Pentose phosphate pathway

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The pentose phosphate pathway (also called Phosphogluconate Pathway, or Hexose Monophosphate Shunt [HMP shunt]) is a 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. The pathway is one of the three main ways the body creates reducing molecules to prevent oxidative stress, accounting for approximately 10% of NADPH production in humans.

1 Oxidative phase
2 Non-oxidative phase
3 See also
- 3.1 External links

[edit] Oxidative phase

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

The overall reaction for this process is:
Glucose 6-phosphate + 2 NADP⁺ + H₂O → ribulose 5-phosphate + 2 NADPH + 2 H⁺ + CO₂

The first step in this series of reactions is the dehydrogenation of glucose 6-phosphate into 6-phosphoglucono-δ-lactone. This reaction is catalyzed by glucose 6-phosphate dehydrogenase. The hydroxyl group located on carbon 1 of glucose 6-phosphate is converted into a keto group, and in the process, NADPH is generated.

The second step is the hydrolysis of 6-phosphoglucono-δ-lactone to 6-phosphogluconate, which is catalyzed by 6-phosphoglucolactonase.

The third step is the oxidative decarboxylation of 6-phosphogluconate to ribulose 5-phosphate. This reaction is catalyzed by 6-phosphogluconate dehydrogenase and NADP⁺ is the electron acceptor, generating another molecule of NADPH.

The final step is the isomerization (catalyzed by phosphopentose isomerase) of ribulose 5-phosphate into ribose 5-phosphate.

The entire set of reactions can be summarized as follows:

Glucose 6-phosphate + NADP+ → 6-phosphoglucono-δ-lactone + NADPH
6-phosphoglucono-δ-lactone + H₂O → 6-phosphogluconate + H⁺
6-phosphogluconate + NADP⁺ → ribulose 5-phosphate + NADPH + CO₂

Oxidative phase of pentose phosphate pathway

[edit] Non-oxidative phase

ribulose 5-phosphate → ribose 5-phosphate (phosphopentose isomerase)
ribulose 5-phosphate → xylulose 5-phosphate (phosphopentose epimerase)
xylulose 5-phosphate + ribose 5-phosphate → glyceraldehyde 3-phosphate + sedoheptulose 7-phosphate (transketolase)
sedoheptulose 7-phosphate + glyceraldehyde 3-phosphate → erythrose 4-phosphate + fructose 6-phosphate (transaldolase)
xylulose 5-phosphate + erythrose 4-phosphate → glyceraldehyde 3-phosphate + fructose 6-phosphate (transketolase)