Heptulosonate 7- phosphate 3-deoxy-D-arabino

Glyphosate also inhibits the activity of a form of 3-deoxy-D-arabino-heptulosonate-7-phosphate synthase (13), however, this inhibition is relatively weak. Effects of glyphosate on the enzymes of aromatic amino acid synthesis have been described in more detail in recent reviews (14-17). 

Figure 1. Hypothetical mechanism for shuttling of intermediates of the common aromatic pathway between plastidic and cytosolic compartments. Enzymes denoted with an asterisk (DAHP synthase-Co, chorismate mutase-2, and cytosolic anthranilate synthase) have been demonstrated to be isozymes located in the cytosol. DAHP molecules from the cytosol are shown to be shuttled into the plastid compartment in exchange for EPSP molecules synthesized within the plastid. Abbreviations C3, phosphoenolpyruvate C4, erythrose 4-P DAHP, 3-deoxy-D-arabino-heptulosonate 7-phosphate EPSP, 5-enolpyruvylshikimate 3-phosphate CHA, chorismate ANT, anthranilate TRP, L-tryptophan PPA, prephenate AGN, L-arogenate TYR, L-tyrosine and PHE, L-phenylalanine.

The product of the DAHP synthase, 3-deoxy-D-arabino-heptulosonate 7-phosphate, is shown in its cyclic hemiacetal form at the beginning of Eq. 25-2. Its conversion to 3-dehydroquinate is a multistep process that is catalyzed by a single enzyme, 14/15 which is the product of E. coli gene am B. The elimination of... 

Scheme 6.4.1. The shikimate biosynthetic pathway. The enzymes involved are (1) 3-deoxy-D-arabino-heptulosonate 7-phosphate synthase, (2) dehydroquinate synthase, (3) 5-...

Phenolic compounds include a wide range of secondary metabolites that are biosynthesised from carbohydrates through the shikimate pathway [14]. This is the biosynthetic route to the aromatic amino acids, phenylalanine, tyrosine, and tryptophan, and only occurs in microorganisms and plants. In the first step, the glycolytic intermediate phosphoenol pyruvate and the pentose phosphate intermediate erythrose-4-phosphate are condensed to 3-deoxy-D-arabino-heptulosonate 7-phosphate (DAHP), a step catalysed by DAHP synthase. Intermediates of the shikimate pathway are 3-dehydroquinate, shikimate, and chorismate (Fig. 1). Phenylalanine is biosynthesised from chorismate, and from phenylalanine all the phenylpropanoids. Quinate is produced from 3-dehydroquinate and incorporated into chlorogenic and isochlorogenic acids (caffeoyl quinic acids) by combination with caffeic acid. Gallic acid is produced from shikimate. 

Fig. (1). Phenolic metabolism. PEP (phospho enol pyruvate) E4P (erythrose 4-phosphate) DAHP (3-Deoxy-D-arabino-heptulosonate 7-phosphate) PAL (phenylalanine ammonia lyase). Stress transcriptionally activates the key enzymes of phenolic metabolism (PAL and DAHP synthase).

Fig. 10. Amino acid homologies between the tyrosine-sensitive and the phenylalanine-sensitive isoenzymes of 3-deoxy-D-arabino-heptulosonate 7-phosphate synthase from Escherichia coli. Reprinted from Reference 28 with permission of the American Society of Biological Chemists.

The nucleotide sequence of the aroF gene of Escherichia coli and the amino acid sequence of the encoded protein, the tyrosine-sensitive 3-deoxy-D-arabino-heptulosonate 7-phosphate synthase. J. 

The metabolic pathway responsible for biosynthesis of aromatic amino acids and for vitamin-like derivatives such as folic acid and ubiquinones is a major enzyme network in nature. In higher plants this pathway plays an even larger role since it is the source of precursors for numerous phenylpropanoid compounds, lignins, auxins, tannins, cyano-genic glycosides and an enormous variety of other secondary metabolites. Such secondary metabolites may originate from the amino acid end products or from intermediates in the pathway (Fig. 1). The aromatic pathway interfaces with carbohydrate metabolism at the reaction catalyzed by 3-deoxy-D-arabino-heptulosonate 7-phosphate (DAHP) synthase, the condensation of erythrose-4-phosphate and PEP to form... 

The first enzyme which is specific for aromatic amino acid biosynthesis is 3-deoxy-D-arabino-heptulosonic 7-phosphate (DAHP) synthase (EC 4.1.2.1.5), which utilizes D-erythrose 4-phosphate (E4P) and phosphoenolpyruvate (PEP) as substrates (Fig. 2). Le Mar chal al.prepared the isosteric homophosphonate of E4P, in which the C-O-P group is replaced by the C-CH2-P group as well as the non-isosteric phosphonate analogue, in which the C-O-P group is replaced by the C-P group (Fig. 3). Both analogues served as substrates for the tyrosine-sensitive DAHP synthase of Escherichia coli, with Vmax values in the ratio 20 3.5 1 for E-4-P, the phosphonate, and the homophosphonate. 

DAHPS 3-Deoxy-D-arabino-heptulosonate-7-phosphate synthase... 

L-Tyrosine biosynthesis starts with the condensation of phosphoenolpyruvate (PEP) and erythrose-4-phosphate (E4P), the intermediates of the glycolytic pathway and pentose phosphate pathway, respectively, which is catalyzed by 3-deoxy-D-arabino-heptulosonate-7-phosphate synthase (DAHPS aroE/aroG/aroH). The resultant 3-deoxy-D-arabino-heptulosonate (DAHP) is converted into chorismate through the shikimate pathway with seven reactions. In plants, prephenate (PPA) is converted into L-arogenate by transamination whereas in E. coli, PPA is converted to p-hydroxyphenylpyruvate (HPP) by prephenate dehydrogenase, which is a bifunctional enzyme that behaves as chorismate mutase/prephenate... 

Kikuchi Y, Tsujimoto K, Kurahashi O (1997) Mutatimial analysis of the feedback sites of phenylalanine-sensitive 3-deoxy-D-arabino-heptulosonate-7-phosphate synthase of Escherichia coli. Appl Environ Mioobiol 63 761—762... 

R.L. Doong, J.E. Gander, R.J. Ganson, R.A. Jensen. The cytostolic isoenzyme of 3-deoxy-D-arabino-heptulosonate 7-phosphate synthase in Spinacia oleracea and other higher plants— extreme substrate ambiguity and other properties. Physiol Planta, 1992, 84, 351-360... 

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