2. Introduction
It refers to all pathways that synthesise new glucose from non-
carbohydrateprecursors.
It occurs in the cytosol.
90% in liver and 10% in the kidneys-only those two tissues can provide
blood glucose by gluconeogenesis because they have the enzyme glucose-
6-phosphatasein their endoplasmic reticulum.
It involves both cytosolicand mitochondrial enzymes and is the main
process maintaining blood glucose level during normal overnight fasting or
between meals.
This metabolicpathway is very important because glucose is the primary
energy source for the brain(since nerve cells store littleamounts of
glycogen) and erythrocytes(dueto lack of mitochondriathusdepend on
glycolysis for energy).
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3. Introduction
Major substrates
- glucogenic amino acids e.g Alanine and aspartate
- Lactate from anaerobicrespiration in RBCs and muscle.
- propionate *principal glucogenic fatty acid producedin the digestion of
carbohydratesby ruminants is a major substratefor gluconeogenesis in
these species
-Glycerol from degradation of fats (lipolysis) in adipose tissue.
N.B- Acetyl-CoAis not a glucogenic precursor for mammals.
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4. Biological importance
The energy requirement of the brain is derived almost entirely from
glucose.
Since nerve cells store very littleglycogen, the brain and certain other
tissues including corneaand red blood cells depend on a constant supplyof
glucose in the blood.
One of the important functions of the liver is to maintain the bloodglucose
level.
Degradation of liver glycogen is the primary source of blood glucose in the
early fasting state.
However, when glycogen stores are depleted, theliver is able to synthesize
glucose from lactate, via gluconeogenesis.
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5. Biological importance
Gluconeogenesis meets the needs of the body for glucose when CHO is not
available in sufficient amountsfrom the diet or from glycogen reserves
A supplyof glucose is necessary especially in the nervous system and
erythrocytes
Hypoglycaemia causes brain dysfunction, which can lead to comma and
death
Gluconeogenesis is also important in maintaining the level of intermediates
of the citric acid cycle even when fatty acids are the main source of acetyl-
coA
It also clears lactateproducedby muscle and erythrocytes
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7. Gluconeogenesis involves glycolysis, the citric
acid cycle and some special reactions
Thermodynamic barriers prevents a simple reversal of glycolysis
The irreversible reactionscatalysed by hexokinase, phosphofructokinase-1
and pyruvate kinase prevent simple reversal of glycolysis for glucose
synthesis
These three stages are bypassed by alternateenzymes specific to
gluconeogenesis
- Pyruvate carboxylase
- Phosphoenolpyruvatecarboxykinase (PEPCK)
- Fructose1,6 - bisphosphatase
- Glucose 6-phosphatase
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11. First bypass reaction
The first bypass reaction involves two phases catalysed by two enzymes
pyruvate carboxylase and phosphoenolpyruvate carboxykinase which
results in the conversion of pyruvate to Phosphoenolpyruvate.
1 .Carboxylation of pyruvate to oxaloacetate
In the mitochondria of liver and kidney cells, pyruvate is carboxylated.
Carboxybiotin is the donorof carboxyl group:
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12. Biotin as a cofactor in the first
bypass reaction
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13. Biotin as a cofactor in the first
bypass reaction
The reaction takes place in two steps
1. Carboxylation of biotin involvingATP
2. Transfer of the carboxyl to pyruvate to form oxaloacetate(OAA)
Pyruvate carboxylase is mitochondrial – it requiresATP and uses biotin as a coenzyme
Biotin binds C𝑂2 from bicarbonate as Carboxybiotin prior to the addition of the C𝑂2
to pyruvate.
Typical biotin-dependent carboxylation reaction
Requires HCO⁻₃ andATP
The biotinyl group serves as a temporary carrier of CO₂ transferring it pyruvate.
N.B -Pyruvate is an ABC enzyme
A- ATP
B- Biotin cofactor
C- carbon dioxide molecule.
The activity of pyruvate carboxylase depends on the presence of an allosteric activator -
acetyl-CoA.
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14. The malate shunt
To be used for gluconeogenesis, the oxaloacetatemust be transferred into
the cytoplasm.
However, mitochondrialack an efficient transporterfor oxaloacetate.
Thus oxaloacetatecannot pass through the inner mitochondrial membrane
and is reduced to malate by mitochondrial malate dehydrogenaseusing
reduced NAD as a coenzyme and transportedinto the cytosol.
