A kinetic model of the branch-point between the methionine and threonine biosynthesis pathways in Arabidopsis thaliana

Authors


  • Enzymes: cystathionine γ-synthase (EC 4.2.99.9; Swiss Prot entry P55217); cystathionine β-lyase (EC 4.4.1.8; Swiss Prot entry P53780); homoserine kinase (EC 2.7.1.39; Swiss Prot entry Q8L7R2); threonine deaminase (EC 4.2.1.16; Swiss Prot entry Q9ZSS6); threonine synthase (EC 4.2.99.2; Swiss Prot entry Q9S7B5); lactate dehydrogenase (EC 1.1.1.27, Swiss Prot entry P13491).

  • Note: The mathematical model described here has been submitted to the Online Cellular Systems Modelling Database and can be accessed at http://jjj.biochem.sun.ac.za/database/curien/index.html free of charge.

G. Curien, Laboratoire de Physiologie Cellulaire Végétale DRDC/CEA-Grenoble, 17 rue des Martyrs, 38054 Grenoble Cedex 9, France. Fax: + 33 4 38 78 50 91, Tel.: +33 4 38 78 23 64, E-mail: gcurien@cea.fr

Abstract

This work proposes a model of the metabolic branch-point between the methionine and threonine biosynthesis pathways in Arabidopsis thaliana which involves kinetic competition for phosphohomoserine between the allosteric enzyme threonine synthase and the two-substrate enzyme cystathionine γ-synthase. Threonine synthase is activated by S-adenosylmethionine and inhibited by AMP. Cystathionine γ-synthase condenses phosphohomoserine to cysteine via a ping-pong mechanism. Reactions are irreversible and inhibited by inorganic phosphate. The modelling procedure included an examination of the kinetic links, the determination of the operating conditions in chloroplasts and the establishment of a computer model using the enzyme rate equations. To test the model, the branch-point was reconstituted with purified enzymes. The computer model showed a partial agreement with the in vitro results. The model was subsequently improved and was then found consistent with flux partition in vitro and in vivo. Under near physiological conditions, S-adenosylmethionine, but not AMP, modulates the partition of a steady-state flux of phosphohomoserine. The computer model indicates a high sensitivity of cystathionine flux to enzyme and S-adenosylmethionine concentrations. Cystathionine flux is sensitive to modulation of threonine flux whereas the reverse is not true. The cystathionine γ-synthase kinetic mechanism favours a low sensitivity of the fluxes to cysteine. Though sensitivity to inorganic phosphate is low, its concentration conditions the dynamics of the system. Threonine synthase and cystathionine γ-synthase display similar kinetic efficiencies in the metabolic context considered and are first-order for the phosphohomoserine substrate. Under these conditions outflows are coordinated.

Ancillary