Thermodynamic analysis of unusually thermostable CutA1 protein from human brain and its protease susceptibility

) was 174kJ/mol at37 C from the denaturant denaturation. The thermo-dynamic analysis showed that the unfolding enthalpyand entropy values of HsCutA1 were considerably lowerthan those of OsCutA1 with a similar stability toHsCutA1, which should be related to flexibility of theunstructured properties in both N- and C-terminals ofHsCutA1. HsCutA1 was almost completely digestedafter 1-day incubation at 37 C by subtilisin, althoughOsCutA1 was hardly digested at the same conditions.These results indicate that easily available fragmentationofHsCutA1withremarkablyhighthermodynamicstabil-ity at the body temperature should be important for itsprotein catabolism in the human cells.Keywords: protein stability/metabolic turnover ofprotein/human CutA1/thermodynamic parameters/unusually high stability.Abbreviations: DSC, differential scanning calorimetry;GuHCl, guanidine hydrochloride; HsCutA1, CutA1from Homo sapiens; OsCutA1, CutA1 from Oryzasativa; PhCutA1, CutA1 from Pyrococcus horikoshii;TtCutA1, CutA1 from Thermus thermophilus.The native state of proteins is only marginally morestable (generally 20 50kJ/mol under physiologicalconditions) than the ensemble of unfolded states (1).This marginal stability is important for the metabolicturnover of proteins in cells because there is a correla-tion between the conformational stability of a proteinand its catabolism (2 4). Furthermore, the formationof amyloids such as amyloidosis, Alzheimer diseaseand prion diseases, changes into stable proteins withresistance to proteolysis and then proves to be fatal(5, 6), suggesting that it can be a disadvantage forliving cells to produce too stable proteins whichexceed physiological demand.Contrary to this presumption, we have found thatthe CutA1 (PhCutA1) from Pyrococcus horikoshiiOT3, a hyperthermophile, has a denaturation tempera-ture (T