Malateis converted back to oxaloacetateby cytosolicmalate
dehydrogenase.
In humans,guinea pig (porcine)and bovine (cow) species, malate
dehydrogenaseis equallydistributedbetween mitochondriaand the cytosol.
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16. The first bypass reaction-phase 2
2.Conversion of oxaloacetate to Phosphoenolpyruvate (PEP)
Oxaloacetateis simultaneouslydecarboxylatedand phosphorylatedby
phosphoenolpyruvatecarboxykinase in the cytosol.
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17. The first bypass reaction-phase 2
The two-step pathway for the formation of phosphoenolpyruvate (the sum of
reactions 1 and 2)
is much more favourable than the reaction catalyzed by pyruvate kinase, because
of the use of a molecule of ATP in the carboxylation reaction 1.
The added molecule of C𝑂2 is then removed to power the endergonic
formation of PEP in the decarboxylation step.
Decarboxylation drives this reaction, which would otherwise be endergonic.
PEP carboxykinase the decarboxylation and phosphorylation to PEP using GTP
as the phosphate donor.
In pigeons and rabbits liver PEPCK is mitochondrial and PEP is transported
into the cytosol for gluconeogenesis
In rats and mouse the enzyme is cytosolic
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18. The first bypass reaction forms PEP
from pyruvate
The first bypass reaction yields phosphoenolpyruvate(PEP)which is an
intermediatein glycolysis.
This is achieved by simple reversal of glycolysis using two enzymes
1. Pyruvatecarboxylase
2. Phosphoenolpyruvatecarboxykinase
Phosphoenolpyruvateenters the gluconeogenesis pathway to generate
glucose.
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19. Second bypass reaction
3.Dephosphorylation of fructose 1,6-bisphosphate
The hydrolysis of fructose 1,6-bisphosphateto fructose 6-phosphateis
catalyzed by fructose1,6-bisphosphatase.
Fructose 1,6 – bisphosphataseis present in liver, kidney and skeletal
muscle
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20. Second bypass reaction
Fructose1,6-bisphosphataseis an allostericenzyme.
Like its glycolytic counterpartphosphofructokinase-1, it participatesin the
regulation of gluconeogenesis.
Both enzymes are reciprocallycontrolledby fructose 2,6-bisphosphatein
the liver.
Fructose2,6-bisphosphatestrongly stimulates phosphofructokinase-1and
inhibits fructose 1,6-bisphosphatase.
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21. Third bypass reaction
In most tissues, gluconeogenesis (if thereis any) ends at glucose 6-
phosphate, free glucose is not generated.
Glucose6-phosphataseis presentonly in the livercells and to a lesser
extent in the kidney, only these tissues can release free glucose into the
blood.
In the third bypass reaction, glucose 6-phosphateis converted to glucose
catalyzed by glucose-6-phosphatase
Glucose 6-phosphataseis present in liver and kidney but absent from
muscle and adipose tissue, which, therefore, cannot export glucose into the
bloodstream.
Glucose-6-phosphatasein an integral membrane protein of endoplasmic
reticulumin the liver and kidney.
The dephosphorylationof glucose 6-phosphatetakes place within the
lumen of endoplasmic reticulum.
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23. Third bypass reaction
SP – 𝐶𝑎2+
-binding stabilizing protein is essential for Glu-6-phosphatase
activity.
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24. Third bypass reaction
Glucose-6-phosphataseis found in the lumen of the endoplasmic reticulum
rather than in the cytoplasm.
Thus, for the final step of gluconeogenesis, G6P must be transported into
the ER, the phosphateis cleaved off, and then glucose and phosphateare
transported back out.
Deficiencies in either glucose-6-phosphataseor any of the three
transportersresult in von Gierke’s disease, with symptoms of
hypoglycemia, lacticacidemia and ketoacidosis after mild fasting.
Note that these two phosphatasereactionsdo not reverse the reciprocal
kinase reactions, becauseATP is not regenerated.
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25. Third bypass reaction
In cycling from glucose to pyruvate to glucose in the Cori cycle, four high-
energy phosphatebondsare hydrolysed.
This expenditureof energy is required to turn an energetically
unfavourableprocess (the reversal of glycolysis, DGo’ = +20 kcal/mol) into
a favourable one (gluconeogenesis, DGo’ = -9 kcal/mol).
The extra cost of nucleotidehydrolysis is borneby the liver, another
example of its altruism toward othertissues.
