Pentose phosphate pathway
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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.
Contents
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* 1 Uses
* 2 Phases
o 2.1 Oxidative phase
o 2.2 Non-oxidative phase
* 3 See also
* 4 References
* 5 External links
[ ] Uses
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.
Oxidative phase of pentose phosphate pathway
Oxidative phase of pentose phosphate pathway
The entire set of reactions can be summarized as follows:
Reactants Products Enzyme Description
Glucose 6-phosphate + NADP+ → 6-phosphoglucono-δ-lactone + NADPH glucose 6-phosphate dehydrogenase Dehydrogenation. The hemiacetal hydroxyl group located on carbon 1 of glucose 6-phosphate is converted into a carbonyl group, generating a lactone, and in the process NADPH is generated.
6-phosphoglucono-δ-lactone + H2O → 6-phosphogluconate + H+ 6-phosphoglucolactonase Hydrolysis
6-phosphogluconate + NADP+ → ribulose 5-phosphate + NADPH + CO2 6-phosphogluconate dehydrogenase Oxidative decarboxylation. NADP+ is the electron acceptor, generating another molecule of NADPH, a CO2, and ribulose 5-phosphate.
ribulose 5-phosphate ribose 5-phosphate Phosphopentose isomerase Isomerization. (Can also be considered part of nonoxidative phase)
The overall reaction for this process is:
Glucose 6-phosphate + 2 NADP+ + H2O → ribose 5-phosphate + 2 NADPH + 2 H+ + CO2
[ ] Non-oxidative phase
The pentose phosphate pathway's Nonoxidative phase
The pentose phosphate pathway's Nonoxidative phase
Reactants Products Enzymes
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