Pentose Phosphate Pathway
Biochemie-6-2-
metabolismus_sacharidů
2
Pentose cycle
Cellular localization: cytoplasm
Tissue localization:
widely in liver, adipose tissue (50% metab. glucose),
erythrocytes, thyroid gland, lactating mammary gland and
others.
(Generally tissues where reduction synthesis are taking
place)
Biochemie-6-2-
metabolismus_sacharidů
3
Importance of pentose cycle
source of NADPH (reductive synthesis, oxygenases
with mixed function, glutathione reduction)
source of ribose-5-P (nucleic acids, nucleotides)
involvement of pentoses received by food into
metabolism
Does not serve to gain energy
Biochemie-6-2-
metabolismus_sacharidů
4
Two parts of pentose cycle
oxidative part irreversible reactions
non-oxidative part (regenerative) reversible reactions
Biochemie-6-2-
metabolismus_sacharidů
Pentose pathway
• A) Introduction
• So far the described matter were related to glucose metabolism, but only those parts in which cell
gaining energy. Pentose phosphate pathway is also one of the metabolic pathways of glucose,
but does not lead to gain energy.
• Runs in a large scale in the liver, adipose tissue (50% glucose metabolism), erythrocytes (very
important source of NADPH + H +), the thyroid gland, lactating mammary gland and other
tissues. Generally, it takes place in the tissues, where reductive syntheses are taking place. In
other tissues only certain parts of this pathway are used.
• Regarding cellular localization, pentose phosphate pathway takes place in the cytosol
• Pentose phosphate pathway:
• is an important source of NADPH + H +, which is used for reducing syntheses, glutathione
reduction and by oxygenases with mixed function
• It is the source of ribose-5-phosphate, which is used for synthesis of nucleic acids and
nucleotides
• allows engagement of pentoses received by food in metabolism (e.g. direct conversion to
nucleotides, or their conversion into hexoses).
• As mentioned in the introduction, this pathway is not an energy source, moreover, does not
consume energy directly.
• We can distinguish two parts of the pentose phosphate pathway:
• oxidizing part in which irreversible reactions take place
• regenerating (nonoxidative) part, which consists of reversible reactions
5
Pentose Phosphate Pathway
Pentose Phosphate Pathway
 Other names:
Phosphogluconate Pathway
Hexose Monophosphate Shunt
 The linear part of the pathway carries out oxidation
and decarboxylation of the 6-C sugar glucose-6-P,
producing the 5-C sugar ribulose-5-P.
Glucose-6-phosphate Dehydrogenase catalyzes
oxidation of the aldehyde (hemiacetal), at C1 of
glucose-6-phosphate, to a carboxylic acid, in ester
linkage (lactone).
NADP+ serves as electron acceptor.
H O
OH
H
OHH
OH
CH2OPO3
2
H
H
OH H O
OH
H
OHH
OH
CH2OPO3
2
H
O
23
4
5
6
1
1
6
5
4
3 2
C
HC
CH
HC
HC
CH2OPO3
2
O O
OH
HO
OH
OH
NADPH+H+
NADP+ H2O H+
1
2
3
4
5
6
Glucose-6-phosphate
Dehydrogenase
6-Phospho-
glucono-
lactonase
glucose-6-phosphate 6-phoshogluconolactone 6-phosphogluconate
6-Phosphogluconolactonase catalyzes hydrolysis of the
ester linkage, resulting in ring opening.
The product is 6-phosphogluconate.
Although ring opening occurs in the absence of a catalyst,
6-Phosphogluconolactonase speeds up the reaction,
decreasing the lifetime of the highly reactive, and thus
potentially toxic, 6-phosphogluconolactone.
H O
OH
H
OHH
OH
CH2OPO3
2
H
H
OH H O
OH
H
OHH
OH
CH2OPO3
2
H
O
23
4
5
6
1
1
6
5
4
3 2
C
HC
CH
HC
HC
CH2OPO3
2
O O
OH
HO
OH
OH
NADPH+H+
NADP+ H2O H+
1
2
3
4
5
6
Glucose-6-phosphate
Dehydrogenase
6-Phospho-
glucono-
lactonase
glucose-6-phosphate 6-phoshogluconolactone 6-phosphogluconate
Phosphogluconate Dehydrogenase catalyzes
oxidative decarboxylation of 6-phosphogluconate, to
yield the 5-C ketose ribulose-5-phosphate.
The OH at C3 (C2 of product) is oxidized to a ketone.
This promotes loss of the carboxyl at C1 as CO2.
NADP+ serves as oxidant.
