Malate Dehydrogenase Isolated from Extremely Halophilic Bacteria of the Dead Sea. 2. Effect of Salt on the Catalytic Activity and Structure

The enzymatic activity and structural stability of the extremely halophilic enzyme malate dehydrogenase (EC 1.1.1.37) isolated from Dead Sea halobacteria depend in a different way on the concentration of inorganic salts. At low salt concentration (<2.0 M NaCl) the enzyme is inactivated in a first-order reaction. When the salt concentration is increased the inactivated enzyme is reactivated in a second-order reaction. The inactivation at low salt concentration is ascribed to dissociation of the dimeric enzyme to inactive subunits and the reactivation at high salt concentration is determined by the rate of reassociation of the subunits to dimers. At low salt concentrations another process participates which causes an apparently irreversible (first order) denaturation. The rate constants of the inactivation and reactivation processes depend in a characteristic manner on salt concentration and salt type (NaCl, KCl, NH₄Cl, and (NH₄)₂SO₄), and on temperature. The thermodynamic analysis suggests that at concentrations below 0.15 M the salt mainly screens the fixed charges of the subunits, whereas at concentrations higher than 0.8 M the salt dominantly stabilizes hydrophobic interactions between the enzyme subunits. If the NaCl concentration is varied in a cyclic manner, the enzymatic activity changes along a hysteresis loop. This observation indicates relatively long-lived metastable states, reflected in the extremely small values of the rate constants of both the inactivation and reactivation process in the range of NaCl concentration where hysteresis exists.