Chorismate mutase-prephenate dehydrogenase from Escherichia coli: positive cooperativity with substrates and inhibitors

Biochemical characterization of prephenate dehydrogenase from the hyperthermophilic bacterium Aquifex aeolicus

A monofunctional prephenate dehydrogenase (PD) from Aquifex aeolicus was expressed as a His-tagged protein in Escherichia coli and was purified by nickel affinity chromatography allowing the first biochemical and biophysical characterization of a thermostable PD. A. aeolicus PD is susceptible to proteolysis. In this report, the properties of the full-length PD are compared with one of these products, an N-terminally truncated protein variant (Delta19PD) also expressed recombinantly in E. coli. Both forms are dimeric and show maximum activity at 95 degrees C or higher. Delta19PD is more sensitive to temperature effects yielding a half-life of 55 min at 95 degrees C versus 2 h for PD, and values of kcat and Km for prephenate, which are twice those determined for PD at 80 degrees C. Low concentrations of guanidine-HCl activate enzyme activity, but at higher concentrations activity is lost concomitant with a multi-state pathway of denaturation that proceeds through unfolding of the dimer, oligomerization, then unfolding of monomers. Measurements of steady-state fluorescence intensity and its quenching by acrylamide in the presence of Gdn-HCl suggest that, of the two tryptophan residues per monomer, one is buried in a hydrophobic pocket and does not become solvent exposed until the protein unfolds, while the less buried tryptophan is at the active site. Tyrosine is a feedback inhibitor of PD activity over a wide temperature range and enhances the cooperativity between subunits in the binding of prephenate. Properties of this thermostable PD are compared and contrasted with those of E. coli chorismate mutase-prephenate dehydrogenase and other mesophilic homologs.

Purification and kinetic analysis of the two recombinant arogenate dehydrogenase isoforms of Arabidopsis thaliana

The present study reports the first purification and kinetic characterization of two plant arogenate dehydrogenases (EC 1.3.1.43), an enzyme that catalyses the oxidative decarboxylation of arogenate into tyrosine in presence of NADP. The two Arabidopsis thaliana arogenate dehydrogenases TyrAAT1 and TyrAAT2 were overproduced in Escherichia coli and purified to homogeneity. Biochemical comparison of the two forms revealed that at low substrate concentration TyrAAT1 is four times more efficient in catalyzing the arogenate dehydrogenase reaction than TyrAAT2. Moreover, TyrAAT2 presents a weak prephenate dehydrogenase activity whereas TyrAAT1 does not. The mechanism of the dehydrogenase reaction catalyzed by these two forms has been investigated using steady-state kinetics. For both enzymes, steady-state velocity patterns are consistent with a rapid equilibrium, random mechanism in which two dead-end complexes, E-NADPH-arogenate and E-NADP-tyrosine, are formed.

New insight into the catalytic mechanism of chorismate mutases from structural studies

Chorismate mutase catalyzes the rearrangement of chorismic acid to prephenic acid, which is the first committed step in the biosynthesis of aromatic amino acids. Its catalytic mechanism has been much studied, but is poorly understood. Recent structural information on enzymes from two species, and on an antibody that catalyzes the same reaction, has shed new light on this topic.