Gluconeogenesis
Cellular Biochemistry and
metabolism 2
CLS 333
Dr. Samah Kotb
Lecturer of Biochemistry
2015
Gluconeogenesis
Dr.Samah Kotb
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Gluconeogenesis
• Gluconeogenesis is the formation of glucose from noncarbohydrate sources e.g lactic acid , amino acids ,
glycerols and propionate.
• occurs mainly in liver .
• Gluconeogenesis occurs to a more limited extent in
kidney & small intestine under some conditions.
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Gluconeogenesis
•
Liver and kidney contains all enzymes of
gluconeogenesis.
• It does not occur in skeletal muscles due to
deficiency of glucose-6-phosphatase.
• It does not occur in heart muscle, smooth
muscles, and adipose tissues due to deficiency of
fructose 1-6 diphosphatase.
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Importance of gluconeogenesis
• Glucose is the only source of energy:
•
•
•
•
•
Nervous system
Skeletal system
Glucose is required :
1) Adipose tissues: as a source of glycerol.
2) Mammary gland: as a source of lactose.
Gluconeogenesis
•
Occurs in all animals, plants, fungi and microbes.
•
Occurs largely in the liver; some in renal cortex.
•
Of 10 enzymatic steps, 7 are reversals of glycolytic
reactions.
glucose
glycolysis
gluconeogenesis
pyruvate
lactate
gluco neo genesis
sugar (re)new make/
create
Gluconeogenesis
• Synthesis of glucose from pyruvate utilizes many of the
same enzymes as Glycolysis.
• Three Glycolysis reactions have such a large negative DG
that they are essentially irreversible.
 Hexokinase (or Glucokinase).
 Phosphofructokinase.
 Pyruvate Kinase.
• These steps must be bypassed in Gluconeogenesis.
• Two of the bypass reactions involve simple hydrolysis
reactions.
Dr.Samah Kotb
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Dr Samah Kotb
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Glucose-6-phosphatase
6 CH OPO 2
2
3
5
O
H
4
OH
H
OH
3
H
H
2
CH2OH
1
OH
OH
glucose-6-phosphate
O
H
H
H2O
H
OH
H
+ Pi
H
OH
OH
H
OH
glucose
Hexokinase or Glucokinase (Glycolysis) catalyzes:
glucose + ATP  glucose-6-phosphate + ADP
Glucose-6-Phosphatase (Gluconeogenesis) catalyzes:
glucose-6-phosphate + H2O  glucose + Pi
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Glucose-6-phosphatase
6 CH OPO 2
2
3
5
O
H
4
OH
H
OH
3
H
H
2
CH2OH
1
OH
OH
glucose-6-phosphate
O
H
H
H2O
H
OH
H
+ Pi
H
OH
OH
H
OH
glucose
Glucose-6-phosphatase enzyme is embedded in the
endoplasmic reticulum (ER) membrane in liver cells.
The catalytic site is found to be exposed to the ER lumen.
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Phosphofructokinase 
6 CH OPO 2
2
3
O
5
H
H
4
OH
6 CH OPO 2
2
3
1CH2OH
ATP
HO
3 OH
H
fructose-6-phosphate
O
ADP
2
5
Pi
H2O
1CH2OPO32
H
H
HO
3 OH
4
OH
2
H
fructose-1,6-bisphosphate
 Fructose-1,6-biosphosphatase
Phosphofructokinase (Glycolysis) catalyzes:
fructose-6-P + ATP  fructose-1,6-bisP + ADP
Fructose-1,6-bisphosphatase (Gluconeogenesis) catalyzes:
fructose-1,6-bisP + H2O  fructose-6-P + Pi
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Bypass of Pyruvate Kinase:
Pyruvate Kinase (last step of Glycolysis) catalyzes:
phosphoenolpyruvate + ADP  pyruvate + ATP
For bypass of the Pyruvate Kinase reaction, cleavage of 2
~P bonds is required.
 DG for cleavage of one ~P bond of ATP is insufficient to drive
synthesis of phosphoenolpyruvate (PEP).
 PEP has a higher negative DG of phosphate hydrolysis than ATP.
