BIOCHEMISTRY: A TEXTBOOK FOR HIGHER EDUCATION — E. S. Severin — 2004

CHAPTER 7. CARBOHYDRATE METABOLISM

XII. The Pentose Phosphate Pathway of Glucose Metabolism

The Pentose Phosphate Pathway, also known as the hexose monophosphate shunt, serves as an alternative pathway for The oxidation of glucose-6-phosphate. It consists of two phases: oxidative and non-oxidative.

In the oxidative phase, glucose-6-phosphate is irreversibly oxidized into a pentose, ribulose-5-phosphate, yielding reduced NADPH.

In the non-oxidative phase, ribulose-5-phosphate is reversibly converted into ribose-5-phosphate and Glycolysis metabolites.

The pentose phosphate pathway provides Cells with ribose for the Synthesis of purine and pyrimidine NUCLEOTIDES, as well as with the hydrogenated coenzyme NADPH, which is utilized in reductive processes.

The overall equation for the pentose phosphate pathway is expressed as follows:

3 Glucose-6-phosphate + 6 NADP⁺ —> 3 CO₂ + 6 (NADPH + H⁺) + 2 Fructose-6-phosphate + Glyceraldehyde-3-phosphate.

The Enzymes of the pentose phosphate pathway, much like those of glycolysis, are localized in the Cytosol.

The pentose phosphate pathway is most active in adipose tissue, the Liver, the adrenal cortex, erythrocytes, the mammary gland during Lactation, and the Testes.

A. Oxidative Stage

In the oxidative phase of the pentose phosphate pathway, glucose-6-phosphate undergoes oxidative decarboxylation, resulting in The formation of pentoses. This stage comprises two dehydrogenation reactions.

The first dehydrogenation reaction—The conversion of glucose-6-phosphate to gluconolactone-6-phosphate—is catalyzed by NADP⁺-dependent glucose-6-phosphate dehydrogenase and is accompanied by the oxidation of the aldehyde group at the first carbon atom and the generation of one molecule of the reduced coenzyme NADPH.

Subsequently, gluconolactone-6-phosphate is rapidly converted into 6-phosphogluconate through the action of the enzyme gluconolactonase.

The enzyme 6-phosphogluconate dehydrogenase catalyzes the second dehydrogenation reaction of the oxidative phase, which also involves decarboxylation. As a result, the carbon chain is shortened by one carbon atom, yielding ribulose-5-phosphate and a second molecule of hydrogenated NADPH (Fig. 7-62).

Class="center">Fig. 7-62. Oxidative stage of the pentose phosphate pathway.

Reduced NADPH inhibits glucose-6-phosphate dehydrogenase, the first enzyme of the oxidative stage of the pentose phosphate pathway. The conversion of NADPH back to its oxidized state, NADP⁺, alleviates this enzymatic inhibition. Consequently, The rate of the corresponding reaction increases, leading to higher NADPH production.

The overall equation for the oxidative stage of the pentose phosphate pathway can be represented as:

Glucose-6-phosphate + 2 NADP⁺ + H₂O —> Ribulose-5-phosphate + 2 (NADPH + H⁺) + CO₂.

The Reactions of the oxidative stage serve as the primary cellular source of NADPH. Hydrogenated Coenzymes supply hydrogen for biosynthetic processes and oxidation-reduction reactions, including cellular defense against reactive oxygen species. As a hydrogen donor, NADPH participates in anabolic processes, such as Cholesterol synthesis. It provides reducing equivalents for cytochrome P₄₅₀, which catalyzes the formation of hydroxyl groups during the synthesis of Steroid Hormones and Bile acids, as well as during the Catabolism of drugs and other xenobiotics (see Chapters 8, 11, and 12). High activity of glucose-6-phosphate dehydrogenase is found in phagocytic leukocytes, where NADPH oxidase utilizes reduced NADPH to generate superoxide radicals from molecular oxygen. The superoxide radical generates other reactive species of oxygen that damage the DNA, Proteins, and Lipids of bacterial cells. The synthesis of Fatty acids from CARBOHYDRATES in the liver is a major pathway for NADPH utilization and ensures the regeneration of the oxidized form, NADP⁺. In the liver, glucose-6-phosphate dehydrogenase—along with Key Enzymes of glycolysis and FATTY ACID Biosynthesis—is induced by an elevated Insulin/Glucagon ratio following a carbohydrate-rich meal.

