The aromatic amino acid pathway branches at L-arogenate in Euglena gracilis.

Abstract

The recently characterized amino acid L-arogenate (Zamir et al., J. Am. Chem. Soc. 102:4499-4504, 1980) may be a precursor of either L-phenylalanine or Ltyrosine in nature. Euglena gracilis is the first example of an organism that uses L-arogenate as the sole precursor of both L-tyrosine and L-phenylalanine, thereby creating a pathway in which L-arogenate rather than prephenate becomes the metabolic branch point. E. gracilis ATCC 12796 was cultured in the light under myxotrophic conditions and harvested in late exponential phase before extract preparation for enzymological assays. Arogenate dehydrogenase was dependent upon nicotinamide adenine dinucleotide phosphate for activity. L-Tyrosine inhibited activity effectively with kinetics that were competitive with respect to Larogenate and noncompetitive with respect to nicotinamide adenine dinucleotide phosphate. The possible inhibition of arogenate dehydratase by L-phenylalanine has not yet been determined. Beyond the latter uncertainty, the overall regulation of aromatic biosynthesis was studied through the characterization of 3-deoxy-Darabino-heptulosonate 7-phosphate synthase and chorismate mutase. 3-Deoxy-D-arabino-heptulosonate 7-phosphate synthase was subject to noncompetitive inhibition by L-tyrosine with respect to either of the two substrates. Chorismate mutase was feedback inhibited with equal effectiveness by either L-tyrosine or Lphenylalanine. L-Tryptophan activated activity of chorismate mutase, a pHdependent effect in which increased activation was dramatic above pH 7.8. LArogenate did not affect activity of 3-deoxy-D-arabino-heptulosonate 7-phosphate synthase or of chorismate mutase. Four species of prephenate aminotransferase activity were separated after ion-exchange chromatography. One aminotransferase exhibited a narrow range of substrate specificity, recognizing only the combination of L-glutamate with prephenate, phenylpyruvate, or 4-hydroxyphenylpyruvate. Possible natural relationships between Euglena spp. and fungi previously considered in the literature are discussed in terms of data currently available to define enzymological variation in the shikimate pathway.

Phenylalanine Biosynthesis in Arabidopsis thaliana

“…In Silico Analysis of Putative PDT and ACT Domains in Bacterial and Plant Genes and Proteins-Although ADT activity has been biochemically detected in bacteria, plants, and green algae (16,22,23,37,38), there are no reports of molecular studies of any putative ADT genes in any organisms, only that of PDTs and CDTs. Our in silico screening, however, resulted in the provisional identification of six putative PDT domaincontaining genes in Arabidopsis (At1g08250, At1g11790, FIGURE 2.…”

Cohesion Group Approach for Evolutionary Analysis of TyrA, a Protein Family with Wide-Ranging Substrate Specificities

“…All photosynthetic bacteria and photosynthetic eukaryotes studied thus far possess L-arogenate-specific, NADP ϩ -specific dehydrogenases. This specificity combination is present in the enzymes from red algae and green algae (9) as well as from Euglena gracilis (14). Coryneform bacteria, other actinomycetes, and Nitrosomonas europeae exemplify bacteria whose possession of L-arogenatespecific dehydrogenases are well documented (see reference 67 and references therein).…”

Purification and Characterization of Chorismate Synthase from Euglena gracilis

“…Unlike other organisms, E. gracilis possesses only a single chorismate mutase isozyme, which is unusually large (31). In contrast to microorganisms, L-arogenate is the last common precursor of both phenylalanine and tyrosine in Euglena (7), which also appears to be the case for higher plants.…”