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Pyruvate Carboxylase
PEP Carboxykinase
O
O
O
O
C
ATP ADP + Pi
C
C
C
O
CH3
C
O
pyruvate
O
GTP GDP
CH2
HCO3
O
O
C
C
CO2
O
oxaloacetate
OPO32
CH2
PEP
Bypass of Pyruvate Kinase (2 enzymes):
Pyruvate Carboxylase (Gluconeogenesis) catalyzes:
pyruvate + HCO3 + ATP  oxaloacetate + ADP + Pi
PEP Carboxykinase (Gluconeogenesis) catalyzes:
oxaloacetate + GTP  PEP + GDP + CO2
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Pyruvate Carboxylase
PEP Carboxykinase
O
O
O
O
C
ATP ADP + Pi
C
C
C
O
CH3
GTP
GDP
O
C
C
CO2
C
O
pyruvate
O
CH2
HCO3
O
O

oxaloacetate
OPO 32
CH2
PEP
• Contributing to spontaneity of the 2-step process:
• Free energy of one ~P bond of ATP is conserved in the
carboxylation reaction.
• Spontaneous
decarboxylation
contributes
to
spontaneity of the 2nd reaction.
• Cleavage of a second ~P bond of GTP also contributes to
driving synthesis of PEP.
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glyceraldehyde-3-phosphate
NAD+ + Pi
Glyceraldehyde-3-phosphate
Dehydrogenase
NADH + H+
Summary of
Gluconeogenesis
Pathway:
Gluconeogenesis
enzyme names in
red.
Glycolysis enzyme
names in blue.
1,3-bisphosphoglycerate
ADP
Phosphoglycerate Kinase
ATP
3-phosphoglycerate
Phosphoglycerate Mutase
2-phosphoglycerate
Enolase
H2O
phosphoenolpyruvate
CO2 + GDP
GTP
oxaloacetate
Pi + ADP
Dr.Samah Kotb
PEP Carboxykinase
HCO3 + ATP
pyruvate
Pyruvate Carboxylase
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Gluconeogenesis
glucose
Pi
Gluconeogenesis
Glucose-6-phosphatase
H2O
glucose-6-phosphate
Phosphoglucose Isomerase
fructose-6-phosphate
Pi
Fructose-1,6-bisphosphatase
H2O
fructose-1,6-bisphosphate
Aldolase
glyceraldehyde-3-phosphate + dihydroxyacetone-phosphate
Triosephosphate
Isomerase
(continued)
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Glycolysis & Gluconeogenesis are both spontaneous.
If both pathways were simultaneously active in a cell, it
would constitute a "futile cycle" that would waste energy.
Glycolysis:
glucose + 2 NAD+ + 2 ADP + 2 Pi 
2 pyruvate + 2 NADH + 2 ATP
Gluconeogenesis:
2 pyruvate + 2 NADH + 4 ATP + 2 GTP 
glucose + 2 NAD+ + 4 ADP + 2 GDP + 6 Pi
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Regulation of Gluconeogenesis
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Phosphofructokinase 
6 CH OPO 2
2
3
O
5
H
H
4
OH
6 CH OPO 2
2
3
1CH2OH
ATP
HO
3 OH
H
fructose-6-phosphate
O
ADP
2
5
Pi
H2O
1CH2OPO32
H
H
HO
3 OH
4
OH
2
H
fructose-1,6-bisphosphate
 Fructose-1,6-biosphosphatase
To prevent the waste of a futile cycle, Glycolysis &
Gluconeogenesis are reciprocally regulated.
Local Control includes reciprocal allosteric regulation by
adenine nucleotides.
 Phosphofructokinase (Glycolysis) is inhibited by ATP and
stimulated by AMP.
 Fructose-1,6-bisphosphatase (Gluconeogenesis) is inhibited by
AMP.
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Regulation of Gluconeogenesis
The opposite effects of adenine nucleotides on
 Phosphofructokinase (Glycolysis)
 Fructose-1,6-bisphosphatase (Gluconeogenesis)
• Insures that when cellular ATP is high (AMP would then
below), glucose is not degraded to make ATP.
• When ATP is high it is more useful to the cell to store
glucose as glycogen.
• When ATP is low (AMP would then be high), the cell
does not expend energy in synthesizing glucose.
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Regulation of Gluconeogenesis
• Global Control in liver cells includes reciprocal effects of
a cyclic AMP cascade, triggered by the hormone glucagon
when blood glucose is low.
