Single amino acid substitutions can further increase the stability of a thermophilic L-lactate dehydrogenase

Lactate dehydrogenases are of considerable interest as stereospecific catalysts in the chemical preparation of enantiomerically pure α-hydroxyacid synthons. For such applications in synthetic organic chemistry it would be desirable to have enzymes which tolerate elevated temperatures for prolonged reaction times, to increase productivity and to extend then applicability to poor substrates. Here, two examples are reported of significant thermostabilizations, induced by sitedirected mutagenesis, of an already thermostable protein, the L-lactate dehydrogenase (EC 1.1.1.27, 35 kDa per monomer subunit) from Bacillus stearothermophilus. Thermal inactivation of this enzyme is accompanied by irreversible unfolding of the native protein structure. The replacement of Argl71 by Tyr stabilizes the enzyme against thermal inactivation and unfolding. This stabilizing effect appears to be based on improved interactions between the subunits in the core of the active dimeric or tetrameric forms of the enzyme. The thermal stability of L-lactate dehydrogenase variants with an active site Arg residue, either in the 171 (wild-type) or in the 102 position, is further increased by sulfate ions. The two stabilizing effects are additive, as found for the Argl71Tyr/ Gln1O2Arg double mutant, for which the stability of the protein in 100 mM sulfate solution reaches that of L-lactate dehydrogenases from extreme thermophiles. All mutant proteins retain significant catalytic activity, both in the presence and absence of stnhilfoing salts, and are viable catalysts in preparative scale reactions.