Transcript Document

Other Pathways of
Carbohydrate Metabolism
Aulanni’am
Laboratory of Biochemistry FMIPA
Brawijaya University
Other pathways of
carbohydrate metabolism
• 1. Gluconeogenesis
• 2. The Glyoxylate pathway
• 3. The Pentose Phosphate Pathway
1. The Gluconeogenesis Pathway
Gluconeogenesis requires 3 bypass reactions.
1st bypass - pyruvate is converted to PEP in 2 steps.
Pyruvate is converted to oxaloacetate before conversion
to phosphoenolpyruvate.
1. Pyruvate carboxylase
A Biotin prosthetic group is required
Acetyl-CoA is an allosteric activator
2. PEP carboxykinase (PEPCK)
Pyruvate Carboxylase has
a biotin prosthetic group
Two phase reaction mechanism of pyruvate carboxylase
Bicarbonate
PEP Carboxykinase catalyzes the GTP-driven
decarboxylation of oxaloacetate to form PEP and GDP.
Gluconeogenesis requires metabolite transport
between mitochondria and cytosol
• Generation of oxaloacetate from pyruvate or citric acid cycle
intermediates occurs only in the mitochondrion.
• Enzymes that convert PEP to glucose are cytosolic.
• Either oxaloacetate must leave the mitochondrion for
conversion to PEP, or the PEP formed must enter the cytosol.
• Oxaloacetate must be converted to malate for which
transport systems exist. The malate dehydrogenase route
results in transport of reducing equivalents from the
mitochondrion to the cytosol.
• Cytosolic NADH is required for gluconeogenesis.
Gluconeogenesis requires metabolite
transport between mitochondria and cytosol
• The difference between the 2 routes involves transport of
NADH.
• The malate dehydrogenase route (Route 2) results in
transport of NADH.
• The aspartate aminotransferase route (Route 1) does not
involve NADH.
• Since cytosolic NADH is required for gluconeogenesis, route
2 is usually required.
• However, if lactate is the gluconeogenic precursor, it is
oxidized to pyruvate generating cytosolic NADH. Therefore,
either route may then be used.
Transport of PEP and oxaloacetate from the mitochondrion to the
cytosol.
2nd and 3rd bypass reactions.
Hydrolytic reactions bypass
PFK and Hexokinase.
At these steps, instead of
generating ATP by reversing
the glycolytic reactions,
FBP and G6P are hydrolyzed,
releasing Pi in exergonic processes
catalyzed by FBPase and
glucose-6-phosphatase.
B. Regulation of gluconeogenesis
• If glycolysis and gluconeogenesis were
uncontrolled, the net effect would be a futile
cycle wastefully hydrolyzing ATP and GTP.
• However, the pathways are reciprocally
regulated so as to meet the needs of the
organism.
• Glycolysis and Gluconeogenesis are
controlled by allosteric interactions and
covalent modifications.
Regulation of glycolysis and gluconeogeneis by
Allosteric Interactions and Covalent Modifications
1. Hexokinase/glucose-6-phosphatase
2. PFK/FBPase
3. Pyruvate kinase/pyruvate carboxylase-PEPCK
C. The Cori Cycle
Through the intermediacy
of the bloodstream, liver
and muscle participate in
a metabolic cycle known
as the Cori cycle.
Hormonal regulation of
[F2,6P] activates
gluconeogenesis in liver
in response to low blood
sugar.
The Cori cycle.
Lactate produced by muscle glycolysis is transported by the
bloodstream to the liver, where it is converted to glucose by
gluconeogenesis.
The bloodstream carries glucose back to the muscles, where
it may be stored as glycogen.
2, The Glyoxylate Pathway
(in plants but not animals).
Two essential enzymes for
The glyoxylate pathway
not found in the citric acid
cycle in animals are:
(1) isocitrate lysase
(2) malate synthase
The overall reaction of the
glyoxylate cycle is the
formation of oxaloacetate
from 2 molecules of
acetyl-CoA. This enables
germinating seeds to
convert stored
triacylglycerols, through
acetyl-CoA, to glucose.
The glyoxylate cycle and
its relationship to the
citric acid cycle.
Electron micrograph of a germinating
cucumber seed, showing a glyoxysome,
mitochondria, and surrounding lipid bodies
Glyoxylate cycle reactions (in glyoxysomes) proceed
simultaneously with, and mesh with those of the citric acid
cycle (in mitochondria).
3. The Pentose Phosphate Pathway
p.617
• Many endergonic reactions, notably the reductive
biosynthesis of fatty acids and cholesterol, as well
as photosynthesis, require NADPH in addition to
ATP.
• Whereas NADH participates in utilizing the free
energy of metabolite oxidation to synthesize ATP,
NADPH is involved in utilizing the free energy of
metabolite oxidation for otherwise endergonic
reductive biosynthesis.
• Cells normally maintains the [NAD+]/[NADH] ratio
near 1000 which favors metabolite oxidation.
The Pentose Phosphate Pathway
• The [NADP+]/[NADPH] ratio is near 0.01
which favors metabolite reduction.
• NADPH is generated by oxidation of G6P via
the pentose phosphate pathway.
• About 30% of glucose oxidation in liver
occurs via the pentose phosphate pathway.
