Download Gluconeogenesis

Ferchmin 2016
GLUCONEOGENESIS
Summary of handout:
Comparison with glycolysis, unique and shared enzymes
Role of biotin in gluconeogenesis (and comparison with vitamin K which is not
involved in gluconeogenesis)
"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 yields 2 ATP/glucose plus - 9 Kcal/mole dissipated.
Gluconeogenesis is really bad news, it consumes the equivalent of 6 ATP/glucose
synthesized. Why would be a need for such a wasteful 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. (By the way, what was a carbohydrate?)
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.
Fatty acids contribute to the fasting organism with ATP through 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 (cause metabolic acidosis). In conclusion: lipids can spare glucose
because they provide for ATP that otherwise would have being synthesized from
glucose. However, 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 necessary glucose during fast.
The complete gluconeogenesis occurs in the liver and kidneys. Glycolysis is
irreversible therefor gluconeogenesis cannot be the reversal of glycolysis. The
enzymes that catalyze the irreversible reactions in glycolysis are overridden in
various ingenious ways in gluconeogenesis.
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We will study gluconeogenesis by comparing it with glycolysis
Left gluconeogenesis------------Right 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.
The first enzyme is pyruvate
carboxylase, the second is
phosphoenolpyruvate
carboxykinase.
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We will analyze some reaction in detail
First, we will consider the exergonic glycolytic reaction catalyzed
by pyruvate kinase and its reversal in gluconeogenesis
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
However, before considering the enzymes we will look at the coenzyme of pyruvate carboxylase
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With reference to pyruvate carboxylase
This part is a “pop up” about a different subject
marginally related to gluconeogenesis but of
practical importance (NBE). There are two vitamins
involved in CO2 incorporation in mammalian tissues:
biotin and vitamin K. In gluconeogenesis biotin is a
cofactor for pyruvate carboxylase.
Biotin binds avidly to AVIDIN.
Vitamin K is involved in the posttranslational
synthesis of γ-carboxyglutamate involved prominently
in blood coagulation.
Biotin is attached to the amino of lysine in carbon ε.
Biotin is a cofactor for
1) Pyruvate carboxylase
2) β-methylcrotonylCoA carboxylase
3) Propionyl CoA carboxylase
4) Acetyl CoA carboxylase
Biotin
Vitamin K
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So, after leaving the detour of biotin we return to pyruvate carboxylase and gluconeogenesis
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. Beware, there are
cataplerotic steps in the Krebs cycle.
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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From PEPA to fructose-1,6bisphosphate all the steps are
shared by glycolysis and
gluconeogenesis and are
reversible.
Most steps of gluconeogenesis take
place in the cytosol but the synthesis
of phosphoenolpyruvic acid (PEPA)
requires the mitochondria. PEPA can
be synthesized from pyruvate or
lactate. In both cases, NADH +H+
must be generated to allow the
reduction of 3-phosphoglyceric acid
by glyceraldehyde-3-phosphate
dehydrogenase.
The figure illustrates both cases.
Why a load of alcohol
inhibits gluconeogenesis and
lowers glucose?
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This graph represents the relationship
between the activity of both enzymes and
the energy status of a muscle cell.
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From previous page, we can see the relationship between
phosphofructokinase and fructose-1,6-phosphatase
In this point we have a
metabolic cycle or futile
cycle that “wastes”
energy but provides
more leverage for
regulation
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Summary of the enzymatic differences between glycolysis and gluconeogenesis
a) Regulatory enzymes
__________________________________________________________________
Glycolysis
Gluconeogenesis
__________________________________________________________________
Hexokinase
Phosphofructokinase
Glucose 6-phosphatase
Pyruvate carboxylase
is located
in liver
Fructose
1,6-bisphosphatase
mitochondrias
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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Integration of gluconeogenesis and glycolysis
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There is a fundamental difference between the role of glycolysis in the
“peripheral” organs and the 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!
Warning: Whoever says that even-
numbered fatty acids are
glucogenic will be decapitated!
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Warning: whoever says that fatty acids
with even number of carbons can
sustain net synthesis of glucose will be
decapitated!
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