References are on the last page

1. THE PENTOSE PHOSPHATE
& URONIC ACID PATHWAY
Dr Manoj Acharya
Department of Biochemistry

2. Overview
1) Pentose phosphate pathway
• Historical aspect
• Reactions of pathway
• Importance
• Glucose 6-phosphate dehydrogenase deficiency
• Regulation
2) Uronic acid pathway
• Reactions of pathway
• Clinical importance

3. Pentose Phosphate Pathway

4. Historical aspect
• 1930: first existence of pentose
phosphate pathway was
obtained by Otto Warburg. He
discovered NADP+ through his
studies on the oxidation of
glucose 6-phosphate to 6-
phosphogluconate
• 1950: PPP was elucidated by
Frank Dickens, Bernard
Horecker, Fritz Lipmann and
Efraim Racker
Bernard Leonard Horecker began his training
in enzymology in 1936 as a graduate student
at University of Chicago in laboratory of T. R.
Hogness. His initial project involved studying
succinic dehydrogenase from beef heart using
the Warburg manometric apparatus. However,
when Erwin Hass arrived from Otto Warburg's
laboratory he asked Horecker to join him in
search for an enzyme that would catalyze
reduction of cytochrome c by reduced NADP.
This was the beginning of Horecker's lifelong
involvement with pentose phosphate
pathway.

5. Introduction
• Occurs in cytosol. The tissues such
as liver, adipose tissue, adrenal
gland, erythrocytes, testes and
lactating mammary gland are
highly active in HMP shunt.
• Of particular importance in these
tissues is the oxidation of glucose
6-phosphate to pentose
phosphates by pentose phosphate
pathway so, also called as
phosphogluconate pathway or the
hexose monophosphate pathway.
• Unique multifunctional
pathway where there are
several interconvertible
substances produced
which may proceed in
different directions in the
metabolic reactions.
• an alternative pathway to
glycolysis and is used to
produce ribose-5-
phosphate and nicotinamide
adenine dinucleotide
phosphate (NADPH)

6. Classification of monosachharides with examples

7. • Begins with the glycolytic
intermediate: Glucose 6-P.
• Two phases
1) Oxidative phase
2) Non oxidative phase
• Ribose 5 phosphate acts as a
precursor in synthesis of
nucleotides, coenzymes, DNA and
RNA
• Demand in actively dividing cells.
• NADPH is used in reductive
biosynthesis
• NADPH maintains reducing
environment on cells.

8. Need/Importance of HMP pathway
1. Ribose-5-phosphate required for the
biosynthesis of DNA and RNA are
provided by this pathway
2. It provides the route for the
interconversion of pentoses to hexoses.
3. It generates NADPH which is required
for the biosynthesis of fatty acids,
cholesterol, steroid hormones and
neurotransmitters.
4. NADPH also keeps the iron of
hemoglobin in ferrous state and prevents
the formation of methemoglobin.
5. NADPH is important for phagocytosis
carried out by WBCs

9. Oxidative phase

10. Isomerization and epimerization reactions

11. Non-oxidative Phase

12. Transketolase mechanism
Fundamentals of Biochemistry Donald Voet, Judith
G. Voet & Charlotte W. Prat

13. Wernicke-Korsakoff Syndrome Is Exacerbated by a Defect in
Transketolase
• Wernicke-Korsakoff syndrome, a mutation in
gene for transketolase results in an enzyme
having an affinity for its coenzyme TPP that is
one-tenth that of normal enzyme.
• Although moderate deficiencies in the vitamin
thiamine have little effect on individuals with
an unmutated transketolase gene, in those
with altered gene, thiamine deficiency drops
level of TPP below that needed to saturate
enzyme.
• Lowering of transketolase activity slows
pentose phosphate pathway, and results WKS.
• More common among alcoholics; chronic
alcohol consumption interferes with
intestinal absorption of some vitamins,
including thiamine

14. Transaldolase mechanism
Fundamentals of Biochemistry Donald Voet, Judith G. Voet & Charlotte W. Prat

15. The metabolism of glucose 6-phosphate by the
pentose phosphate pathway is coordinated with
glycolysis
• G6PD enzyme is allosterically
stimulated by NADP+ and
strongly inhibited by NADPH.
The ratio of NADPH:NADP+ is
the primary mode of
regulation for the enzyme and
is normally about 100:1 in liver
cytosol.
• Glucose 6 phosphate is
metabolized by both glycolytic
and pentose phosphate
pathway
• The cytoplasmic concentration
of NADP+ plays a key role in
determining the fate of glucose
6- phosphate
The rate of PPP is controlled by
level of NADP+
The flow of glucose 6 phosphate
depends upon the need for
NADPH, ribose 5 phosphate and
ATP

16. 1) Much more ribose 5-
phosphate than NADPH
is required
2) The needs for NADPH
and for ribose 5-phosphate
are balanced

17. 3) Much more NADPH than
ribose 5-phosphate is
required
4) Both NADPH and ATP are
required

18. Glucose-6-Phosphate Dehydrogenase
• G-6-PD is an oxidoreductase
that catalyzes the oxidation of
glucose-6- phosphate to 6-
phosphogluconate or the
corresponding lactone.
• Reaction is important as first
step in pentose-phosphate
shunt of glucose metabolism
with ultimate production of
NADPH
Tissue Source
adrenal cortex, spleen, thymus,
lymph nodes, lactating
mammary gland, and
erythrocytes.
Little activity is found in normal
serum.
“Most of the interest of G-6-PD focuses
on its role in the erythrocyte.”

