Ferchmin 2017
GLUCONEOGENESIS
Summary of handout:
Comparison with glycolysis, unique and shared enzymes
"Reversal" of pyruvate kinase. Participation of the mitochondria
"Reversal" of Phosphofructokinase
"Reversal" of hexokinase
The Cori and alanine cycles
Regulation. Role of insulin and glucagon in glycolysis and
gluconeogenesis.
Glycogenic and ketogenic compounds
Metabolic role of gluconeogenesis
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COMPARISON BETWEEN GLYCOLYSIS AND GLUCONEOGENESIS
The overall reaction of gluconeogenesis is:
COOH
|
2 CO + 4 ATP + 2 GTP + 2 NADH + 2H+ + 2 H2O ➔ glucose + 4 ADP + 2 GDP + 6 Pi + 2 NAD+
|
CH3
ΔG°'= -9 Kcal/mole
The overall reaction of glycolysis is:
COOH
|
+
Glucose + 2 ADP + 2 Pi + 2 NAD ➔ 2 CO + 2 ATP + 2 NADH + 2H+ + 2 H2O
|
CH3
ΔG°'= -20 Kcal/mole
Glycolysis yield 2 ATP/glucose plus a net dissipated -20 Kcal/mole.
Gluconeogenesis is really bad news, it consumes the equivalent of 6 ATP/glucose
synthesized. Why would be a need for such a metabolic pathway?
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Gluconeogenesis is the synthesis of glucose from precursors that are not sugars,
like lactate, pyruvate, glycerol or glycogenic amino acids. The synthesis of glucose
from other sugars simply is not gluconeogenesis. The neo means de novo from
non-carbohydrate molecules like amino acids, glycerol, lactate, etc…
There is no gluconeogenesis from fatty acids except the rare ones with odd
number of carbons that have a minute contribution to the synthesis of glucose.
The main contribution of fatty acids to gluconeogenesis is to provide material for exam questions.
Fatty acids contribute during fasting with ATP produced in β-oxidation and
oxidation of ketone bodies in the Krebs cycle. Ketone bodies only partially
substitute for glucose and are synthesized by a pathway different from
gluconeogenesis. Ketone bodies are potentially dangerous in the absence of
glucose (you will study this later). In conclusion: lipids can spare glucose because
they provide for ATP that otherwise would be synthesized in glycolysis. But lipids
do not substitute for glucose.
We need about l60 grams of glucose per day, 120 grams are needed for the
brain and 40 grams for muscle, erythrocytes, eye lens cells, kidneys
medulla, etc. Approximately 200 grams are stored in hepatic glycogen.
Gluconeogenesis provides the indispensable glucose during fasing.
The complete gluconeogenesis occurs in liver and a small fraction in kidney. Since
glycolysis is irreversible gluconeogenesis cannot be the reversal of glycolysis. The
enzymes that catalyze the irreversible reactions in glycolysis are overridden in
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various ingenious ways in gluconeogenesis.
We will study gluconeogenesis by comparing it with glycolysis
By using an enzyme that
catalyzes the opposite
also irreversible step!!!
By using an enzyme that
catalyzes the opposite
also irreversible step!!!
How do your reverse
an irreversible
metabolic step?
How do your reverse
an irreversible
metabolic step?
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The last glycolytic step
catalyzed by pyruvate
kinase is irreversible, the
free energy change is
high, -7.5 Kcal/mole.
To reverse this step in
gluconeogenesis two
enzymes are used and
the process takes place
in two cellular
compartments.
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Enzymatic differences between glycolysis and gluconeogenesis
a) Regulatory enzymes
__________________________________________________________________
Glycolysis
Gluconeogenesis
__________________________________________________________________
Hexokinase
Pyruvate carboxylase
is located in liver
mitochondria
Glucose 6-phosphatase
Phosphofructokinase 1
Fructose 1,6bisphosphatase
Pyruvate
Pyruvate kinase
carboxylase
Phosphoenolpyruvate
carboxykinase
__________________________________________________________________
b) The remaining enzymes are shared by both pathways
__________________________________________________________________
Essential concept: Pathways for breakdown and synthesis of a particular
metabolite are always different, utilizing unique enzymes in one or more steps.
The difference usually is in the regulatory enzymes.
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First we will consider the reversal in gluconeogenesis of the exergonic glycolytic
reaction catalyzed by pyruvate kinase. The reaction is shown below:
The above exergonic reaction is overcome by an input of energy and
of two complex reactions that regenerate phosphoenolpyruvate.
The two enzymes involved are:
a) Pyruvate carboxylase
b) Phosphoenolpyruvate carboxykinase
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PYRUVATE CARBOXYLASE is exclusively hepatic.
The reaction catalyzed by pyruvate carboxylase takes place in 2 steps:
STEP 1: Enz-Biotin + ATP + CO2 ➔ Enz-Carboxybiotin + ADP + Pi
This first step requires CH3-CO-CoA (acetyl~S-CoA)
STEP 2: Enz-Carboxybiotin + pyruvate ➔ Enz-Biotin + oxaloacetate
This is an anaplerotic reaction (re-supplying). It provides oxaloacetate
for the Krebs cycle and for gluconeogenesis.
The requirement for CH3-CO~S-CoA is a manifestation of the need of
oxaloacetate for the TCA cycle or the abundance of CH3-CO-CoA produced by a
lipid rich diet that calls for storage of glucogenic intermediaries.
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The next step is the synthesis of phosphoenolpyruvic acid from oxaloacetate
The synthesis of PEPA reverses the
effect of pyruvate kinase
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Glycolysis can not go on without NAD+
reciprocally gluconeogenesis
can not go on without NAD+H+
Gluconeogenesis takes place in the cytosol and in the
mitochondria. There are two pathways to generate
PEPA (phosphoenolpyruvic acid). In both pathways
NADH must be generated to allow the activity of
glyceraldehyde-3-phosphate dehydrogenase in the
reduction of 3-phosphoglyceric acid.
From PEPA to fructose-1,6bisphosphate all the steps are
shared by glycolysis and
gluconeogenesis and are
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reversible.
This graph represents the relationship
between the activity of both enzymes and
the energy status of a muscle cell.
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Integration of gluconeogenesis and glycolysis
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There is a fundamental difference between the role of glycolysis in the
“peripheral” organs and liver.
In liver the role of glycolysis is to make you FAT!!!! In muscle is to make you
run!!!
Ethanol and fatty acids are not glucogenic (odd number fatty acids
contribute insignificantly to gluconeogenesis).
Glycerol, the ketoacids of most amino acids, lactate and pyruvate ARE
glucogenic.
Galactose, fructose, etc are not glucogenic. They are monosaccharides in
equilibrium with glucose!
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