|
type |
Journal Article |
authors |
Curien, G.; Job, D.; Douce, R.; Dumas, R. |
title |
Allosteric activation of Arabidopsis threonine synthase by S- adenosylmethionine |
journal |
Biochemistry |
Activity |
4.2.3.1 |
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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