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

Unusually stable proteins are a disadvantage for the metabolic turnover of proteins in cells. The CutA1 pro-teins from Pyrococcus horikoshii and from Oryza sativa (OsCutA1) have unusually high denaturation tempera-tures (Td) of nearly 150 and 100 C, respectively, at pH 7.0. It seemed that the CutA1 protein from the human brain (HsCutA1) also has a remarkably high stability. Therefore, the thermodynamic stabilities of HsCutA1 and its protease susceptibility were examined. The Td was remarkably high, being over 95C at pH 7.0. The unfolding Gibbs energy (G0H2O) was 174 kJ/mol at 37C from the denaturant denaturation. The thermo-dynamic analysis showed that the unfolding enthalpy and entropy values of HsCutA1 were considerably lower than those of OsCutA1 with a similar stability to HsCutA1, which should be related to flexibility of the unstructured properties in both N- and C-terminals of HsCutA1. HsCutA1 was almost completely digested after 1-day incubation at 37C by subtilisin, although OsCutA1 was hardly digested at the same conditions. These results indicate that easily available fragmentation ofHsCutA1 with remarkably high thermodynamic stabil-ity at the body temperature should be important for its protein catabolism in the human cells