Structure and Activity of Malate Dehydrogenase from the Extreme Halophilic Bacteria of the Dead Sea. 2. Inactivation, Dissociation and Unfolding at NaCl Concentrations below 2 M. Salt, Salt Concentration and Temperature Dependence of Enzyme Stability

Abstract

The stability of malate dehydrogenase [EC 1.1.1.37] from extreme halophilic bacteria increases with increasing salt concentration and with decreases in temperature. Stabilization by various salts, at high salt concentrations, follows the Hofmeister series. The enzyme inactivation rates closely match dissociation of the dimeric enzymes into monomeric subunits and unfolding of the polypeptide chains, as followed by velocity sedimentation, light scattering and circular dichroism measurements. The .alpha.-helix content goes to zero upon denaturation. Unusual water and salt binding properties of the native enzyme are believed to be largely lost on enzyme dissociation and unfolding. These properties thus seem to be associated with the intact structure of the enzyme.