• Phosphorylation of enzymes & regulatory proteins in
liver by Protein Kinase A (cAMP Dependent Protein
Kinase) results in
 inhibition of glycolysis
 stimulation of gluconeogenesis,
• making glucose available for release to the blood.
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Glycogen
Pyruvate
Gluconeogenesis
X
Glucose-1-P
Glucose-6-P
Glucose + Pi
Glucose-6-Pase
X
Glycolysis
Pathway
Summary of effects of glucagon-cAMP cascade in liver:
 Gluconeogenesis is stimulated.
 Glycolysis is inhibited.
 Glycogen breakdown is stimulated.
 Glycogen synthesis is inhibited.
 Free glucose is formed for release to the blood.
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Regulation of Gluconeogenesis
Enzymes relevant to these pathways
phosphorylated by Protein Kinase A include:
that
are
 Pyruvate Kinase, a glycolysis enzyme that is
inhibited when phosphorylated.
 CREB (cAMP response element binding protein)
which activates, through other factors, transcription
of the gene for PEP Carboxykinase, leading to
increased gluconeogenesis.
 A bi-functional enzyme that makes and degrades an
allosteric regulator, fructose-2,6-bisphosphate.
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Reciprocal regulation by fructose-2,6bisphosphate
 Fructose-2,6-bisphosphate stimulates Glycolysis.
Fructose-2,6-bisphosphate allosterically activates the
Glycolysis enzyme Phosphofructokinase.
Fructose-2,6-bisphosphate
also
activates
transcription of the gene for Glucokinase, the liver
variant of Hexokinase that phosphorylates glucose to
glucose-6-phosphate, the input to Glycolysis.
 Fructose-2,6-bisphosphate
allosterically
inhibits
the
gluconeogenesis
enzyme
Fructose-1,6-bisphosphatase.
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The Cori Cycle
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Cori Cycle
• The Cori Cycle operates during exercise.
• For a brief burst of ATP utilization, muscle cells utilize
~P stored as phosphocreatine.
• Once phosphocreatine is exhausted, ATP is provided
mainly by Glycolysis, with the input coming from
glycogen breakdown and from glucose uptake from the
blood.
• (Aerobic fat metabolism , is more significant during a
lengthy period of exertion such as a marathon run.)
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Cori Cycle
Liver
Glucose
2 NAD+
2 NADH
6 ~P
2 Pyruvate
2 NADH
2 NAD+
2 Lactate
Blood
Muscle
Glucose
2 NAD+
2 NADH
2 ~P
2 Pyruvate
2 NADH
2 NAD+
2 Lactate
• Lactate produced from pyruvate passes via the blood to
the liver, where it may be converted to glucose.
• The glucose may travel back to the muscle to fuel
Glycolysis.
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Cori Cycle
Liver
Glucose
2 NAD+
2 NADH
6 ~P
2 Pyruvate
2 NADH
2 NAD+
2 Lactate
Blood
Muscle
Glucose
2 NAD+
2 NADH
2 ~P
2 Pyruvate
2 NADH
2 NAD+
2 Lactate
The Cori cycle costs 6 ~P in liver for every 2 ~P made
available in muscle. The net cost is 4 ~P.
Although costly in ~P bonds, the Cori Cycle allows the
organism to accommodate to large fluctuations in energy
needs of skeletal muscle between rest and exercise.
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Cori Cycle
• The equivalent of the Cori Cycle also operates during
cancer.
• If blood vessel development does not keep equivalent
with growth of a solid tumor, decreased O2
concentration within the tumor leads to activation of
signal processes that result in a shift to anaerobic
metabolism.
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Liver
Blood
Glucose
2 NAD+
2 NADH
6 ~P
Cancer Cell
Glucose
2 NAD+
2 NADH
2 ~P
2 Pyruvate
2 NADH
2 NAD+
2 Lactate
2 Pyruvate
2 NADH
2 NAD+
2 Lactate
Energy dissipation by the Cori Cycle, which expends 6 ~P
in liver for every 2 ~P produced via Glycolysis for utilization
within the tumor, is thought to contribute to the weight
loss that typically occurs in late-stage cancer even when
food intake remains normal.
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