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B6db references: 98422275

type Journal Article
authors Curien, G.; Job, D.; Douce, R.; Dumas, R.
title Allosteric activation of Arabidopsis threonine synthase by S- adenosylmethionine
journal Biochemistry
ui 98422275
year (1998)
volume 37
number 38
pages 13212-21.
keywords Allosteric Site/genetics
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.
last changed 2002/11/12 16:17

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