Quinate dehydrogenase Quinic acid

The linking molecules for both pathw s are 3-dehydroquinic acid and 3-dehydroshikimic acid. 3-dehydro-quinic acid can be formed in a one-step reaction catalysed by quinate-dehydrogenase from quinic acid, an abundant plant metabolite (Figure 2). 

Figure 2. The biosynthesis of 3-dehydroquinic acid and 3-dehydroshikimic acid from quinic acid (1... quinate-dehydrogenase 2... dehydrogenase).

Quinic acid is also found throughout the plant kingdom, usually with shikimic acid. It occurs as the free acid in many plants and its phenolic esters are also very common. The simplest assumption for the biosynthesis of quinic acid is through the reduction of 3-dehydroquinic acid, catalyzed by quinate dehydrogenase. However, because of the rare occurrence of this enzyme, the biosynthesis of quinic acid still remains to be explained. 

Mitsuhashi and Davis 213) also discovered a dehydrogenase that oxidizes quinic acid to dehydroquinic acid in certain mutants of Aerdbactor. In this instance DPN is the required pyridine nucleotide. The bacterial strains that contain this enzyme respond to quinate as a growth factor. This dehydrogenase was purified in the same manner and to about the same extent as dehydroshikimic reductase. The pH optimum of quinic dehydrogenase was found to be at 9.8 Km values obtained were 4.9 X 10" M for quinic acid and 1.4 X 10 M for DPN at pH 9.4. 

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