Although NADPH is also generated during the oxidation of malate to Pyruvate and carbon dioxide (mediated by NADP⁺-dependent malate dehydrogenase) and the dehydrogenation of isocitrate (mediated by NADP⁺-dependent isocitrate dehydrogenase), cellular requirements for reducing equivalents are met primarily by the pentose phosphate pathway in most instances.

The reactions of the oxidative pathway proceed only if the reduced coenzyme NADPH is converted back to its initial oxidized state, NADP⁺, via NADPH-dependent dehydrogenases (i.e., provided that hydrogenated NADPH is consumed in reductive processes). If cellular demands for NADPH are low, ribose-5-phosphate is synthesized via the reversible reactions of the non-oxidative stage of the pentose phosphate pathway, utilizing glycolysis metabolites—glyceraldehyde-3-phosphate and fructose-6-phosphate—as precursors.

B. Non-Oxidative Stage

The non-oxidative phase of the pentose phosphate pathway comprises a series of reversible reactions that convert ribulose-5-phosphate into ribose-5-phosphate and xylulose-5-phosphate, followed by The transfer of carbon units to glycolysis intermediates: fructose-6-phosphate and glyceraldehyde-3-phosphate. These conversions are catalyzed by epimerase, isomerase, transketolase, and transaldolase. Transketolase utilizes Thiamine diphosphate as a coenzyme. Because the non-oxidative stage of the pentose phosphate pathway lacks dehydrogenation reactions, it is exclusively dedicated to pentose synthesis.

Ribulose-5-phosphate serves as a substrate for two enzymes. Ribulose-5-phosphate 3-epimerase alters the stoichiometric position of a single hydroxyl group at the third carbon atom, converting ribulose-5-phosphate into xylulose-5-phosphate. Another enzyme, ribulose-5-phosphate isomerase, catalyzes the conversion of ribulose-5-phosphate into ribose-5-phosphate (Fig. 7-63). The ribose-5-phosphate produced in the non-oxidative phase supplies cells with the ribose required for nucleotide synthesis, which act as precursors and structural components for dehydrogenase coenzymes and Nucleic Acids.

Fig. 7-63. Conversions of ribulose-5-phosphate.

Transketolase and transaldolase catalyze the transfer of two- and three-carbon units, respectively, using a ketose as the carbon donor and an aldose as the acceptor. These reactions proceed in two steps: first, a carbon fragment is cleaved from the donor molecule, and second, this fragment is transferred to the acceptor molecule. In the non-oxidative phase of the pentose phosphate pathway, transketolase catalyzes two reactions. In the first reaction (Fig. 7-64), transketolase cleaves the C–C bond between the keto group and the adjacent carbon atom in xylulose-5-phosphate, converting the ketosugar into an aldose, glyceraldehyde-3-phosphate, which contains two fewer carbon atoms. The resulting two-carbon fragment remains covalently bound to the coenzyme thiamine diphosphate at the Active Site of the enzyme. Next, the enzyme transfers the two-carbon fragment to the aldehyde group of an aldosugar, yielding a new ketose: sedoheptulose-7-phosphate.

Fig. 7-64. Two-carbon fragment transfer reaction catalyzed by transketolase.

Transaldolase transfers a three-carbon unit from sedoheptulose-7-phosphate to glyceraldehyde-3-phosphate, producing erythrose-4-phosphate and fructose-6-phosphate (Fig. 7-65).

Fig. 7-65. Reaction catalyzed by transaldolase.

This reaction is analogous to the aldolytic Cleavage in glycolysis, except that here the three-carbon fragment containing the keto group is transferred to the aldosugar glyceraldehyde-3-phosphate, whereas in glycolysis the keto fragment is released as dihydroxyacetone phosphate.

In the subsequent transketolase-catalyzed reaction, a two-carbon unit is transferred from xylulose-5-phosphate to erythrose-4-phosphate. The products of this reaction are fructose-6-phosphate and glyceraldehyde-3-phosphate (Fig. 7-66).

Fig. 7-66. Reaction catalyzed by transketolase.

