The document discusses the hexose monophosphate shunt (HMP shunt), an alternative pathway to glycolysis and the TCA cycle for oxidizing glucose. The HMP shunt occurs in the cytosol and is more anabolic in nature, producing NADPH and pentose through oxidative and non-oxidative phases. Tissues with high HMP shunt activity like the liver, adipose tissue, and lactating mammary glands use the NADPH produced for biosynthesis of fatty acids and steroids.

1. HEXOSE MONOPHOSPHATE SHUNT
HMP Shunt is an alternative pathway to glycolysis and TCA cycle for the oxidation of
glucose. Hexose monophosphate pathway or HMP shunt is also called pentose
phosphate pathway or phosphogluconate pathway. HMP shunt is more anabolic in
nature, since it is concerned with the biosynthesis of NADPH and pentose.
The enzymes of HMP shunt are located in the cytosol, Tissues such as liver, adipose
tissue, adrenal gland, erythrocytes, testes and lactating mammary gland, are highly active
in HMP shunt.
As Most of these tissues are involved in the biosynthesis of fatty acids and steroids
which are dependent on the supply of NADPH.

2. REACTIONS OF THE PATHWAY
The sequence of reactions of HMP shunt is divided into two phases-oxidative and non-oxidative.
A. Oxidative phase: Glucose 6-phosphate dehydrogenase (G6PD) is an NADP-dependent enzyme that
converts glucose 6-phosphate to 6-phosphogluconolactone. The 6-phosphogluconolactone is then
hydrolysed by the gluconolactone hydrolase to 6-phosphogluconate.
The next reaction involving the synthesis of NADPH is catalysed by 6-phosphogluconate
dehydrogenase to produce 3 keto 6-phosphogluconate which then undergoes decarboxylation to give
ribulose 5-phosphate.
B. Non-oxidative phase:
1. Ribulose 5-phosphate is acted upon by an epimerase to produce xylulose 5-phosphate while ribose 5-
phosphate ketoisomerase converts ribulose 5-phosphate to ribose 5-phosphate

4. GLYCOLYSIS

5. Erythrose-
4-
phosphate
GLYCOLYSIS

6. 2.The enzyme transketolase catalyses the transfer of two carbon moiety from xylulose 5-
phosphate to ribose 5-phosphate to give a 3-carbon glyceraldehyde 3-phosphate and a 7-
carbon sedoheptulose 7-phosphate (TPP-Coenzyme).
3.Transaldolase brings about the transfer of a 3-carbon fragment (active dihydroxyacetone)
from sedoheptulose 7-phosphate to glyceraldehyde 3-phosphate to give fructose 6-
phosphate and four carbon erythrose 4-phosphate.
4.Transketolase acts on xylulose 5-phosphate and transfers a 2-carbon fragment
(glyceraldehyde) from it to erythrose 4-phosphate to generate fructose 6-phosphate and
glyceraldehyde 3-phosphate.

7. The overall reaction may be represented as
6 Glucose 6-phosphate + 12 NADP+ + 6H2O ----->6CO2 + 12 NADPH + 12H+ + 5
Glucose 6-phosphate
Note 1.
For the complete oxidation of glucose 6-phosphate to 6CO2, we have to start with 6 molecules of
glucose 6-phosphate. Of these 6 molecules of glucose 6-phosphate, 5 molecules of glucose 6-
phosphate are regenerated with the production of l2 NADPH.

8. Note 2.
Fructose 6-phosphate and glyceraldehyde 3-phosphate can be further catabolized through glycolysis and
citric acid cycle.