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Table of Contents
Hexose Monophosphate Pathway
The HMP is a route for the cell to produce NADPH which is required for the biosynthetic reaction.
- NADPH is crucial to keep glutathione in its reduced form to prevent oxidative damage to the cell. Because when glutathione gets oxidised it starts degradation inside the cell.
- The major pathway for aerobic respiration of glucose is through glycolysis and Kreb's Cycle but HMP shunts an equally important alternate pathway of glucose catabolism.
- The Hexose Monophosphate Pathway takes place in the cytosol of most organisms but
- In plants, HMP shunt normally occurs in Plastids.
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Process
The process is divided into two distinct parts i.e.
1. Oxidative - it involves irreversible reaction.
2. Non-oxidative - it involves reversible reaction.
1. In the first step one molecule NADPH is produced when the
Glucose 6 phosphate is dehydrogenated to
6-phosphoglucono-lactone in the presence of glucose 6-phosphate
dehydrogenase.
2. 6-phosphoglucono-lactone is hydrolysed into 6-phosphogluconate in
the presence of 6-phosphogluconolactonase. This step requires water
molecules, which may be obtained from the Electron Transport chain.
3. 6-phosphogluconate is converted into ribulose 5-phosphate in the
presence of 6-phosphogluconate dehydrogenase by oxidative
decarboxylation, producing the 2nd molecule of NADPH, and CO2 release readily.
4. Ribulose-5-phosphate isomerises into ribose-5-phosphate in
the presence of ribose-5-phosphate isomerase.
5. Another enzyme, phosphopentose epimerase, isomerises
ribulose-5-phosphate
into xylulose 5-phosphate at the same time.
6. Transketolase enzyme transfers a carbon group from ketose
(xylulose-5-phosphate) to the aldose (ribose-5-phosphate), and
the products obtained are glyceraldehyde 3-phosphate and
sedoheptulose 7-phosphate.
7. Transaldolase again transfers a carbon group from
sedoheptulose 7-phosphate (ketose) to
glyceraldehyde 3-phosphate (aldose), and the products obtained are
erythrose 4-phosphate and fructose 6-phosphate. The key
point is the number of carbon molecules numbers is maintained (10 molecules)
in above and this step.
8. A carbon from xylulose 5-phosphate is transferred to erythrose
4-phosphate in the presence of transketolase to obtain
glyceraldehyde 3-phosphate and fructose 6-phosphate. The
substrate can further participate in glycolysis.
End product
NADPH - required for the biosynthetic reaction. An important component
within phagolysosomes in the immune response
Rbose-5-phosphate, which is used in the synthesis of nucleic acid and
nucleotides,
Erythrose-4-phosphate, is used for the synthesis of aromatic amino
acids.
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Hexose Monophosphate Pathway
Plant Physiology
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