Pentose phosphate pathway

The pentose phosphate pathway (also called Phosphogluconate Pathway, or Hexose Monophosphate Shunt [HMP shunt]) is a cytosolic process that serves to generate NADPH and the synthesis of pentose (5-carbon) sugars. There are two distinct phases in the pathway. The first is the oxidative phase, in which NADPH is generated, and the second is the non-oxidative synthesis of 5-carbon sugars. This pathway is an alternative to glycolysis. While it does involve oxidation of glucose, its primary role is anabolic rather than catabolic.

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Uses

Located exclusively in the cytoplasm, the pathway is one of the three main ways the body creates molecules with reducing power, accounting for approximately 60% of NADPH production in humans.

One of the uses of NADPH in the cell is to prevent oxidative stress. It reduces the coenzyme glutathione, which converts reactive H₂O₂ into H₂O. If absent, the H₂O₂ would be converted to hydroxyl free radicals, which can attack the cell.

It is also used to generate hydrogen peroxide for phagocytes.^[1]

Phases

Oxidative phase

In this phase, two molecules of NADP⁺ are reduced to NADPH, utilizing the energy from the conversion of glucose-6-phosphate into ribulose 5-phosphate.

The entire set of reactions can be summarized as follows:

The overall reaction for this process is:

Glucose 6-phosphate + 2 NADP⁺ + H₂O → ribulose 5-phosphate + 2 NADPH + 2 H⁺ + CO₂

Non-oxidative phase

Regulation

Glucose-6-phosphate dehydrogenase is the rate-controlling enzyme of this pathway. It is allosterically stimulated by NADP⁺. The ratio of NADPH:NADP⁺ is normally about 100:1 in liver cytosol. This makes the cytosol a highly-reducing environment. Formation of NADP⁺ by a NADPH-utilizing pathway, thus, stimulates production of more NADPH.

See also

• G6PDH deficiency - A hereditary disease that disrupts the pentose phosphate pathway
• NADPH
• RNA

References

1. ^ Immunology at MCG 1/cytotox