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 increa sed thermostability, the temperature dependence of the free energy of unfol ding was determined for these CheY homologues using denaturant-induced unfo lding experiments. This allowed comparison of T. maritima CheY with B. subt ilis CheY and determination of the thermodynamic qualities responsible for the enhanced thermostability of T. maritima CheY. The stability of the ther mophilic CheY protein is a direct result of the increased enthalpy contribu tion at the temperature of zero entropy, T-s, and the decreased heat capaci ty change upon unfolding, resulting in a decreased dependence of the free e nergy of unfolding on temperature. It was found that neither purely entropi c nor purely enthalpic contributions alone (as reflected by T (s)) were suf ficient to account for the increase in stability.