tailieunhanh - Báo cáo khoa học: A kinetic model of the branch-point between the methionine and threonine biosynthesis pathways in Arabidopsis thaliana

This work proposes a model of the metabolic branch-point between the methionine and threonine biosynthesis path-ways inArabidopsis thalianawhich involves kinetic compe-tition for phosphohomoserine between the allosteric enzyme threonine synthase and the two-substrate enzyme cysta-thionine c-synthase. Threonine synthase is activated by S-adenosylmethionine and inhibited by AMP. Cystathio-ninec-synthase condenses phosphohomoserine to cysteine via a ping-pong mechanism. | Eur. J. Biochem. 270 4615-4627 2003 FEBS 2003 doi A kinetic model of the branch-point between the methionine and threonine biosynthesis pathways in Arabidopsis thaliana Gilles Curien Stephane Ravanel and Renaud Dumas Laboratoire de Physiologie Cellulaire Végétale DRDC CEA-Grenoble France 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 y-synthase. Threonine synthase is activated by S-adenosylmethionine and inhibited by AMP. Cystathionine y-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 fluxof phosphohomoserine. The computer model indicates a high sensitivity of cystathionine fluxto enzyme and S-adenosylmethionine concentrations. Cystathionine fluxis sensitive to modulation of threonine fluxwhereas the reverse is not true. The cystathionine y-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 y-synthase display similar kinetic efficiencies in the metabolic context considered