Other Pathways of Carbohydrate Metabolism

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Other Pathways of Carbohydrate Metabolism

• Aulanniam
• Laboratory of Biochemistry FMIPA
• Brawijaya University

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Other pathways of carbohydrate metabolism

• 1. Gluconeogenesis
• 2. The Glyoxylate pathway
• 3. The Pentose Phosphate Pathway

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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)
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Pyruvate Carboxylase has a biotin prosthetic group
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Two phase reaction mechanism of pyruvate
carboxylase
Bicarbonate
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PEP Carboxykinase catalyzes the
GTP-driven decarboxylation of oxaloacetate to
form PEP and GDP.
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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.

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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.

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Transport of PEP and oxaloacetate from the
mitochondrion to the cytosol.
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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-phosph
atase.
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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.

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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
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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.
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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.
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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.
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The glyoxylate cycle and its relationship to
the citric acid cycle.
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Electron micrograph of a germinating cucumber
seed, showing a glyoxysome, mitochondria, and
surrounding lipid bodies
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Glyoxylate cycle reactions (in glyoxysomes)
proceed simultaneously with, and mesh with those
of the citric acid cycle (in mitochondria).
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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.

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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.

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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-5-phosphate (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

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1
2
3
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• Oxidative reactions (reactions 1-3) that form
• NADPH and ribulose-5-phosphate (Ru5P).

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(2) Isomerization and epimerization
reactions (reactions 4 and 5) that transform Ru5P
either to ribose-5-phosphate (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.
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Ru5P
2.
3.
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NONOXIDATIVE REACTIONS BY TRANSKETOLASE AND
TRANSALDOLASE CONVERT PENTOSE PHOSPHATES BACK
INTO HEXOSE PHOSPHATES
Pentose Phosphate Pathway
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The glucose-6-phosphate dehydrogenase
reaction (reaction 1). This enzyme is specific
for NADP and Is strongly inhibited by NADPH.
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The phosphogluconate dehydrogenase
reaction (reaction 3).
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Ribulose-5-phosphate isomerase and
ribulose-5-phosphate epimerase reactions both
involve endiolate intermediates.
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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).

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Transketolase utilizes the coenzyme thiamine
pyrophosphate (TPP) to stabilize the carbanion
formed on the cleavage of the C2-C3 bond of Xu5P.
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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.

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Transaldolase contains an essential Lys residue
that forms a Schiff base with S7P to facilitate
an aldol cleavage reaction.
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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)
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A schematic diagram showing the pathway leading
from 6 pentoses (5C) to 5 hexoses (6C).
Pentose Phosphate Pathway
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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).

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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.

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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.