The pentose phosphate pathway
Metabolism of fructose and galactose
The uronic acid pathway
The synthesis of amino sugars and
glycosyl donors in glycoprotein synthesis
Biochemistry I
Lecture 5 2008 (J.S.)
2
The pentose phosphate pathway
(also called the phosphogluconate pathway)
is one of the secondary pathways of glucose catabolism.
It leads to two special products in animal tissues:
NADPH is a carrier of chemical energy in the form of reducing power
for reductive syntheses and hydroxylations catalysed by
monooxygenases, and some other important reductions.
and ribose 5-phosphate used in the biosynthesis of nucleic acids.
It does not serve to generate ATP energy.
The pathway is highly active in the cytoplasm of the liver, adipose tissue,
mammary gland, and the adrenal cortex. Other tissues less active in
synthesizing fatty acids, such as skeletal muscle, are virtually lacking in the
pentose phosphate pathway.
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PHASE 2
of interconversion
(nonoxidative)
2 NADPH + H+
Nucleotide
synthesis
4
1 The oxidative phase is irreversible
Glucose 6-phosphate dehydrogenase
is the regulated key enzyme of the pathway: In the cytosol of a liver cell from a well
fed rat the ratio NADP+
/NADPH is about 0.014. An increase of this ratio stimulates
the activity of G-6-P dehydrogenase.
(For comparison, the ratio NAD+
/NADH is 700 under the same conditions, at much
higher concentrations of NAD+
+ NADH.)
Lactonase
The first oxidative step is the dehydrogenation of the cyclic form
of glucose 6-P (a hemiacetal) to the lactone of 6-P gluconic acid:
5
The second oxidative reaction (dehydrogenation at carbon 3)
is accompanied with decarboxylation:
6-phosphogluconate dehydrogenase reaction
6
Three pentose 5-phosphates are required for the regeneration
of the glucose-pathway intermediates – two molecules of
xylulose 5-phosphate and one molecule of ribose phosphate
2 The interconversion phase is fully reversible
It begins with isomerization or epimerization of ribulose 5-phosphate:
PO3
2–
4-Epimerase
Isomerase
PO3
2–
PO3
2–
5-P
5-P
5-P
to regeneration
of glycolysis intermediates
(if there is no need for
nucleotide synthesis)
to nucleotide synthesis
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The first transketolase reaction (the transfer of C2 to ribose 5-P) :
Transketolase has a thiamine diphosphate prosthetic group.
C5 + C5 C3 + C7
The1st and the 2nd
pentose 5-phosphate
8
The transaldolase reaction (the transfer of C3 to glyceraldehyde 3-P) :
C3 + C7 C6 + C4
An intermediate
of glycolysis
9
Intermediates of the glycolytic pathway
C4 + C5 C6 + C3
The second transketolase reaction (the transfer of C2 to C4):
The 3rd
pentose 5-phosphate
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Interconversion (non-oxidative) phase
Oxidative phase
The summary of the pentose phosphate pathway:
3 Glucose 6-P + 6 NADP+
6 NADPH+H+
+ 2 Fructose 6-P + Glyceraldehyde 3-P + 3 CO2
3 C6 2 C6 + C3 + 3
C
or in the cells which need ribose 5-P for synthesis of nucleotides
3 Glucose 6-P + 6 NADP+
6 NADPH + H+
+ 3 Ribose 5-P + 3 CO2
3 C6 3 C5 + 3 CO2
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Some cells in certain states require much more NADPH (for reductive syntheses, e.g. in
adipose tissue) than ribose 5-phosphate, but they do not require the intensive production
of pentose 5-phosphates
- then most of the pentose 5-phosphates is regenerated into glycolysis
intermediates.
With respect to the energy charge, the glycolysis intermediates are either catabolized to
gain energy,
or they are used for regeneration of glucose 6-phosphate (e.g. for biosynthesis of
glycogen) by the gluconeogenic pathway.
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The cell requires much more ribose 5-phosphate(to synthesize nucleotides) than NADPH
and does not require NADPH for reductive syntheses (e.g. skeletal muscle)
- the reversal of the interconversion phase
X
The cell spends NADPH very intensively in reductive syntheses
as well as in biosynthesis of nucleotides, the needs are balanced
- the pentose 5-phosphates do not enter the interconversion phase
13
14
Glucose 6-phosphate dehydrogenase
plays a role in protection against reactive oxygen species (ROS).
Reduced glutathione is required to "detoxify“ ROS with a free
sulfanyl group, it maintains the normal reduced state in the cell.
Glutathione (reduced form, abbr. GSH )
γ-Glutamyl- cysteinyl- glycine
-
OOC
N
N COO
-
O
H
CH2
SH
O
H
N
+
H3
Glutathione, oxidized by ROS (GSSH) is reduced by NADPH generated
by Glc-6-P dehydrogenase in the pentose phosphate pathway.
Cells with reduced levels of Glc-6-P dehydrogenase are especially sensitive
to oxidative stress. This stress is most acute in red blood cells.