C
HC
CH
HC
HC
CH2OPO3
2
O O
OH
HO
OH
OH
1
2
3
4
5
6
CH2OH
C
HC
HC
CH2OPO3
2
OH
OH
1
2
3
4
5
O
NADP+
NADPH + H+
CO2
Phosphogluconate
Dehydrogenase
6-phosphogluconate ribulose-5-phosphate
 NADPH, a product of the Pentose Phosphate
Pathway, functions as a reductant in anabolic
(synthetic) pathways, e.g., fatty acid synthesis.
 NAD+ serves as electron acceptor in catabolic
pathways, in which metabolites are oxidized.
The resultant NADH is reoxidized by the respiratory
chain, producing ATP.
N
R
H
C
NH2
O
N
R
C
NH2
O
H H
+
2e
+ H
+
NADP+
NADPH
Reduction of NADP+
(as with NAD+)
involves transfer of 2e
and 1H+ to the
nicotinamide moiety.
NAD+ & NADP+ differ
only in the presence of
an extra phosphate on
the adenosine ribose of
NADP+.
This difference has little
to do with redox activity,
but is recognized by
substrate-binding sites
of enzymes.
It is a mechanism for
separation of catabolic
and synthetic pathways.
H
C
NH2
O
CH2
H
N
H
OH OH
H H
O
OP
O
HH
OH OH
H H
O
CH2
N
N
N
NH2
OP
O
O

O
+
N
O
nicotinamide
adenine
esterified to
Pi in NADP+
Nicotinamide
Adenine
Dinucleotide
Regulation of Glucose-6-phosphate Dehydrogenase:
 Glucose-6-phosphate Dehydrogenase is the
committed step of the Pentose Phosphate Pathway.
This enzyme is regulated by availability of the
substrate NADP+.
 As NADPH is utilized in reductive synthetic
pathways, the increasing concentration of NADP+
stimulates the Pentose Phosphate Pathway, to
replenish NADPH.
The rest of the pathway converts ribulose-5-P to the 5-C
product ribose-5-P, or to 3-C glyceraldehyde-3-P & 6-C
fructose-6-P.
Additional enzymes include an Isomerase, Epimerase,
Transketolase, and Transaldolase.
Epimerase interconverts
stereoisomers
ribulose-5-P and
xylulose-5-P.
Isomerase converts the
ketose ribulose-5-P to
the aldose ribose-5-P.
Both reactions involve
deprotonation to an
endiolate intermediate
followed by specific
reprotonation to yield
the product.
Both reactions are
reversible.
C
C
C
CH2OPO3
2
O
OHH
OHH
CH2OH
C
C
C
CH2OPO3
2
O
HHO
OHH
CH2OH
C
C
C
CH2OPO3
2
OH
OHH
OHH
HC O
H
ribulose-5-
phosphate
xylulose-5-
phosphate
ribose-5-
phosphate
Epimerase
Isomerase
Transketolase & Transaldolase catalyze transfer of
2-C or 3-C molecular fragments respectively, in each
case from a ketose donor to an aldose acceptor.
D. E. Nicholson has suggested that the names of these
enzymes should be changed, since
 Transketolase actually transfers an aldol moiety
(glycoaldehyde), and
 Transaldolase actually transfers a ketol moiety
(dihydroxyacetone).
However the traditional enzyme names are used here.
 Transketolase transfers a 2-C fragment from xylulose-
5-P to either ribose-5-P or erythrose-4-P.
 Transketolase utilizes as prosthetic group thiamine
pyrophosphate (TPP), a derivative of vitamin B1.
Pyruvate Dehydrogenase of Krebs Cycle also utilizes
TPP as prosthetic group.
C
C
C
CH2OPO3
2
O
HHO
OHH
CH2OH
C
C
C
CH2OPO3
2
OH
OHH
OHH
HC O
H C
C
C
CH2OPO3
2
OH
OHH
OHH
C H
H
HC
C
CH2OPO3
2
O
OHH
C
CH2OH
O
HO
+ +
xylulose- ribose- glyceraldehyde- sedoheptulose-
5-phosphate 5-phosphate 3-phosphate 7-phosphate
Transketolase
 TPP binds at the active site in a “V” conformation.
 H+ dissociates from the C between N & S in the
thiazolium ring.
 The aminopyrimidine amino group is near the
dissociable H+, & serves as H+ acceptor.
This H+ transfer is promoted by a Glu residue adjacent
to the pyrimidine ring.
thiamine pyrophosphate (TPP)
N
NH3C NH2
CH2
S
C
N
H3C
CH2 O P O P O
O
O
CH2
H
O O
+
acidic H+
aminopyrimidine
moiety
thiazolium
ring
The thiazolium
carbanion reacts
with the carbonyl
C of xylulose-5-P
to form an addition
compound.