19. • The enzyme is active as a
tetramer or dimer
• The monomer of G6PD
consists of 515
aminoacids and
molecular weight of 59
kDa.
• The G6PD gene is located
at the telomeric region
of long arm of X
chromosome
• Maintain NADPH in reduced form.
• Adequate concentration of NADPH is required to regenerate sulfhydryl-
containing proteins, such as glutathione, from oxidized to reduced state.
• Glutathione in the reduced form, in turn, protects hemoglobin from
oxidation by agents that may be present in the cell
doi:10.1016/S0140-6736(08)60073-2

20. Glucose 6-phosphate dehydrogenase deficiency
• NADPH enables cells to counterbalance oxidative stress
• Pentose phosphate shunt provides the reducing power of red cell in
the form of NADPH , maintaining glutathione in reduced form (GSH)
via closely linked glutathione pathway.
• GSH protects the red cells from oxidative damage;
inadequate supplies result in peroxidation of red cell membrane,
denaturation of haemoglobin and its precipitation as Heinz bodies,
resulting in reduced cell deformability and intravascular haemolysis
• G6PD deficiency is an X-linked, hereditary genetic defect caused by
mutations in the G6PD gene
• Most common clinical manifestation: neonatal jaundice and acute
hemolytic anemia
• Present in more than 400 million people worldwide

21. Epidemiology of G6PD deficiency
Luzzatto L, Ally M, Notaro R. Glucose-6-phosphate dehydrogenase deficiency. Blood. 2020
Sep 10;136(11):1225-1240.

22. The disorder can result in several
different clinical manifestations, one
of which is drug-induced hemolytic
anemia.
When exposed to an oxidant drug
such as primaquine, an antimalarial
drug, affected individuals experience
a hemolytic episode.
The severity of the hemolysis is
related to the drug concentration.
C15H21N3O
https://pubchem.ncbi.nlm.nih.gov/compound/
Primaquine

23. NADPH enables cells to counterbalance
oxidative stress.
Heinz body supravital
staining
Patient presented with severe NNJ requiring exchange
blood transfusion; he then had severe anemia requiring
frequent blood tranfusions. At age of 9 he was
splenectomized, whereupon he became transfusion-
independent. Note the persistent reticulocytosis,
remitted after splenectomy. The underlying unique
variant was G6PD Harilaou (class I: F216L) Luzzatto L, Ally M, Notaro R. Glucose-6-
phosphate dehydrogenase deficiency. Blood.
2020 Sep 10;136(11):1225-1240.

24. G6PD and Malaria
• In vitro studies show that growth of one malaria parasite,
Plasmodium falciparum, is inhibited in G6PD- deficient
erythrocytes.
• The parasite is very sensitive to oxidative damage and is killed by
a level of oxidative stress that is tolerable to a G6PD- deficient
human host.
• When G6PD deficient red cells are infected by P falciparum they
are sensed by macrophages as abnormal at an early stage, and
therefore they are removed: this seems a highly plausible
protective mechanism
Luzzatto L, Ally M, Notaro R. Glucose-6-phosphate dehydrogenase deficiency. Blood.
2020 Sep 10;136(11):1225-1240.

25. Uronic Acid Pathway

26. • Another alternative glucose oxidation pathway, does not lead to
the formation of ATP.
• In liver, it catalyzes the conversion of glucose to glucuronic acid,
ascorbic acid (except in human beings and other species) and
pentose.
• Source of glucuronic acid for conjugation of several endogenous
and exogenous substances like bilirubin, steroids, drugs before
excretion as glucuronides in urine and bile.
• GAG synthesis- Heparin, Hyaluronic acid, Dermatan sulfate.

28. Uronic acid pathway: Significance
UDP Glucose UDP Glucoronic acid
• Synthesis of Glycogen
• Metabolism of Galactose
• Lactose synthesis
• Formation of UDP-
Glucoronic acid
• Biosynthesis of ascorbic
acid in lower animals
• Detoxification reactions
(bilirubin and steroid hormone)
• Biosynthesis of GAG
(hyaluronic acid and heparin)
• Biosythesis of L- iduronic acid
- An epimer of Glucoronic acid
- (heparin sulphate and
dermatan sulphate)

29. Clinical importance of Uronic acid pathway
Disruption of uronic acid pathways can be caused by
enzyme defect and administration of certain drugs.
1. Essential pentosuria :
• An inherited disorder.
• Due to deficiency of enzyme L- Xylitol
dehydrogenase or Xylulose kinase.
• Appearance of xylulose and excretion in urine
.
2. Oxalosis:
• Parenteral administration of xylitol may lead
to oxalosis, calcium oxalate deposition in
brain and kidney.

30. References
• Wilson J.L Biochemistry Lubert Stryer
• Lehninger Principles of Biochemistry
• Harper's Illustrated Biochemistry
• Fundamentals of Biochemistry Donald Voet, Judith G. Voet & Charlotte
W. Prat
• Luzzatto L, Ally M, Notaro R. Glucose-6-phosphate dehydrogenase
deficiency. Blood. 2020 Sep 10;136(11):1225-1240.
• https://pubchem.ncbi.nlm.nih.gov/compound/Primaquine