Allosteric activation of Arabidopsis threonine synthase by S-adenosylmethionine.
Plant threonine synthase, in contrast to its bacterial counterpart, is strongly stimulated by S-adenosylmethionine via a noncovalent interaction [Giovanelli et al. (1984) Plant. Physiol. 76, 285-292]. The mechanism of activation remained, however, largely unknown. To further characterize this unusual role for S-adenosylmethionine, the Arabidopsis thaliana threonine synthase was overexpressed in Escherichia coli, purified to homogeneity, and then used for kinetic and enzyme-bound pyridoxal 5'-phosphate fluorescence equilibrium-binding experiments. We observed that the activating effect of S-adenosylmethionine results from an 8-fold increase in the rate of catalysis and from a 25-fold decrease in the Km value for the O-phosphohomoserine substrate. The data can be well fitted by a kinetic model assuming binding of two S-adenosylmethionine molecules on the native enzyme. We suggest that the dramatic modifications of the enzyme kinetic properties originate most presumably from an allosteric and cooperative transition induced by S-adenosylmethionine. This transition occurs at a much faster rate in the presence of the substrate than in its absence.
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Unité mixte CNRS/Rhône-Poulenc (UMR 41), Rhône-Poulenc Agrochimie, Lyon, France.
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D000495:Allosteric Site / Q000235:genetics; D017360:Arabidopsis / Q000201:enzymology*; D019757:Carbon-Oxygen Lyases / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D004789:Enzyme Activation / Q000235:genetics; D006714:Homoserine / Q000031:analogs & derivatives / Q000378:metabolism; D007700:Kinetics; D008954:Models, Biological; D008956:Models, Chemical; D010940:Plant Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D011485:Protein Binding; D011994:Recombinant Proteins / Q000737:chemistry / Q000378:metabolism; D012436:S-Adenosylmethionine / Q000378:metabolism*; D013050:Spectrometry, Fluorescence
D010940:Plant Proteins; D011994:Recombinant Proteins; C011066:O-phosphohomoserine; D006714:Homoserine; D012436:S-Adenosylmethionine; D019757:Carbon-Oxygen Lyases; C023160:threonine synthase