HMP SHUNT-UNIT 2.pdf,PHYSIOLOGY MICROBIAL

HEXOSE MONOSE PATHWAY

1. HMP
SHUNT
PHYSIOLOGY-UNIT 2

2. INTRODUCTION
Hexose monophosphate pathway or HMP shunt is also called pentose phosphate
pathway or phosphogluconate pathway.
This is an alternative pathway to glycolysis and TCA cycle for the oxidation of
glucose.
It can operate either aerobically or anaerobically and is important in both
biosynthesis and catabolism
The pathway starts with glucose 6-phosphate. As such, no ATP is directly utilized
or produced in HMP pathway.
It is a unique multifunctional pathway, since there are several interconvertible
substances produced which may proceed in different directions in the metabolic
reactions.
The enzymes of HMP shunt are located in the cytosol.
The tissues such as liver, adipose tissue, adrenal gland, erythrocytes, testes and
lactating mammary gland, are highly active in HMP shunt.
Most of these tissues are involved in the biosynthesis of fatty acids and steroids
which are dependent on the supply of NADPH.

3. REFER HMP PATHWAY
IMAGE IN WIKIPEDIA

4. OXIDATIVE PHASE
Glucose 6-phosphate dehydrogenase (G6PD) is an
NADP-dependent enzyme that converts glucose 6-
phosphate to 6-phosphogluconolactone.
The latter 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.

5. NON OXIDATIVE PHASE
The non-oxidative reactions are concerned with the interconversion of three,
four, five and seven carbon monosaccharides.
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.
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.
Transketolase is dependent on the coenzyme thiamine pyrophosphate (TPP)
and Mg2+ ions.
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.
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.

6. Fructose 6-phosphate and glyceraldehyde 3-phosphate
can be further catabolized through glycolysis and citric
acid cycle. Glucose may also be synthesized from these
two compounds.
OVERALL REACTION
3 glucose 6-phosphate + 6NADP +3H2O ⎯⎯→ 2 fructose 6-
phosphate+ glyceraldehyde 3-phosphate+ 3CO2
+6NADPH +6H
NET EQUATION
Glucose 6-phosphate +12NADP +7H2O ⎯⎯→ 6CO2+
12NADPH +12H+ Pi

7. SIGNIFICANCE
1. NADPH from the pentose phosphate pathway serves as a source of
electrons for the reduction of molecules during biosynthesis.
2. The pathway produces two important precursor metabolites:
erythrose 4-phosphate, which is used to synthesize aromatic amino
acids and vitamin B6 (pyridoxal) and ribose 5-phosphate, which is a
major component of nucleic acids.
3.Intermediates in the pentose phosphate pathway may be used to
produce ATP.
1.
Glyceraldehyde 3-phosphate from the pathway can enter the
three-carbon phase of the EmbdenMeyerhof pathway and be
converted to pyruvate, as ATP is produced by substrate-level
phosphorylation.
2.
Pyruvate may be oxidized in the tricarboxylic acid cycle to provide
more energy.
3.