Gluconeogenesis

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Gluconeogenesis
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What is the Significance of Gluconeogenesis?
Some tissues absolutely require a constant supply
of glucose. The brain, for example. When glucose
is absent (during fasting) and when liver stores
of glycogen are depleted, gluconeogenesis becomes
essential.
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Gluconeogenesis Reverse Glycolysis?
Gluconeogenesis is essentially glycolysis in
reverse. Many glycolytic reactions are near
equilibrium under physiological conditions (DG
0), but som exhibit large negative free
energies. These must be overcome in
gluconeogenesis.
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Free Energies of Glycolysis in Heart Muscle Under
Standard (DG) and Physiological (DG) Conditions
Arrows indicate glycolytic reactions near
equilibrium under physiological conditions.
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Comparison of Glycolytic and Gluconeogenic
Pathways I Specific Gluconeogenic Enzymes
Compensate Energetically Prohibitive Glycolytic
Steps
Free energies (in the gluconeogenic direction) in
liver shown in parentheses.
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Comparison of Glycolytic and Gluconeogenic
Pathways II Specific Gluconeogenic Enzymes
Compensate Energetically Prohibitive Glycolytic
Steps
The middle steps of glycolysis are more or less
readily reversible underphysiological conditions.
Free energies (in the gluconeogenic direction) in
liver shown in parentheses.
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Comparison of Glycolytic and Gluconeogenic
Pathways III Specific Gluconeogenic Enzymes
Compensate Energetically Prohibitive Glycolytic
Steps
The very favourable substrate level phosphorylatio
n of ADP by phospho- enolpyruvate and pyruvate
kinase is overcome in gluconeogenesis by two
coupled reactions. Note the occurrence of a
citric acid cycle intermediate in the
gluconeogenic pathway Implications?
Free energies (in the gluconeogenic direction) in
liver shown in parentheses.
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Citric Acid Cycle (and Other) Sources of Carbon
for Gluconeogenesis
Note the points at which amino acids may enter
the citric acid cycle. Connection to amino acid
biosynthesis and degredation. Reason for muscle
wasting during extended periods of
fasting. What about fats?
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The Two Enzyme Conversion of Pyruvate to
Phosphoenolpyruvate
PEPCK Phospho-Enol Pyruvate Carboxy Kinase
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The Biotin Cofactor in Pyruvate Carboxylase
First Transformation
Biotin (Vitamin B7) is readily carboxylated. Carb
oxy-biotin in pyruvate carboxylase is the source
of the activated carboxylate added to pyruvate to
generate oxaloacetate.
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Capture of Bicarbonate Ion Activation of
Pyruvate Carboxylase (First Transformation)
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Transient Generation of a Pyruvate Enolate
Nucleophile and a Carbon Dioxide Electrophile
Results in the Formation of Oxaloacetate (First
Transformation)
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• Things Happening at the Pyruvate Carboxylase
  Active Site
• (First Transformation)
• Bicarbonate capture by biotin.
• Uses ATP.
• Pyruvate enolate formation.
• Therefore
• ATP binding site
• Pyruvate binding site
• Carbonate binding site
• Presence of biotin
• All of these species are brought together

