Purification, Catalytic Properties, and Thermal Stability of Threo-Ds-3-Isopropylmalate Dehydrogenase Coded by leuB Gene from an Extreme Thermophile, Thermus thermophilus Strain HB81

Abstract

Threo-Ds-3-isopropylmalate dehydrogenase coded by the leuB gene from an extreme thermophile, Thermus thermophilus strain HB8, was expressed in Escherichia coli carrying a recombinant plasmid. The thermostable enzyme thus produced was extracted from the E. coli cells, purified, and crystallized. The enzyme was shown to be a dimer of identical subunits of molecular weight (4.0±0.5)×l0⁴ The K_m for threo-Ds-3-isopropyl-malate was estimated to be 8.0×10₋₅ M and that for NAD 6.3×10₋₄ M. The optimum pH at 75°C in the presence of 1.2 M KCl was around 7.2. The presence of Mg₂₊ or Mn₂₊ was essential for the enzyme action. The enzyme was activated about 30-fold by the addition of 1 M KCl or RbCl. The high salt concentration decelerated the thermal unfolding of the enzyme, and accelerated the aggregation of the unfolded protein. Based on these effects, the molecular mechanism of the unusual stability of the enzyme is discussed.