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Pentose phosphate pathway
The penthose phosphate pathway (also known as the Hexose Monophosphate 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.
The overall reaction for this process is:
Glucose 6-phosphate + 2 NADPH+ + H2O → ribose 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 a lactonase.
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
Synthesis of sugar
- 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 (transaldolase)
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