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The Reaction at Phosphoenolpyruvate Carboxy
Kinase (PEPCK) (Second Transformation)
Decarboxylation of oxaloacetate and formation of
pyruvate enolate.
Nucleophilic attack by pyruvate enolate on
g-phosphate of GTP generates phosphoenol pyruvate.
Recall Phosphoenolpyruvate a trapped pyruvate
enolate. Think of oxaloacetate as another trapped
pyruvate enolate.
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Problem Citric Acid Cycle metabolites occur in
the mitochondrial matrix. Glycolysis (and
gluconeogenesis) are cytosolic processes. Answer
Various metabolite transporters are
available. Some are boring. A metabolite goes
from one side to the other. Some are much more
interesting. Another connection to amino acid
metabolism.
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The Aspartate Malate Shunt
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Amino Transferases, Deaminations, and the
Pyridoxine Cofactor
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Deamination by Aspartate Amino Transferase I
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Deamination by Aspartate Amino Transferase II
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Deamination by Aspartate Amino Transferase III
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Gluconeogenesis Reverse Glycolysis?
Gluconeogenesis is essentially glycolysis in
reverse. Many glycolytic reactions are near
equilibrium under physiological conditions (DG
0), but some exhibit large negative free
energies. These must be overcome in
gluconeogenesis.
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The Second Glycolytic Bypass Occurs at
Fructose-1,6-Bisphosphatase 1 FBPase 1
In liver, the reverse glycolytic reaction (at
phosphofructokinase) would consume 24.5
kJ/mol. However the fructose-1.6-bisphosphatase
reaction liberates 8.6 kJ/mol.
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The Third Glycolytic Bypass in Gluconeogenesis
Occurs at Glucose-6-phosphatase
In liver, the reverse glycolytic reaction (at
hexokinase) would consume 32.9 kJ/mol. However
the glucose-6-phosphatase reaction liberates 5.1
kJ/mol.
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Comparison of Overall Glycolytic and
Gluconeogenic Reactions
Glycolysis DG -63 kJ/mol
glucose 2 ADP 2 Pi 2 NAD
2 pyruvate 2 ATP 2 NADH 2 H 2 H2O
Gluconeogenesis DG -16 kJ/mol
2 pyruvate 4 ATP 2 GTP 2 NADH 2 H 4 H2O
glucose 4 ADP 2 GDP 6 Pi 2 NAD
Gluconeogenesis is energetically expensive, but
it must be so such that it is effectively
irreversible.
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The Glyoxylate Cycle Another Process Involving
Glycolytic Enzymes and Metabolites.
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The Glyoxylate Cycle

• Related to the citric acid cycle many of the
  same reactions.
• Found in plants, fungi, bacteria not animals.
• Net reaction is the formation of succinate from
  the
• condensation of 2 acetyl-CoA groups.
• This is the reason animals cant synthesize
  glucose from fat.
• Given the connections among the citric acid cycle
  and
• gluconeogenesis, it is therefore a means of
  synthesizing
• glucose and other carbohydrates from
• fat and (b) other 2 carbon units.
• E.g. Mycobacterium tuberculosis feeds on host
  fat to produce its carbohydrdates.

2 acetyl-CoA 2 NAD FAD
succinate 2 CoA 2 NADH FADH2 2 H
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Components of the Glyoxylate Cycle
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The Glyoxylate Cycle I
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The Glyoxylate Cycle Demonstration of
Connections to the Citric Acid Cycle
Note that the decarboxylation reactions of the
citric acid cycle are bypassed. Glyoxyla
te cycle allows assembly of larger molecules from
two carbon units.
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The Pentose Phosphate Pathway Glucose oxidation
pathway. Alternative to glycolysis. However,
biosynthetic (anabolic), not catabolic.
Generates, 3, 4, 5, 7 carbon sugars. Generates
ribose-5-phosphate nucleotide
synthesis. Generates NADPH. NADPH figures in
many biosynthetic processes amino acid
metabolism fatty acid metabolism synthesis of
glucose in the dark cycle of photosynthesis
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The Pentose Phosphate Pathway Consists of two
stages an oxidative stage which produces
NADPH a non-oxidative stage producing 3, 4, 6,
and 7 carbon sugars (metabolic precursors) Linked
to glycolysis through common metabolites glucose
-6-phosphate fructose-6-phosphate glyceraldehyde-3
-phosphate
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The NADP Cofactor
NADP Structurally similar to NAD Same redox
center. But plays fundamentally different
biochemical role. NAD energy production,
electron transport glucose oxidation. NADP re
ductive biosynthesis, anti-oxidant esp. in
RBC, glucose oxidation (in pentose phosphate
pathway)
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The Oxidative Stage of the Pentose Phosphate
Pathway Production of 2 NADPH A Decarboxylation
Production of Ribulose-5-phosphate
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The Five Carbon Sugars of the Pathway, and Their
Inter-conversions
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Transketolase transfers 2 carbon units
from ketoses to aldoses.
(ketose)
(aldose)
transketolase

(ketose)
(aldose)
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transketolase
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Recall TPP Facile Ylid Formation
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A Second Transketolase Reaction
transketolase
xylulose-5-phosphate erythrose-4-phosphate
glyceraldehyde-3-phosphate fructose-6-phosphate
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Transaldolase Transfers 3 Carbon Units From a
Ketose to an Aldose TransAldolase. TransALDOLASE.
Mechanism?
(aldose)
(ketose)
transaldolase
(ketose)
(aldose)
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(No Transcript)
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The transketolase reaction. Work through the
details on your own.
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The transaldolase reaction. Work through the
details on your own.
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Overview of Carbohydrate Metabolism