15
Regeneration of the oxidized form of glutathione (GS-SG) is catalyzed by
glutathione reductase:
GS-SG + NADPH + H+
→ 2 GSH + NADP+
Glucose 6-phosphate dehydrogenase deficiency
in red blood cells is an inherited defect affecting hundred of
millions of people (e.g. 11 % among Afroamericans). The
deficiency is quite benign in the absence of oxidative stress.
The generation of peroxides, e.g. after eating fava beans (of the
Mediterranean plant Vicia faba) or taking an antimalarial drug
pamaquine, may be a cause of severe haemolysis, destruction of
red blood cells and anaemia.
On the other hand, this enzyme deficiency protect against
falciparum malaria. The parasites causing this disease require
reduced glutathione and the products of the pentose phosphate
cycle for optimal growth.
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Metabolism of fructose
CH2–OH
CH2–OH
C=O
HO–CH
CH–OH
CH–OH
D-Fructose
17
ide
Fructose is present in many different fruits and in honey.
A considerable quantities of this sugar are ingested
chiefly in the form of sucrose:
18
Fructose is metabolized mostly in the liver:
Glycolysis
or reconversion to
glucose
Fructose metabolism is not subject to
the insulin control as that of glucose,
it bypasses the phosphofructokinase
reaction and is very rapid.
So unpredictable quantities of
intermediates can enter the glycolytic
pathway
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In the intestinal mucosa, muscle, and adipose tissue,
a part of fructose may enter directly into glycolysis:
Glycolysis
20
Some tissues (e.g. gonads) are able to synthesize fructose
from glucose through the polyol metabolic pathway:
D-Glucose D-Glucitol D-Fructose
aldose reductase
NADPH + H+
NADP+
NAD+
NADH + H+
alditol dehydrogenase
If the blood concentration of a monosaccharide is very high (e.g. glucose
in diabetes mellitus or galactose in galactosaemia), the polyol pathway
produces alditols (glucitol and galactitol, resp.) that may cause cataract
formation (a cataract is the clouding of the normally clear lens of the eye).
21
Defects in fructose metabolism
Essential fructosuria – lack in fructokinase
is without any serious consequences: blood fructose concentration is abnormally
high and fructose is excreted into the urine.
Hereditary fructose intolerance – low activity of aldolase B;
fructose 1-phosphate may accumulate in the liver to such an extent that most
of the inorganic phosphate is removed from the cytosol. Oxidative phosphorylation
is inhibited and hypoglycaemia also appears (Fru-1-P inhibits both glycolysis and
gluconeogenesis). The intake of fructose and sucrose must be restricted.
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Metabolism of galactose
Galactose occurs as component of lactose in milk and in dairy products.
Hydrolysis of lactose in the gut yields glucose and galactose.
CH=O
CH2–OH
CH–OH
HO–CH
HO–CH
CH–OH
D-Galactose β-D-Galactopyranose
23
Transformation of galactose into glucose in the liver
UDP-Glucose
UDP-Galactose
Glucose 1-phosphate
Gal-1-P uridyltransferase UDP-Gal 4-epimerase
UTP
PPi
Glc-1-P
Glc-6-P
Glucose Glycolysis
Glycogen
UMP
H2O
24
Defect in galactose catabolism
Galactosaemia is the hereditary deficiency of either galactokinase
or (mostly) Gal-1-P uridyltransferase. It can have fatal results for
children if they are not quickly put on a lactose-free diet.
Afflicted infants fail to thrive, they vomit after consuming milk, very common is the
disturbance of the liver function and retarded mental development. A cataract formation is
caused by accumulation of galactitol in the lens of the eye.
Lactose intolerance
Many adults are intolerant of milk because they are "deficient" in
lactase bound in the membrane of cells in the intestinal mucosa.
The decrease in lactase is normal during development in all mammals,
usually to about 5 – 10 % of the level at birth (this decrease is not as
pronounced with some groups of people, most notably Northern
Europeans.)
Micro organisms in the colon ferment the unresorbed lactose to lactic acid that is
osmotically active and causes diarrhoea, while also generating methane and hydrogen –
the gas creates uncomfortable feeling of gut distension.
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Galactose and N-acetylgalactosamine
are important constituents of
glycoproteins, proteoglycans, and glycolipids.
In the syntheses of those compounds in all types of cells, the galactosyl and
N-acetylgalactosyl groups are transferred from UDP-galactose and UDP-N-acetylgalactose
by the action of UDP-galactosyltransferase.
Biosynthesis of lactose
occurs only in the lactating mammary gland
The specificity of UDP-galactosyltransferase is modified by a regulatory protein
α-lactalbumin, which is synthesized in the mammary gland due to steep decrease
of hormonal levels just before the birth. α-Lactalbumin binds onto the transferase
and changes its specificity so that it begins to catalyze the transfer of galactosyl
from UDP-Gal to glucose and production of lactose.
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Entry points in glycolysis for fructose and galactose
27
UDPG dehydrogenase
2 NAD+
2 NADH+H+
The uronic acid pathway
is an alternative oxidative pathway for glucose.
