Model Info:

Title:

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

Brief Description:

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

Authors:

Gilles Curien, Ste´ phane Ravanel and Renaud Dumas

Affiliations:

Laboratoire de Physiologie Cellulaire Ve´ge´tale DRDC/CEA-Grenoble, France

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 c-synthase. Threonine synthase is activated by S-adenosylmethionine and inhibited by AMP. Cystathionine c-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 purifed enzymes. The computer model showed a partial agreement with the in vitro results. The model was subsequently improved and was then found consistent with fuxpartition in vitro and in vivo. Under near physiological conditions, S-adenosylmethionine, but notAMP,modulates the partition of a steady-state .uxof phosphohomoserine. The computer model indicates a high sensitivity of cystathionine flux to enzyme and S-adenosylmethionine concentrations. Cystathionine .uxis sensitive to modulation of threonine .uxwhereas the reverse is not true. The cystathionine c-synthase kinetic mechanism favours a low sensitivity of the .uxes to cysteine. Though sensitivity to inorganic phosphate is low, its concentration conditions the dynamics of the system. Threonine synthase and cystathionine c-synthase display similar kinetic efficiencies in the metabolic context considered and are first-order for the phosphohomoserine substrate. Under these conditions out-fows are coordinated.

Journal:

Eur. J. Biochem. 270, 1-13 (2003)