STRUCTURAL FEATURES RESPONSIBLE FOR KINETIC THERMAL-STABILITY OF A CARBOXYPEPTIDASE FROM THE ARCHAEBACTERIUM SULFOLOBUS-SOLFATARICUS

Investigations were performed on the structural features responsible f or kinetic thermal stability of a thermostable carboxypeptidase from t he thermoacidophilic archaebacterium Sulfolobus solfataricus which had been purified previously and identified as a zinc metalloprotease [Co lombo, D'Auria, Fusi, Zecca, Raia and Tortora (1992) Eur. J. Biochem. 206, 349-357]. Removal of Zn2+ by dialysis led to reversible activity loss, which was promptly restored by addition of 80 muM ZnCl2 to the a ssay mixture. For the first-order irreversible thermal inactivation th e metal-depleted enzyme showed an activation energy value of 205.6 kJ . mol-1, which is considerably lower than that of the holoenzyme (494. 4 kJ . mol-1). The values of activation free energies, enthalpies and entropies also dropped with metal removal. Thermal inactivation of the apoenzyme was very quick at 80-degrees-C, whereas the holoenzyme was stable at the same temperature. These findings suggest a major stabili zing role for the bivalent cation. Chaotropic salts strongly destabili zed the holoenzyme, showing that hydrophobic interactions are involved in maintaining the native conformation of the enzyme. However, the in activation rate was also increased by sodium sulphate, acetate and chl oride, which are not chaotropes, indicating that one or more salt brid ges concur in stabilizing the active enzyme. Furthermore, at the extre mes of the pH-stability curve, NaCl did not affect the inactivation ra te, confirming the stabilizing role of intramolecular ionic bonds, as a pH-dependent decrease in stability is likely to occur from breaking of salt bridges involved in maintaining the native conformation of the protein.