Allosteric activation of Arabidopsis threonine synthase by S-adenosylmethionine

Biochemistry. 1998 Sep 22;37(38):13212-21. doi: 10.1021/bi980068f.

Abstract

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.

MeSH terms

  • Allosteric Site / genetics
  • Arabidopsis / enzymology*
  • Carbon-Oxygen Lyases / chemistry
  • Carbon-Oxygen Lyases / genetics
  • Carbon-Oxygen Lyases / metabolism*
  • Enzyme Activation / genetics
  • Homoserine / analogs & derivatives
  • Homoserine / metabolism
  • Kinetics
  • Models, Biological
  • Models, Chemical
  • Plant Proteins / chemistry
  • Plant Proteins / genetics
  • Plant Proteins / metabolism
  • Protein Binding
  • Recombinant Proteins / chemistry
  • Recombinant Proteins / metabolism
  • S-Adenosylmethionine / metabolism*
  • Spectrometry, Fluorescence

Substances

  • Plant Proteins
  • Recombinant Proteins
  • O-phosphohomoserine
  • Homoserine
  • S-Adenosylmethionine
  • Carbon-Oxygen Lyases
  • threonine synthase