Irreversible Oxidative Reactions

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Chapter: Biochemistry : Pentose Phosphate Pathway and Nicotinamide Adenine Dinucleotide Phosphate

The oxidative portion of the pentose phosphate pathway consists of three reactions that lead to the formation of ribulose 5-phosphate, CO2, and two molecules of NADPH for each molecule of glucose 6-phosphate oxidized.


IRREVERSIBLE OXIDATIVE REACTIONS

The oxidative portion of the pentose phosphate pathway consists of three reactions that lead to the formation of ribulose 5-phosphate, CO2, and two molecules of NADPH for each molecule of glucose 6-phosphate oxidized (Figure 13.2). This portion of the pathway is particularly important in the liver, lactating mammary glands, and adipose tissue, which are active in the NADPH-dependent biosynthesis of fatty acids; in the testes, ovaries, placenta, and adrenal cortex, which are active in the NADPH-dependent biosynthesis of steroid hormones; and in red blood cells (RBCs), which require NADPH to keep glutathione reduced.

 

A. Dehydrogenation of glucose 6-phosphate

Glucose 6-phosphate dehydrogenase (G6PD) catalyzes an irreversible oxidation of glucose 6-phosphate to 6-phosphogluconolactone in a reaction that is specific for oxidized NADP (NADP+) as the coenzyme. The pentose phosphate pathway is regulated primarily at the G6PD reaction. NADPH is a potent competitive inhibitor of the enzyme, and, under most metabolic conditions, the ratio of NADPH/NADP+ is sufficiently high to substantially inhibit enzyme activity. However, with increased demand for NADPH, the ratio of NADPH/NADP+ decreases, and flux through the cycle increases in response to the enhanced activity of G6PD. Insulin upregulates expression of the gene for G6PD, and flux through the pathway increases in the absorptive state.

 

B. Formation of ribulose 5-phosphate

6-Phosphogluconolactone is hydrolyzed by 6-phosphogluconolactone hydrolase. The reaction is irreversible and not rate limiting. The oxidative decarboxylation of the product, 6-phosphogluconate, is catalyzed by 6-phosphogluconate dehydrogenase. This irreversible reaction produces a pentose sugar–phosphate (ribulose 5-phosphate), CO2 (from carbon 1 of glucose), and a second molecule of NADPH (see Figure 13.2).


Figure 13.2 Reactions of the pentose phosphate pathway. Enzymes numbered above are: 1, 2) glucose 6-phosphate dehydrogenase and 6-phosphogluconolactone hydrolase,3) 6-phosphogluconate dehydrogenase, 4) ribose 5-phosphate isomerase, 5) phosphopentose epimerase, 6 and 8) transketolase (coenzyme: thiamine pyrophosphate), and 7) transaldolase. Δ2C = two carbons are transferred in transketolase reactions; Δ2C = three carbons are transferred in the transaldolase reaction. This can be represented as: 5C sugar Δ2C, + 5C sugar 7C sugar + 3C sugar Δ2C, 4C sugar + 6C sugar. NADP(H) = nicotinamide adenine dinucleotide phosphate; P = phosphate.

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