• Ribose-5-phosphate (R5P) produced by this
pathway is required for nucleotide
biosynthesis.
Pentose phosphate pathway
There are 3 stages:
(1) Oxidative reactions (reactions 1-3) that form NADPH and
ribulose-5-phosphate (Ru5P).
(2) Isomerization and epimerization reactions
(reactions 4 and 5) that transform Ru5P either to ribose-5phosphate (R5P) or to xylulose-5-phosphate (Xu5P).
(3) A series of C-C bond cleavage and formation reactions
(reactions 6-8) that convert 2 Xu5P and 1 R5P to 2 F6P
and 1 glyceraldehyde-3-phosphate (GAP).
The overall reaction of the pathway is:
3G6P + 6NADP+ + 3H2O  6NADPH + 6H+ + 3CO2 +2F6P +GAP
1
2
3
(1) Oxidative reactions (reactions 1-3) that form
NADPH and ribulose-5-phosphate (Ru5P).
(2) Isomerization and epimerization reactions
(reactions 4 and 5) that transform Ru5P either to ribose-5phosphate (R5P) or to zylulose-5-phosphate (Xu5P).
(3) A series of C-C bond cleavage and formation reactions
(reactions 6-8) that convert 2 Xu5P and 1 R5P to 2 F6P and 1
glyceraldehyde-3-phosphate (GAP). These reactions require
transketolase and transaldolase.
Transketolase transfers 2C units.
Transaldolase transfers 3C units.
Ru5P
2.
3.
NONOXIDATIVE REACTIONS BY TRANSKETOLASE
AND TRANSALDOLASE CONVERT PENTOSE
PHOSPHATES BACK INTO HEXOSE PHOSPHATES
Pentose
Phosphate
Pathway
The glucose-6-phosphate dehydrogenase reaction
(reaction 1). This enzyme is specific for NADP+ and
Is strongly inhibited by NADPH.
The phosphogluconate dehydrogenase reaction
(reaction 3).
Ribulose-5-phosphate isomerase
and ribulose-5-phosphate
epimerase reactions both involve
endiolate intermediates.
Carbon-carbon bond cleavage and
formation reactions
• Transketolase catalyzes the transfer of C2 units
• TPP is a cofactor in the transfer of C2 units.
• A C2 unit is transferred from Xu5P to R5P yielding
GAP and sedoheptulose-7-phosphate (S7P) (a 7
carbon sugar).
Transketolase utilizes the coenzyme thiamine pyrophosphate
(TPP) to stabilize the carbanion formed on the cleavage of the
C2-C3 bond of Xu5P.
Transaldolase Catalyzes the
Transfer of C3 Units.
• Transaldolase catalyzes the transfer of a C3 unit.
• A C3 unit is transferred from S7P to GAP yielding
erythrose-4-phosphate (E4P) and F6P.
• The reactions occurs by an aldol cleavage.
Transaldolase contains an essential Lys residue that forms a
Schiff base with S7P to facilitate an aldol cleavage reaction.
Carbon-carbon bond formations and cleavages that
convert 3 C5 sugars to 2 C6 and 1 C3 sugar in the
pentose phosphate pathway.
(6) Transketolase (2 carbon transfer)
(7) Transaldolase (3 carbon transfer)
(8) Transketolase (2 carbon transfer)
A schematic diagram showing the pathway leading
from 6 pentoses (5C) to 5 hexoses (6C).
Pentose
Phosphate
Pathway
Control of the pentose phosphate pathway
• When the need for NADPH exceeds that of R5P in nucleotide
biosynthesis, excess R5P is converted to glycolytic
intermediates. GAP and F6P are consumed through
glycolysis and oxidative phosphorylation or recycled by
gluconeogenesis to form G6P. In the latter case, 1 G6P can
be converted, via 6 cycles of pentose phosphate pathway and
gluconeogenesis, to 6 CO2 and 12 NADPH.
• When R5P is needed more than NADPH, F6P and GAP can
be diverted from the glycolytic pathway for use in synthesis of
R5P by the reversal of the transaldolase and transketolase
reactions.
• Flux through the pathway and thus the rate of NADPH
production is controlled by rate of glucose-6-phosphate
dehydrogenase reaction (the first committed step). The
activity is regulated by the NADP+ concentration (substrate
availability).
Glucose-6-phosphate dehydrogenase deficiency
• NADPH is required for many reductive processes in
addition to biosynthesis. Erythrocyte membrane integrity
requires reduced glutathione (GSH) to eliminate H2O2 and
organic hydroperoxides. Peroxides are eliminated by
glutathione peroxidase using GSH and yielding glutatione
disulfie (GSSG).
• GSH is regenerated by NADPH reduction of GSSG
catalyzed by glutathione reductase. Therefore, a steady
supply of NADPH is vital for erythrocyte integrity.
Glucose-6-phosphate dehydrogenase deficiency
• Primaquine, an antimalarial agent, causes hemolytic
anemia in glucose-6-phosphate dehydrogenase mutants.
• Under most conditions, the erythrocytes have sufficient
enzyme activity for normal function.
• However, primaquine and similar agents stimulate peroxide
formation, thereby increasing the demand for NADPH to a
level that mutant cells cannot meet.
• About 400 million people are deficient in G6PD, which
makes this condition the most common human
enzymopathy.