Phosphogluconic acid dehydrogenase

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 carboxylase. 6PGD. 

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

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. 

After oxidation, the phosphogluconic acid undergoes decarboxylation. In this reaction catalyzed by 6-phos-phogluconate dehydrogenase, as in that catalyzed by the malic enzyme, no free intermediate has been detected, and, again, the reaction presumably occurs on a single protein. The oxidative carboxylation is NADP-dependent and yields D-ribulose-5-phosphate, CO2, and NADPH as final products. This is the only step in the cycle in which CO2 is formed. 

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