title: [Kinetic and allosteric properties of highly-purified, biosynthetic L-threonine dehydrogenase from brewer's yeast Saccharomyces carlsbergensis].
abstract: Kinetic and allosteric propeties of highly purified "biosynthetic" L-threonine dehydratase from brewer's yeast S. carlbergensis were studied at three pH values, using L-threonine and L-serine as substrates. It was shown that the plot of the initial reaction rate (v) versus initial substrate concentrations ([S]0 pH 6.5 is hyperbolic (Km=5.0.10-2M), while these at pH 7.8 and 9.5 have a faintly pronounced sigmoidal shape with fast occurring saturation plateaus ([S]0.5= 1.0.10-2 and 0.9.10-2M, respectively). the ratios between L-threonine and L-serine dehydratation rates depend on pH. The kinetic properties and the dependence of substrate specificity on pH suggest that the enzyme molecule undergoes pH-induced (at pH 7.0) conformational changes. The determination of pK values of the enzyme functional groups involved in L-threonine binding demonstrated that these groups have pK is approximately equal to 7.5 and 9.5. The latter group was hypothetically identified as a epsilon-NH2-group of the lysine residue. High concentrations of the allosteric inhibitor (L-isoleucine) decrease the rates of L-threonine and L-serine dehydratation and induce the appearance (at pH 6.5) or increase (at pH 7.9 and 9.5) of homotropic cooperative interactions between the active sites in the course of L-threonine dehydratation. The enzyme inhibition by L-isoleucine increases with a decrease of L-threonine concentrations. Low L-isoleucine concentrations, as well as the enzyme activator (L-valine) stimulate the enzyme at non-saturating substrate concentrations (when L-threonine or L-serine are used as substrates) without normalization of (v) versus [S]0 plots. The maximal activation of the enzyme is observed at pHG 8.5--9.0. It is assumed that the molecule of "biosynthetic" L-threonine dehydratase from brewer's yeast contains two types of sites responsible for the effector binding, i.e., "activatory" and "inhibitory" ones.
mesh_terms: D000429:Alcohol Oxidoreductases / Q000378:metabolism*; D000494:Allosteric Regulation; D006863:Hydrogen-Ion Concentration; D007532:Isoleucine / Q000378:metabolism; D007700:Kinetics; D012440:Saccharomyces / Q000201:enzymology*; D012694:Serine / Q000378:metabolism; D013379:Substrate Specificity; D014633:Valine / Q000378:metabolism
chemical_list: D007532:Isoleucine; D012694:Serine; D000429:Alcohol Oxidoreductases; C023159:L-threonine 3-dehydrogenase; D014633:Valine
vernacular_title: Kineticheskie i allostericheskie svoĭstva vysokoochishchennoĭ "biosinteticheskoĭ" L-treonindegidratazy iz pivnykh drozhzheĭ Saccharomyces carlsbergensis.