Two NADPH Molecules Are Generated by the Pentose Phosphate Pathway

Two NADPH Molecules Are Generated by the Pentose Phosphate Pathway 

NADPH is required for many biosynthetic sequences. It is generated in different kinds of cells by a variety of reactions, including an NADP+-linked oxidation of malate to pyruvate and C02 and transfer of hydride ion from NADH to NADP+ in a mitochondrial reaction that is driven by metabolic energy. However, in many cases, including in the mammalian liver, a major part of the NADPH requirement is met by oxidation of glucose-6-phosphate to ribulose-5-phosphate and C02. The four electrons that are released by the oxidation are transferred to two molecules of NADP+. 

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. 

At this point the metabolic function of the sequence, when it is serving to supply electrons for biosynthesis, is fulfilled. Two molecules of NADPH are generated for each molecule of glucose-6-phosphate oxidized. It is only necessary to convert ribulose-5-phosphate, the end product of the 

Stage 1 of the pentose phosphate pathway. Net Reaction Glucose-6 phosphate + 2 NADP+ — Ribulose-5-phosphate + C02 + 

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