It supplies glucuronic acid, and in most animals (not in humans,
other primates, and guinea pigs) ascorbic acid.
Glucuronic acid is finally metabolized to the pentoses which can
be reconverted to intermediates of glycolysis.
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H2O
UDP
Synthesis of glycoseaminoglycans
Conjugation with xenobiotics
HC–OH
HC–OH
HC–OH
HO–CH
COOH
CH2OH
HC–OH
HO–CH
HO–CH
HO–CH
CH2OH
COOH
≡
COOH
CH=O
HC–OH
HC–OH
HC–OH
HO–CH D-Glucuronic acid
(free)
NADPH+H+
L-Gulonic acid
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HC–OH
HO–CH
HO–CH
HO–CH
CH2OH
COOH
L-Gulonic acid
H2O
L-Gulonolactone
O2
2-Keto-L-gulonic acid
L-Ascorbic acid
L-Xylulose (a pentulose)
Xylitol
D-Xylulose
D-Xylulose 5-phosphate
CO2
NAD+
NADH+H+
3-keto-L-gulonic acid
which is further metabolized in the interconversion
phase of the pentose phosphate pathway to the
intermediates of glycolysis.
ATP
(NADPH+H+
)
(NAD+
)
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Synthesis of amino sugars
Fructose 6-phosphate Glucosamine 6-phosphate
(2-Amino-2-deoxyglucosamine 6-phosphate)
CH–
CH=O
NH2
CH–OH
CH2–O– P
HO–CH
CH–OH
CH–OH
CH2–O– P
HO–CH
CH–OH
C=O
CH2–OH
Glutamic acid
Aminotransferase
Glutamine
The basic amino groups –NH2 of amino sugars are nearly always "neutralized“ by acetylation
in the reaction with acetyl-coenzyme A, so that they exist as N-acetylhexosamines.
Unlike amines, amides (acetamido groups) are nor basic.
31
CH3CO
CH2
C=O
COOH
HC–OH
HO–CH
HC–OH
CH2–OH
-NH–CH
HC–
OH
Synthesis of sialic acids
Sialic acids is the group name used for various
acylated derivatives of neuraminic acid (N- as well as O-acylated).
(Neuraminic acid is 5-amino-3,5-dideoxy-nonulosonic acid.)
The most common sialic acid is N-acetylneuraminic acid:
32
N-Acetylmannosamine 6-phosphate
Phosphoenolpyruvate
N-Acetylneuraminic acid 9-phosphate
CH2
C=O
COO–
HC–OH
HO–CH
HC–OH
CH2–O–P
CH3CO–NH–CH
HC–OH
HO–CH
HC–OH
CH2–O–P
CH3CO–NH–CH
HC–OH
HC=O
CH2
=
COO–
C–O–P
Pi
+
Synthesis of sialic acid:
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Saccharides found in glycoproteins and glycolipids
Abbreviation:
Hexoses: Glucose Glc
Galactose Gal
Mannose Man
Acetyl hexosamines: N-Acetylglucosamine GlcNAc
N-Acetylgalactosamine GalNAc
Pentoses: Xylose Xyl
Arabinose Ara
Deoxyhexose
(Methyl pentose): L-Fucose Fuc
Sialic acids: N-Acetylneuraminic acid NeuNAc
(predominant)
34
Examples of saccharidic component of glycolipids or glycoproteins:
Ceramide (sphingolipid) or protein
Blood group substance A
NeuNAc NeuNAc
Bi-antennary component of a plasma-type
(N-linked) oligosaccharide
The boxed area encloses
the pentasaccharide core
common to all N-linked
glycoproteins.
35
Glycosyl donors in glycoprotein synthesis
Before being incorporated into the oligosaccharide chains, monosaccharides involved in
the synthesis of glycoproteins are activated by formation of nucleotide sugars,
similarly to formation of UDP-glucose in the reaction of glucose 1-phosphate with UTP.
The glycosyls of these compounds can be transferred to suitable acceptors provided
appropriate transferases are available.
Glucose 6-P Glucose 1-P UDP-Glucose UDP-Galactose
UDP-Glucuronic acid UDP-Xylose
Fructose 6-P Mannose 6-P Mannose 1-P GDP-Mannose
GDP-L-Fucose
N-Acetylglucosamine 6-P N-Acetylglucosamine 1-P UDP-N-Acetylglucosamine
UDP-N-Acetylmannosamine UDP-N-Acetylgalactosamine
N-Acetylneuraminic acid CMP-N-Acetylneuraminic acid
CTP
UTP
GTP
UTP
36
A brief survey of major pathways in saccharide metabolism
GLUCOSE
Glc-6-P
Fru-6-P
Fru-1,6-bisP
Gra-3-P
GALACTOSE
Gal-1-P
Glc-1-P
GLYCOGEN
UDP-Glc
UDP-Gal
UDP-GlcUA
GlcUA
CO2
Xyl-5-P
CO2FRUCTOSE
Glucitol
Fru-1-P
Glyceraldehyde
PYRUVATE
Oxaloacetate
Lactate
ACETYL-CoA