The energy used in the process is obtainedfrom β-oxidation of fatty acids.
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29. Glycerol as a glucogenic precursor
Glycerol (from mobilized reserve fat) enters the gluconeogenesis as
dihydroxyacetonephosphate:an intermediate of glucose and it is converted
back to glucose by a simple reversal of glycolytic steps that result in its
formation from glucose
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30. Lactate as a glucogenic precursor
Lactate from fermentation in RBCs and muscle is taken up by the liver.
Lactate dehydrogenaseoxidises lactateto pyruvate which enters the
gluconeogenesis pathway to form phosphoenolpyruvatean intermediate of
glycolysis.
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32. Glucogenic amino acids
After transamination or deamination, glucogenic amino acids yield
either pyruvate or intermediates of the citric acid cycle.
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33. Propionate - a precursor of
gluconeogenesis
Propionateis a major source of glucose in ruminants and enters
gluconeogenesis via the citric acid cycle
Dietary odd chain fatty acids upon oxidation yield propionatea substrateof
gluconeogenesis in the human liver
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34. Propionate - a precursor of
gluconeogenesis
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35. The Cori’s cycle and the Glucose-
Alanine cycle
Gluconeogenesis in the liver transforms part of the lactateformed by active
skeletal muscle to glucose.
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36. The Cori’s cycle and the Glucose-
Alanine cycle
When the body performs highly strenuousmuscular activity, the amount of
oxygen intake becomes disproportional(much less) to the energy
requirement of the muscle.
An anaerobic metabolic process therefore takes place producinglactate
from pyruvate. In the Cori cycle, lactateaccumulated in the muscle cells is
taken up by the liver.
The liver performs gluconeogenesis, to convert lactateback to glucose.
Gluconeogenesis reverses both the processes of glycolysis and fermentation
that the body had performed to producelactate.
Of the amino acids transportedfrom muscle to the liver during starvation,
alaninepredominates.
The glucose-alaninecycle transportsglucose from liver to muscle with
formation of pyruvate, followed by transamination to alanine
Alanine is transportedback to the liver, followed by gluconeogenesis back
to glucose.
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38. Regulation of Gluconeogenesis
The enzymes of glycolysis and gluconeogenesis in the liver are reciprocally
regulated so that either glucose is converted to pyruvate or pyruvate is
converted to glucose.
Fructose-2,6-bisphosphate, which we have already seen serves to activate
phosphofructokinase, is an inhibitor of fructose-1,6-bisphosphatase.
Because of this reciprocal effect, only one of the two enzymes is active at any
given time.
The liver also contains glucokinase inhibitor protein, which is activated by
fructose-6- phosphate.
When bound to F6P, glucokinase inhibitor protein sequesters and inactivates
glucokinase, shutting down the first step in glycolysis.
There is no equivalent inhibitory protein for hexokinase, so accumulation of
F6P shuts down glycolysis and enables activation of gluconeogenesis only in
the liver.
Allosteric regulation of gluconeogenesis and glycolysis is summarized below:
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42. Regulation of Gluconeogenesis-
Hormonal control of gluconeogenesis
As with glycolysis, glucagon-dependentprotein phosphorylationalso
regulates enzyme activities in gluconeogenesis.
Recall that F-2,6-BPlevels are regulated by glucagon, with high glucagon
(low blood sugar) favouring conversion of F-2,6-BPback into F6P
In addition, glucagon activates lipases is adipose tissue, promoting release
of fatty acids into the bloodstream.
These fatty acids are broken down in the mitochondriaof liver, resultingin
high concentrationsof acetyl CoA.
Acetyl CoAacts as an allostericactivator of pyruvate carboxylase.
Both insulin and glucagon regulate the transcription of bypass enzymes:
insulin inhibits transcription of phosphoenolpyruvatecarboxykinase, and
glucagon activates its transcription.
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43. Regulation of Gluconeogenesis
Glucagon and the availability of substratesmainly regulate
gluconeogenesis
Glucagon & glucocorticoid↑ gluconeogenesis.
Insulin inhibit gluconeogenesis
Glucogenic amino acids have stimulating effect on key gluconeogenic
enzymes
* Acetyl CoA promotes gluconeogenesis
Starvation → excessive lipolysis in adipose tissues
Acetyl CoAaccumulates in the liver, acetyl CoA stimulate gluconeogenic
enzymes.
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