Dehydroquinate pathway reactions

The shikimate pathway begins with a coupling of phosphoenolpyruvate (PEP) and D-erythrose 4-phosphate to give the seven-carbon 3-deoxy-D-arabino-heptulo-sonic acid 7-phosphate (DAHP) through an aldol-type condensation. Elimination of phosphoric acid from DAHP, followed by an intramolecular aldol reaction, generates the first carbocyclic intermediate, 3-dehydroquinic acid. Shikimic acid (394) is... 

The dehydration of 3-dehydroquinate to 3-dehydroshiki-mate catalyzed by 3-dehydroquinase (EC 4.2.1.10) is common to two metabolic pathways, the inicrobial biosynthetic shikimate pathway and the catabolic quinate pathway. This reaction is catalyzed by two completely different enzymes either a Type I enzyme that catalyzes a syn elimination or a Type II enzyme that catalyzes an anti elimination. 

During the elucidation of the aromatic biosynthetic pathway, compounds were found that appear to be made in side reactions. These include quinic acid, which is reversibly made from dehydroquinic acid through the action of a DPN-specific dehydrogenase. This enzyme is found in Aerobacter, but not in E. coli. 5-Phosphoshikimic acid is also accumulated by certain coli mutants, but has no known metabolic func-... 

Mutant strains of Escherichia coli and Aerobacter aerogenes were described which had a quintuple requirement of aromatic substrates (L-phenylalanine, L-tyrosine, L-tryptophan, 4-amino-benzoate and 4-hydroxybenzoate) for growth. Certain of these mutants were found to accumulate (—)-shikimic acid (4) in their culture filtrates and other mutants, blocked in earlier reactions in the pathway, were able to utilise (—)-shikimic acid (4) to replace the aromatic sutetrates. These observations established with great probability that (—)-shikimic add was a common precursor for each of these aromatic compounds. Experiments of this type permitted each of the intermediate in the common pathway, 3-dehydroquinic add (10), 3-dehydroshikimic add (11), (—)-shikimic add (4), shikimic add-3-phosphate (12), 5-enolpyruvylshikimic add-3-phosphate (13) and chorismic acid (14), to be isolated and characterised and for the pathway... 

Dehydroquinate synthetase, the enzyme responsible for the cyclisation of DAHP (9) to give 3-dehydroquinate (10), the first cyclic intermediate in the shikimate pathway, was obtmned in partially purified form from Escherichia coli. The enzyme required Co and NAD" " (but not NADP" ) for full activity. No intermediates were isolable when these cofactors were removed but it was observed in a kinetic analysis of the enzymic transformation that the release of orthophosphate, the disappearance of DAHP and the formation of 3-dehydroquinate aU proceeded at the same rate. These observations indicated, it was suggested, that one enzyme was responsible for the whole sequence of reactions necessary for the convosion. On the basis of these observations, Sprinson and his collaborators formulated a working hypothesis for the steps involved in the cyclisation of the substrate DAHP and this is discussed in more detail later. 

The reactions of the shikimate pathway pose a number of interesting questions of both a mechanistic and stereochemical nature. Detailed studies have been made of the DAHP synthetase, 3-dehydroquinate dehydratase, 5-enolpyruvylshikimate-3-phosphate synthetase and chorismate synthetase reactions which have added further important knowledge to this area of molecular biology. Whilst these investigations have done nothing to detract from the important dictum that cells obey the laws of chemistry , they have nevertheless revealed some of the distinctive facets of enzyme chemistry and have highlighted some of the important differences between enzyme and relat chemically catalysed reactions. 

Partial conversion of 3-dehydroquinic acid to 3-dehydroshikimic acid (38) can be achieved by mild acid or base treatment and in contrast to the parallel enzymic conversion, which forms an integral part of the shikimate pathway, the mechanism for this reaction has been shown to involve a tram elimination of Hjax and the hydroxyl at C-1. 

The shikimic acid pathway begins with phosphoenolpyruvate which is obtained from glycolysis, and D-erythrose-4-phosphate, which comes from the pentose phosphate cycle. The two are linked to form an intermediate with 7 C atoms which cyclizes to 5-dehydroquinic acid. The latter exists in equilibrium with quinic acid. The pathway proceeds via 5-dehydroshikimic acid and shikimic acid to 5-phosphoshikimic acid. An additional phosphoenolpyruvate unit is now attached to the last-mentioned compound. The product of this reaction is converted, in several steps, to chorismic acid. 

Figure 2.2. Enzymes and reactions of the shikimate pathway A, DAMP synthase B, dehydroquinate synthase C, dehydroquinase D, shikimate dehydrogenase E, shikimate kinase F, EPSP synthase G, chorismate synthase.

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