Thermodynamic Basis for the Stabilities of Three CutA1s from Pyrococcus horikoshii, Thermus thermophilus, and Oryza sativa, with Unusually High Denaturation Temperatures^†

In order to elucidate the stabilization mechanism of CutA1 from Pyrococcus horikoshii (PhCutA1) with a denaturation temperature of nearly 150 °C, GuHCl denaturation and heat denaturation were examined at neutral and acidic pHs. As a comparison, CutA1 proteins from Thermus thermophilus (TtCutA1) and Oryza sativa (OsCutA1) were also examined, which have lower optimum growth temperatures of 75 and 28 °C, respectively, than that (98 °C) of P. horikoshii. GuHCl-induced unfolding and refolding curves of the three proteins showed hysteresis effects due to an unusually slow unfolding rate. The midpoints of refolding for PhCutA1, TtCutA1 and OsCutA1 were 5.7 M, 3.3 M, and 2.3 M GuHCl, respectively, at pH 8.0 and 37 °C. DSC experiments with TtCutA1 and OsCutA1 showed that the denaturation temperatures were remarkably high, 112.8 and 97.3 °C, respectively, at pH 7.0 and that the good heat reversibility was amenable to thermodynamic analyses. At acidic pH, TtCutA1 showed higher stability to both heat and denaturant than PhCutA1. Combined with the data for DSC and denaturant denaturation, the unfolding Gibbs energy of PhCutA1 could be depicted as a function of temperature. It was experimentally revealed that (1) the unusually high stability of PhCutA1 basically originates from a common trimer structure of the three proteins, (2) the stability of PhCutA1 is superior to those of the other two CutA1s over all temperatures above 0 °C at neutral pH, due to the decrease in both enthalpy and entropy, and (3) ion pairs of PhCutA1 contribute to the unusually high stability at neutral pH.