BIVALENT-CATIONS STABILIZE YEAST ALCOHOL-DEHYDROGENASE-I

The thermostability of yeast alcohol dehydrogenase (ADH) I is strongly dependent on the presence of NaCl, a salt that is almost neutral on t he Hofmeister scale, which suggests that solvent-accessible electrosta tic repulsion might play a role in the inactivation of the enzyme. Mor eover, CaCl2 and MgCl2 are able to stabilize the enzyme at millimolar concentrations. Ca2+ stabilizes yeast ADH I by preventing the dissocia tion of the reduced form of the enzyme and by preventing the unfolding of the oxidized form of the enzyme. An analysis of several chimaeric ADHs suggests that Ca2+ is fixed by the Asp-236 and Glu-101 side chain s in yeast ADH I, but that Ca2+ can be displaced by replacing Met-168 by an Arg residue, as suggested by a three-dimensional model of the en zyme structure. These results indicate that electrostatic repulsion ca n cause protein unfolding and/or dissociation. It is proposed that yea st ADH I binds Mg2+ in vivo.