Pentose phosphate pathway

The pentose phosphate pathway (PPP, also known as the hexose monophosphate pathway) is an oxidative metabolic pathway located in the cytoplasm, which, like glycoly-sis, starts from glucose 6-phosphate. It supplies two important precursors for anabolic pathways: NADPH+H+, which is required for the biosynthesis of fatty acids and isopren-oids, for example (see p. 168), and ribose 5-phosphate, a precursor in nucleotide biosynthesis (see p. 188).

A. Pentose phosphate pathway: oxidative part 3

The oxidative segment of the PPP converts glucose 6-phosphate to ribulose 5-phosphate. One CO2 and two NADPH+H+ are formed in the process. Depending on the metabolic state, the much more complex regenerative part of the pathway (see B) can convert some of the pentose phosphates back to hexose phosphates, or it can pass them on to glycol-ysis for breakdown. In most cells, less than 10% of glucose 6-phosphate is degraded via the pentose phosphate pathway.

B. Reactions 3

[1] The oxidative part starts with the oxidation of glucose 6-phosphate by glucose-6-phosphate dehydrogenase. This forms NADPH+H+ for the first time. The second product, 6-phosphogluconolactone, is an intramolecular ester (lactone) of 6-phospho-gluconate.

[2] A specific hydrolase then cleaves the lactone, exposing the carboxyl group of 6-phosphogluconate.

[3] The last enzyme in the oxidative part is phosphogluconate dehydrogenase [3], which releases the carboxylate group of 6-phospho-gluconate as CO2 and at the same time oxidizes the hydroxyl group at C3 to an oxo group. In addition to a second NADPH+H+, this also produces the ketopentose ribulose 5-phosphate. This is converted by an isomer-ase to ribose 5-phosphate, the initial compound for nucleotide synthesis (top).

The regenerative part of the PPP is only shown here schematically. A complete reaction scheme is given on p. 408. The function of the regenerative branch is to adjust the net production of NADPH+H+ and pentose phosphates to the cell's current requirements. Normally, the demand for NADPH+H+ is much higher than that for pentose phosphates. In these conditions, the reaction steps shown first convert six ribulose 5-phosphates to five molecules of fructose 6-phosphate and then, by isomerization, regenerate five glucose 6-phosphates. These can once again supply NADPH+H+ to the oxidative part of the PPP. Repeating these reactions finally results in the oxidation of one glucose 6-phosphate into six CO2. Twelve NADPH+H+ arise in the same process. In sum, no pentose phosphates are produced via this pathway.

In the recombination of sugar phosphates in the regenerative part of the PPP, there are two enzymes that are particularly important:

[5] Transaldolase transfers C3 units from sedoheptulose 7-phosphate, a ketose with seven C atoms, to the aldehyde group of glyc-eraldehyde 3-phosphate.

[4] Transketolase, which contains thiamine diphosphate, transfers C2 fragments from one sugar phosphate to another.

The reactions in the regenerative segment of the PPP are freely reversible. It is therefore easily possible to use the regenerative part of the pathway to convert hexose phosphates into pentose phosphates. This can occur when there is a high demand for pentose phosphates—e.g., during DNA replication in the S phase of the cell cycle (see p. 394).

Additional information

When energy in the form of ATP is required in addition to NADPH+H+, the cell is able to channel the products of the regenerative part of the PPP (fructose 6-phosphate and glyceraldehyde 3-phosphate) into glycolysis. Further degradation is carried out via the tri-carboxylic acid cycle and the respiratory chain to CO2 and water. Overall, the cell in this way obtains 12 mol NADPH+H+ and around 150 mol ATP from 6 mol glucose 6-phos-phate. PPP activity is stimulated by insulin (see p. 388). This not only increases the rate of glucose degradation, but also produces additional NADPH+H+ for fatty acid synthesis (see p. 168).

- A. Pentose phosphate pathway: oxidative part

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