Since all reactions of the non-oxidative stage are reversible, ribose-5-phosphate can be formed not only via the isomerization of the oxidative phase product ribulose-5-phosphate by isomerase, but also from the glycolytic intermediates fructose-6-phosphate and glyceraldehyde-3-phosphate. The sequence of transformations leading to the formation of ribose-5-phosphate from such glycolytic products can be represented as follows:

2 Fructose-6-phosphate + Glyceraldehyde-3-phosphate —> 2 Xylulose-5-phosphate + Ribose-5-phosphate

2 Xylulose-5-phosphate —> 2 Ribulose-5-phosphate

2 Ribulose-5-phosphate —> 2 Ribose-5-phosphate.

The net result of the METABOLISM of 3 molecules of ribulose-5-phosphate in the non-oxidative phase of the pentose phosphate pathway is the formation of 2 molecules of fructose-6-phosphate and 1 molecule of glyceraldehyde-3-phosphate. Subsequently, fructose-6-phosphate and glyceraldehyde-3-phosphate can be converted into glucose. Accounting for a stoichiometric coefficient of 2, the formation of 5 glucose molecules (containing 30 carbon atoms) requires 4 molecules of fructose-6-phosphate and 2 molecules of glyceraldehyde-3-phosphate (together also containing 30 carbon atoms), or alternatively, 6 molecules of ribulose-5-phosphate. Thus, the non-oxidative pathway can be viewed as a process that channels pentoses back into the hexose pool.

B. The Pentose Phosphate Cycle

The oxidative phase of pentose formation and the non-oxidative phase (the pathway returning pentoses to hexoses) together constitute a cyclic process.

This process can be described by the overall equation:

6 Glucose-6-phosphate + 12 NADP⁺ + 2 H₂O —> 5 Glucose-6-phosphate + 12 NADPH + 12 H⁺ + 6 CO₂.

This means that 6 molecules of glucose yield 6 molecules of ribulose-5-phosphate (a pentose) and 6 molecules of CO₂. The enzymes of the non-oxidative phase convert 6 molecules of ribulose-5-phosphate into 5 molecules of glucose (a hexose). When these reactions run sequentially, the only useful product is NADPH, generated during the oxidative phase of the pentose phosphate pathway. This process is referred to as the pentose phosphate cycle (Fig. 7-67).

Fig. 7-67. The pentose phosphate cycle in adipose tissue.

The pentose phosphate pathway enables cells to produce the NADPH required for lipid synthesis without accumulating pentoses.

The energy released during glucose breakdown is transformed into the energy of a high-energy hydrogen donor, NADPH. Hydrogenated NADPH serves as a hydrogen source for reductive biosyntheses, while the energy of NADPH is converted and stored in newly synthesized substances, such as fatty acids, and is released during their catabolism for cellular use.

G. Glucose-6-phosphate dehydrogenase deficiency in erythrocytes

The non-enzymatic oxidation of Hemoglobin (Fе²⁺) to methemoglobin (Fе³⁺) leads to the single-electron reduction of oxygen, yielding a reactive anion radical—superoxide O_2-, which serves as a precursor to other reactive oxygen species: hydrogen peroxide Н₂O₂ and the hydroxyl radical ОН^-. Reactive oxygen species are potent oxidizing agents capable of causing severe damage to DNA molecules, proteins, and unsaturated lipids.

Erythrocytes, like most cells, contain a thiol-containing tripeptide—Glutathione (γ-glutamyl-cysteinyl-Glycine). The reduced form of glutathione (G-SH) features an SH group (Fig. 7-68) that can act as an electron donor in reduction reactions. Catalyzed by the enzyme glutathione peroxidase, reduced glutathione converts a hydrogen peroxide molecule into a Water molecule while becoming oxidized itself (G-SS-G). The regeneration of reduced glutathione is driven by glutathione reductase, utilizing hydrogen-donating NADPH. For erythrocytes, the pentose phosphate pathway serves as the sole source of NADPH, whereas other Tissues possess an alternative pathway involving NADP-dependent malate dehydrogenase (malic enzyme).

Fig. 7-68. Reduction of glutathione by glutathione reductase. A — Structure of glutathione; B — reduction of glutathione.

The interaction of reduced glutathione with hydrogen peroxide in erythrocytes protects the Cysteine residues in hemoglobin protomers from oxidation. A genetic deficiency in glucose-6-phosphate dehydrogenase leads to a drop in reduced NADPH levels, which in turn severely depletes reduced glutathione and correspondingly increases the accumulation of reactive oxygen species within The Cell. Under these conditions, the oxidation of SH groups in hemoglobin molecules triggers the formation of cross-linked Disulfide Bonds and the aggregation of hemoglobin protomers, resulting in the formation of Heinz bodies (see Section 14). The presence of Heinz bodies impairs membrane elasticity, causing erythrocytes to lose their ability to deform as they pass through capillaries. This compromises membrane integrity and leads to hemolysis. Furthermore, certain drugs—such as the antimalarial agent primaquine and sulfonamides—also diminish the capacity of erythrocytes to counteract reactive oxygen species.