Hyperthermophilic proteins possess many ion pairs on their surface. To reveal the role of the ion pairs, O-6-methylguanine-DNA methyltransferase from Thermococcus kodakaraensis KOD1 (Tk-MGMT) was studied as a model protein. The maximum free-energy changes of the protein in 0.1 and 0.5 M NaCl at pH 7.0 were 61.7 kJ mol(-1) at 31.5degreesC and 77.4 kJ mol(-1) at 39.7degreesC, respectively. On the other hand, mid points of the thermal unfolding temperatures in 0.1 and 0.5 M NaCl at pH 7.0 were 94.8degreesC and 90.1degreesC, respectively. The results suggest that the protein-surface ion pairs contribute to thermal stability (T-m), rather than thermodynamic stability (DeltaG).