Dehydroshikimate formation

Biosynthesis of Tea Flavonoids. The pathways for the de novo biosynthesis of flavonoids in both soft and woody plants (Pigs. 3 and 4) have been generally elucidated and reviewed in detail (32,51). The regulation and control of these pathways in tea and the nature of the enzymes involved in synthesis in tea have not been studied exhaustively. The key enzymes thought to be involved in the biosynthesis of tea flavonoids are 5-dehydroshikimate reductase (52), phenylalanine ammonia lyase (53), and those associated with the shikimate/arogenate pathway (52). At least 13 enzymes catalyze the formation of plant flavonoids (Table 4). 

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... 

Figure 12 Reactions leading to the formation of 4-hydroxybenzoate. (41) 3-dehydroshikimate (42) shikimate (4) CHA, chorismate (9) prephenate (10) 4-hydroxyphenylpyruvate (12) tyrosine (43) 4-hydroxyphenyllactate (44) 4-hydroxycinnamate or 4-coumarate (45) 4-coumaroyl-CoA (46) /3-hydroxythioester of 4-coumaroyl-CoA (47) acetyl-CoA. (48) 4-hydroxybenzaldehyde (4-HBA) (11) 4-hydroxybenzoate (4-HB).

Rifamycin S derived from [l- C]glycerate showed enhanced n.m.r. signals for C-3 and C-8 which is consistent with incorporation by way of intermediates on the shikimate pathway (Scheme 22). ° Greater enhancement of C-8 by [l- C]glycerate and of C-1 by [l- C]glucose was observed, compared respectively with C-3 and C-10. This indicates that C-1 derives from the methylene carbon of phosphoenol-pyruvate rather than C-4 of tetrose phosphate and that C-8 derives from the carboxy-group of phosphoenolpyruvate. It follows then that C-9 and C-10 of rifamycin S (193) would be the location of the double bond of a dehydroshikimate intermediate. Michael addition to this double bond as in (194) allows completion of the naphthoquinone moiety of rifamycin S in an analogous fashion to the formation of the menaquinones. ... 

In this process, sugars, obtained from biomass, are fermented at low pH into cis-muconic acid. The process of microbial muconic adic formation was already described by Frost and coworkers, who developed E. coli WNl/pWN2.248 that synthesized 36.8 g/L of c/s,ci>muconic acid in 22% (mol/mol) yield from glucose after 48 h of culturing under fed-batch fermentation conditions [147]. This strain did not possess the aroE encoded shikamate dehydrogenase preventing the cells to convert 3-dehydroshikimic acid into shikimic acid which is available for production of cis,cis-muconic acid. Optimization of microbial cis.m-muconic acid synthesis required expression of three enzymes not typically found in E. coli. A recent patent application by Bui et al. describes a productivity of 59 g/L cis muconic acid from 248 g/L glucose by a modified E. coli. in a 20 L fermenter in 88 h. 

The products accumulated and isolated from the cultures of mutants are sometimes not those of the actual pathway, but modified products derived from intermediates involved. For example, in Neurospora mutants blocked after the formation of dehydroshikimic acid, only protocatechuic and vanillic acids (derived from dehydroshikimic acid) accumulate. In other instances, an intermediate or a derivative of an intermediate may be converted by a pathway that usually... 

Fig. 143. Formation of dehydroquinic, dehydroshikimic, shikimic and chorismic acids and secondary products derived from these compounds...

It may be appropriate to say that in the above synthesis, use of unmodified E. coli gives the amino acids, L-phenylalanine, L-tyrosine and L-tryptophan via the formation of shikimic acid from dehydroshikimic acid (Scheme 6). 

Further support for the operation of the shikimate pathway in higher plants has been provided by studies on enzymes isolated and characterised from plant sources. DAMP synthetase activity has thus been demonstrated in extracts of several plant tissues. An enzyme preparation was also obtained from sweet potato which catalysed the formation of DAHP from D-erythrose-4-phosphate (7) and phosphoenolpyruvate (8) and had properties very similar to those enzymes isolated from bacterial sources. Nandy and Ganguli have similarly demonstrated the presence of DAHP synthetase activity in mung bean Phaseolus aureus) by showing that extracts of mung bean seedlings converted a mixture of the two substrates (7 and 8) to 3-dehydroshikimate (11). 

Adachi O, Ano Y, Toyama H, Matsushita K (2008b) A novel 3-dehydroquinate dehydratase catalyzing extracellular formation of 3-dehydroshikimate by oxidative fermentation of Gluconobacter oxydans IFO 3244. Biosci Biotechnol Biochem 72(6) 1475-1482. Epub 2008 Jun 7... 

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