Heptulosonate 7- phosphate

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). 

Deoxy-D-araZu o-heptulosonate-7-phosphate synthetase (DAHPS) is the initial enzyme in the pathway responsible for the synthesis of aromatic compounds in microorganisms and plants. It catalyses the reaction shown in Equation (4) ... 

This enzyme [EC 4.6.1.3] catalyzes the conversion of 3-deoxy-(2raZ mo-heptulosonate 7-phosphate to form 3-dehydroquinate and orthophosphate. The enzyme requires cobalt and the hydrogen atoms located on C7 of the substrate are retained on C2 of the product. 

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... 

The six carbons of the benzene ring of the aromatic amino acids are derived from the four carbons of erythrose 4-phosphate and two of the three carbons of phosphoenolpyruvate (PEP). The initial step in the pathway (Fig. 25-1, step a) is the condensation of erythrose 4-P with PEP and is catalyzed by 3-deoxy-D-arafrmo-heptulosonate-7-phosphate (DAHP) synthase. Closely analogous to an aldol condensation, the mechanism provides a surprise.10 When PEP containing lsO in the oxygen bridge to the phospho group reacts, the lsO is retained in the eliminated phosphate biochemical intuition would suggest that it should stay in the... 

The shikimate pathway is common to both plants and microorganisms (Figure 3-3). Shikimate is synthesized from the substrates phosphoewo/pyruvate (3.9) and erythrose 4-phosphate (3.17). These two precursors are derived from glycolysis and the pentose phosphate pathway, respectively, and are condensed to 3-deoxy-D-ara6/ o-heptulosonate 7-phosphate (DAHP 3.18) by the enzyme DAHP synthase. The subsequent steps result in the formation of 3-dehydro-quinate (3.19) by the enzyme 3-dehydroquinate synthase, 3-dehydroshikimate... 

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-...

PEP) shikimate is biosynthesized via 3-deoxy-D-arafeino-heptulosonate 7-phosphate (DAHP), dehydroquinate (DHQ), and dehydroshikimate (DHS) (Scheme 6.4.1). 

Herrmann, K. M., Schultz, J. and Hermodson, M. A. (1980). Sequence homology between the tyrosine-sensitive 3-deoxy-D-ara zn( -heptulosonate 7-phosphate synthase from Escherichia coli and hemerythrin from Sipunculida. Journal of Biological Chemistry, 255, 7079-81. 

Bromopyruvic acid inhibits S-deoxy-D-erj t ro-hexulosonate 6-phosphate aldolase " and 3-deoxy-D-ar-a6mo-heptulosonate 7-phosphate synthetase " in a similar way. The enzymes are protected from inhibition by pyruvate and phospho-enol pyruvate respectively. Since aldolases must possess a nucleophilic group to initiate the reaction, this type of inhibition should be a general property of aldolases. 

Figure 5 The shikimate-chorismate pathway. DAHP, 3-deoxy-o-arabino-2-heptulosonate-7-phosphate EPSP,...

Deoxy-D-arahmo-heptulosonate 7-phosphate (DAHP) synthase is the first enzyme of the common aromatic biosynthetic pathway in bacteria and plants (212). Three such isozymes have been identified in E. coli, which are sensitive to phenylalanine, tyrosine, and tryptophan, respectively. The tyrosine-sensitive DAHP synthase has been found to contain approximately one iron per mole of enzyme and to exhibit an absorption maximum at... 

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