N+ in the thiazole
ring acts as an e
sink, promoting
C-C bond cleavage.
N
NH3C NH2
CH2
S
C
N
H3C
CH2
+
C
C
C
CH2OPO3
2
CH2OHHO
HHO
OHH
N
NH3C NH2
CH2
S
C
N
H3C
CH2
+
C
C
C
CH2OPO3
2
O
HHO
OHH
 CH2OH
CH2OPO2OPO3
2
CH2OPO2OPO3
2
TPP
xylulose-5-P
active site
intermediate
Transketolase
subsequent
cleavage
The 3-C aldose
glyceraldehyde-3-P
is released.
A 2-C fragment
remains on TPP.
Completion is by
reversal of these
steps.
The 2-C fragment
condenses with
one of the aldoses
erythrose-4-P (4-C)
or ribose-5-P (5-C)
to form a ketose-P
product.
N
NH3C NH2
CH2
S
C
N
H3C
CH2
+
C
C
C
CH2OPO3
2
CH2OHHO
HHO
OHH
N
NH3C NH2
CH2
S
C
N
H3C
CH2
+
C
C
C
CH2OPO3
2
O
HHO
OHH
 CH2OH
CH2OPO2OPO3
2
CH2OPO2OPO3
2
TPP
xylulose-5-P
active site
intermediate
Transketolase
subsequent
cleavage
 Transfer of the 2-C fragment to the 5-C aldose
ribose-5-phosphate yields sedoheptulose-7-phosphate.
 Transfer of the 2-C fragment instead to the 4-C aldose
erythrose-4-phosphate yields fructose-6-phosphate.
C
C
C
CH2OPO3
2
O
HHO
OHH
CH2OH
C
C
C
CH2OPO3
2
OH
OHH
OHH
HC O
H C
C
C
CH2OPO3
2
OH
OHH
OHH
C H
H
HC
C
CH2OPO3
2
O
OHH
C
CH2OH
O
HO
+ +
xylulose- ribose- glyceraldehyde- sedoheptulose-
5-phosphate 5-phosphate 3-phosphate 7-phosphate
Transketolase
Transaldolase catalyzes transfer of a 3-C
dihydroxyacetone moiety, from sedoheptulose-7-phosphate
to glyceraldehyde-3-phosphate.
Transaldolase has an a,b barrel structure.
CH2OH
C
CH
HC
HC
HC
H2C
OH
OH
OPO3
2
OH
HO
O
HC
HC
HC
H2C
O
OH
OPO3
2
OH
HC
HC
H2C
O
OPO3
2
OH
H2C
C
CH
HC
HC
H2C
OH
OPO3
2
OH
OH
HO
O
sedoheptulose- glyceraldehyde- erythrose- fructose-
7-phosphate 3-phosphate 4-phosphate 6-phosphate
Transaldolase
+ +
In Transaldolase, the e-amino group of a lysine residue
reacts with the carbonyl C of sedoheptulose-7-P to form
a protonated Schiff base intermediate.
CH2OH
C
CH
HC
HC
HC
H2C
OH
OH
OPO3
2
OH
HO
O
Enz-Lys NH2
CH2OH
C
CH
HC
HC
HC
H2C
OH
OH
OPO3
2
OH
HO
Enz-Lys N
OH
+
HC
HC
HC
H2C
O
OH
OPO3
2
OH
CH2OH
C
CHO
N
+
H

+ H+
H
H
Enz-Lys
sedoheptulose-
7-phosphate
Schiff base
intermediate
Transaldolase
erythrose-4-
phosphate
Completion of the reaction is by reversal, as the carbanion
attacks instead the aldehyde carbon of the 3-C aldose
glyceraldehyde-3-P to yield the 6-C fructose-6-P.
Aldol
cleavage
releases
erythrose-4-
phosphate.
The Schiff
base
stabilizes the
carbanion
on C3.
CH2OH
C
CH
HC
HC
HC
H2C
OH
OH
OPO3
2
OH
HO
O
Enz-Lys NH2
CH2OH
C
CH
HC
HC
HC
H2C
OH
OH
OPO3
2
OH
HO
Enz-Lys N
OH
+
HC
HC
HC
H2C
O
OH
OPO3
2
OH
CH2OH
C
CHO
N
+
H

+ H+
H
H
Enz-Lys
sedoheptulose-
7-phosphate
Schiff base
intermediate
Transaldolase
erythrose-4-
phosphate
The diagram at
right summarizes
flow of 15 C atoms
through Pentose
Phosphate Pathway
reactions by which
5-C sugars are
converted to 3-C
and 6-C sugars.
