COMPARATIVE THERMOSTABILITY OF GLUCOSE-DEHYDROGENASE FROM HALOFERAX-MEDITERRANEI - EFFECTS OF SALTS AND POLYOLS

The effect of temperature and pH on thermoinactivation kinetics of glu cose dehydrogenase from Haloferax mediterranei has been studied in the presence of different monovalent sails (LiCl, LiBr, NaCl, NaBr, KCl, KBr, NH4Cl, and NH4Br) and polyols (glycerol, erythrytol, xylitol, and sorbitol) concentrations. The stabilization degree of salts followed the rank of the Hofmeister series, and the product of the Setchenov co nstant (K-s) times the concentration of solute (C-s) was useful to pre dict the enzyme stability in the presence of salt solutions. Polyols s tabilized the halophilic enzyme as much as salts. For an equal polyol concentration, the thermostability increased in the range glycerol < e rythritol < xylitol < sorbitol. The overall hydroxyl group concentrati on proved to be a good parameter for correlating the protective effect of polyols with the polyol nature. Thermoinactivation of the halophil ic glucose dehydrogenase in the presence of NaCl and sorbitol was comp ared with that of a nonhalophilic glucose dehydrogenase in terms of th e transition state theory . The free activation energy was, in all cas es, enthalpy driven, and hydrogen-bond and/or ionic-binding interactio ns are the main forces involved in protein stabilization. The halophil ic enzyme showed, in general, lower free activation energies for the d eactivation process. The adaptation of the enzyme to a halophilic envi ronment led to an enzyme with higher activity at high salt concentrati ons, but such an increase in enzyme activity was not related to an enh ancement in enzyme thermostability.