HORT640 - Metabolic Plant Physiology
Aromatic amino acid biosynthesis
Terminal reactions of phenylalanine and tyrosine synthesis
There is considerable evidence that the aromatic amino acids phenylalanine, tyrosine, and tryptophan are synthesized in the plastids of higher plants. However, a cytosolic isoform exists of the enzyme catalyzing the first step of the branch of the pathway that is specific for the synthesis of phenylalanine and tyrosine; chorismate mutase (CM) (Eberhard et al, 1996ab). A cDNA clone encoding a cytosolic CM isozyme was identified from Arabidopsis thaliana by complementing a CM-deficient Escherichia coli strain (Eberhard et al, 1996ab). The plastidic CM is activated by tryptophan and inhibited by phenylalanine and tyrosine, whereas the cytosolic isozyme is insensitive (Eberhard et al, 1996ab). The existence of a cytosolic CM isozyme implies that either a cytosolic pathway (partial or complete) for the biosynthesis of phenylalanine and tyrosine exists, or that prephenate, originating from chorismate in the cytosol, is utilized for the synthesis of metabolites other than these two aromatic amino acids (Eberhard et al, 1996ab).
The bifunctional enzyme chorismate mutase/prephenate dehydratase (EC 22.214.171.124/126.96.36.199), which is encoded by the pheA gene of Escherichia coli, is subject to strong feedback inhibition by L-phenylalanine (Gething and Davidson, 1976; Nelms et al, 1992). Chorismate mutase and prephenate dehydrogenase activities are also associated with a single bifunctional enzyme, hydroxyphenylpyruvate synthase, encoded by the tyrA gene in E. coli (Christopherson, 1997; Christendat and Turnbull, 1999). In contrast, Bacillus subtilis possesses a monofunctional chorismate mutase encoded by the aroH gene (Gray et al, 1990; Mattei et al, 1999).
Christendat D, Turnbull JL 1999 Identifying groups involved in the binding of prephenate to prephenate dehydrogenase from Escherichia coli. Biochemistry 38: 4782-4793.
Christopherson RI 1997 Partial inactivation of chorismate mutase-prephenate dehydrogenase from Escherichia coli in the presence of analogues of chorismate. Int. J. Biochem. Cell Biol. 29: 589-594.
Eberhard J, Bischoff M, Raesecke HR, Amrhein N, Schmid J 1996a Isolation of a cDNA from tomato coding for an unregulated, cytosolic chorismate mutase. Plant Mol. Biol. 31: 917-922.
Eberhard J, Ehrler TT, Epple P, Felix G, Raesecke HR, Amrhein N, Schmid J 1996b Cytosolic and plastidic chorismate mutase isozymes from Arabidopsis thaliana: molecular characterization and enzymatic properties. Plant J. 10: 815-821.
Gething MJ, Davidson BE 1976 Chorismate mutase/prephenate dehydratase from Escherichia coli K12. 2. Evidence for identical subunits catalysing the two activities. Eur. J. Biochem. 71: 327-336.
Gray JV, Golinelli-Pimpaneau B, Knowles JR 1990 Monofunctional chorismate mutase from Bacillus subtilis: purification of the protein, molecular cloning of the gene, and overexpression of the gene product in Escherichia coli. Biochemistry 29: 376-383.
Mattei P, Kast P, Hilvert D 1999 Bacillus subtilis chorismate mutase is partially diffusion-controlled. Eur. J. Biochem. 261: 25-32.
Nelms J, Edwards RM, Warwick J, Fotheringham I 1992 Novel mutations in the pheA gene of Escherichia coli K-12 which result in highly feedback inhibition-resistant variants of chorismate mutase/prephenate dehydratase. Appl. Environ. Microbiol. 58: 2592-2598.
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