IS = Isomerase
EP = Epimerase
TK = Transketolase
TA = Transaldolase
(3) ribulose-5-P
ribose-5-P (2) xylulose-5-P
glyceraldehyde-3-P
sedoheptulose 7 P
fructose-6- P
erythrose-4-P
fructose-6-P
glyceraldehyde-3-P
IS EP
TK
TK
TA
The balance sheet below summarizes flow of 15 C
atoms through Pentose Phosphate Pathway reactions by
which 5-C sugars are converted to 3-C and 6-C sugars.
C5 + C5  C3 + C7 (Transketolase)
C3 + C7  C6 + C4 (Transaldolase)
C5 + C4  C6 + C3 (Transketolase)
____________________________
3 C5  2 C6 + C3 (Overall)
Glucose-6-phosphate may be regenerated from either
the 3-C glyceraldehyde-3-phosphate or the 6-C
fructose-6-phosphate, via enzymes of Gluconeogenesis.
Ribulose-5-P may be converted to ribose-5-phosphate,
a substrate for synthesis of nucleotides and nucleic acids.
The pathway also produces some NADPH.
Depending on needs of a cell for ribose-5-phosphate,
NADPH, and ATP, the Pentose Phosphate Pathway can
operate in various modes, to maximize different products.
There are three major scenarios:
2 NADP+
2 NADPH + CO2
glucose-6-P ribulose-5-P ribose-5-P
Pentose Phosphate Pathway producing
NADPH and ribose-5-phosphate
Glyceraldehyde-3-P and fructose-6-P may be
converted to glucose-6-P for reentry to the linear
portion of the Pentose Phosphate Pathway,
maximizing formation of NADPH.
2 NADP+
2 NADPH + CO2
glucose-6-P ribulose-5-P ribose-5-P
fructose-6-P, &
glyceraldehyde-3-P
Pentose Phosphate Pathway producing
maximum NADPH
Glyceraldehyde-3-P and fructose-6-P, formed from 5-C
sugar phosphates, may enter Glycolysis for ATP synthesis.
The pathway also produces some NADPH.
2 NADP+
2 NADPH + CO2
glucose-6-P ribulose-5-P ribose-5-P
fructose-6-P, &
glyceraldehyde-3-P
to Glycolysis
for production of ATP
Pentose Phosphate Pathway producing
NADPH and ATP
Ribose-1-phosphate generated during catabolism of
nucleosides also enters Glycolysis in this way, after first
being converted to ribose-5-phosphate.
Thus the Pentose Phosphate Pathway serves as an entry
into Glycolysis for both 5-carbon & 6-carbon sugars.
2 NADP+
2 NADPH + CO2
glucose-6-P ribulose-5-P ribose-5-P
fructose-6-P, &
glyceraldehyde-3-P
to Glycolysis
for production of ATP
Pentose Phosphate Pathway producing
NADPH and ATP
Glutathione is a tripeptide that includes a Glu linked by
an isopeptide bond involving the side-chain carbonyl group.
Its functional group is a cysteine thiol.
One role of glutathione is degradation of hydroperoxides,
that arise spontaneously in the oxygen-rich environment
in red blood cells.
Hydroperoxides can react with double bonds in fatty acids
of membrane lipids, making membranes leaky.
H3N+
H
C CH2 CH2
COO
C
O
N
H
CH
CH2
SH
C
O
N
H
CH2 COO
-glutamyl-cysteinyl-glycine
Glutathione
Glutathione Peroxidase catalyzes degradation of organic
hydroperoxides by reduction, as two glutathione molecules
(represented as GSH) are oxidized to a disulfide.
2GSH + ROOH  GSSG + ROH + H2O
Glutathione Peroxidase uses the trace element selenium as
functional group.
The enzyme's primary structure includes an analog of
cysteine, selenocysteine, with Se replacing S.
H3N+
H
C CH2 CH2
COO
C
O
N
H
CH
CH2
SH
C
O
N
H
CH2 COO
-glutamyl-cysteinyl-glycine
Glutathione
Regeneration of reduced glutathione requires NADPH,
produced within erythrocytes in the Pentose Phosphate
Pathway.
Glutathione Reductase catalyzes:
GSSG + NADPH + H+  2 GSH + NADP+
Genetic deficiency of Glucose-6-P Dehydrogenase can
lead to hemolytic anemia, due to inadequate [NADPH]
within red blood cells.
The effect of partial deficiency of Glucose-6-phosphate
Dehydrogenase is exacerbated by substances that lead to
increased production of peroxides (e.g., the antimalarial
primaquine).