Dehydroquinate pathway activity

Enzymes involved as catalysts in each of the steps from 3-dehydro-quinate (10) to chorismate (14) in the conunon pathway have all been subsequently isolated and characterised from bacterial mutants. Methods of assay for each form of activity have been described . Mitsuhashi and Davis first isolated 3-dehydroquinate dehydratase (E.C. 4.1.2.10) the enzyme which is responsible for the dehydration step (10 11). With a partially purified extract they showed that the... 

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. 

Quinic acid, known to be widely distributed in plants, has to be considered as a possible intermediate in aromatic biosynthesis. It has been reported to exhibit a low-order growth activity for a Neurospora mutant (S06). Secondary mutants of Aercbactor that responded well to dehydroquinic acid also responded equally well to quinic acid ( 07). E. coli mutants that utilized dehydroquinic acid, on the contrary, did not respond to quinic acid. In addition no Aerabactor auxotroph block between quinic acid and dehydroquinic could be found. In view of this and the enzymic evidence presented below, quinic acid is considered not to be on the direct pathway of aromatic biosynthesis. 

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