capacity change upon unfolding, resulting in a decreased dependence of the free energy of unfolding on temperature. It was found that neither purely entropic nor purely enthalpic contributions alone (as reflected by T(s)) were sufficient to account for the increase in stability.