TEMPERATURE-INDUCED DENATURATION OF BETA-GLYCOSIDASE FROM THE ARCHAEON SULFOLOBUS-SOLFATARICUS

The beta-glycosidase isolated from the extreme thermophilic archaeon S ulfolobus solfataricus, grown at 87 degrees C, is a tetrameric protein with a molecular mass of 240 kDa, This enzyme is barely active at 30 degrees C and has optimal activity, over 95 degrees C, at pH 6.5, Its thermal stability was investigated at pH 10.1 and 10.6 by means of fun ctional studies, circular dichroism and differential scanning calorime try. There was no evidence of thermal activation of the enzyme and the temperature-induced denaturation was irreversible and not well repres ented by the two-state transition model. A more complex process occurr ed, involving the dissociation and unfolding of subunits, and subseque nt nonspecific association and/or aggregation, Denaturation temperatur e was around 85 degrees C, depending on protein concentration. The den aturation enthalpy change was between 7,500 and 9,800 kJ . mol(-1), de pending on the pH. The collapse of the native structure around 85 degr ees C was confirmed by circular dichroism measurements and time-depend ent activity studies, Finally, preliminary investigations were perform ed on the recombinant enzyme expressed in Escherichia coli.