Phosphogluconic acid

Natural derivatives of sugars. The aldehyde group of an aldose can be oxidized readily to a carboxyl group to form an aldonic acid. Among the several aldonic acids that occur naturally is 6-phosphogluconic acid, which is pictured here as the 6-phosphogluconate ion ... 

In a certain group of bacteria, still another pathway (Entner-Doudomff pathway) for the utilization of glucose has been studied. Here glucosc-6-phosphate is oxidized to 6-phosphogluconic acid which is dehydrated to 2-keto-3-deoxy-6-phusphogluconic acid. This substance is then split to pyruvic acid and glyceraldehyde-3-phosphatc l which alsu can be converted to pyruvic acid). 

The four reactions involved in this conversion are shown in figure 12.31. The first oxidation, catalyzed by glu-cose-6-phosphate dehydrogenase at C-1, converts the hemi-acetal derivative of the aldehyde group to the lactone of the corresponding acid, 6-phosphogluconic acid. After hydrolysis of the lactone, the second oxidation at C-3, converts the secondary alcohol to a ketone. The expected product, 3-keto-6-phosphogluconic acid, is decarboxylated yielding ribulose-5-phosphate. 

The oxidative decarboxylation of 6-phosphogluconic acid occurs in a stepwise mechanism involving a )8-keto acid intermediate 64). However, it appears that no metal ion is required in the reaction (66). 2-Deoxy-6-phosphogluconate is also a substrate for this enzyme, but the reaction is about two orders of magnitude slower than reaction of the natural substrate. The keto acid produced in the initial oxidation is released into solution and subsequently undergoes slow enzyme-catalyzed decarboxylation (67). 

The first step in the HMP pathway (Fig. 17.6) is the oxidation of glucose-6-phosphate by an NADP+-dependent dehydrogenase. The product, 6-phospho-gluconolactone, is converted to 6-phosphogluconic acid by a lactonase. A second oxidative step mediated by 6-phosphogluconic acid dehydrogenase. 

Phosphogluconic acid dehydrogenase, 6-Phosphogluconic dehydrogenase, 6-Phosphogluconic acid carboxylase] (1.1.1.44) is present. 

Evidence has been presented that the primary oxidation product of glucose-6-phosphate is the 5-lactone of 6-phosphogluconic acid (Fig. 9). This product was observed to react rapidly with hydroxylamine to form a hydroxamic acid in a manner analogous to the reactivity of 5-glucono-... 

Fio. 9. The dehydrogenation of glucose-6-phosphate to 6-phosphogluconic acid via gluconolactone-6-phosphate. 

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