Transcript Gluconeogenesis by Dr Tarek

Gluconeogenesis
Dr. Tarek A Salem
Objectives of this lecture
• Definition of gluconeogenesis
• The steps of gluconeogenesis
• Site of occurrence and importance of
gluconeogenesis.
• The sources of carbon atoms used in gluconeogenesis
Definition
• Glucneogenesis is making a new glucose from
non-carbohydrate precursors
• In other words:
– Create new glucose from the products of its
breakdown
Site of gluconeogenesis
• Major site of gluconeogenesis: Liver (90%)
• Secondary site: Kidney cortex and in small
intestine under some conditions. (10 %)
• It takes place in the mitochondria and cytoplasm.
• The production of glucose is necessary for use as a
fuel source by the brain, testes, erythrocytes,
kidney medulla, lens and cornea of the eye and
exercising muscle.
Overview
Synthesis of glucose from pyruvate utilizes many
of the same enzymes as Glycolysis.
Three Glycolysis reactions have such a large
negative ΔG that they are essentially irreversible
 Hexokinase (or Glucokinase)
 Phosphofructokinase
 Pyruvate Kinase.
These steps must be bypassed in Gluconeogenesis.
First Bypass Reaction:
Convervsion of Pyruvate to Phosphoenolpyruvate
• Enzymes involved:
Pyruvate carboxylase
PEP carboxykinase
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
CO2
O
oxaloacetate
C
OPO32
CH2
PEP
In Mitochondria
• Pyruvate carboxylase (PC) presents in the mitochondria
of liver and kidney but absent in muscle
• ATP, biotin, Mn++ and CO2 are required.
Transport of Oxaloacetate into cytosol as
Malate
In cytosol
Summary of the first bypass
The overall equation for this set
of bypass reactions
• Pyruvate + ATP + GTP + HCO3•
phosphoenolpyruvate + ADP + GDP + Pi + H+ + CO2
• Thus the synthesis of one molecule of PEP requires an
investment of 1 ATP and 1 GTP.
• Note: when either pyruvate or the ATP/ADP ratio is
high, the reaction is pushed toward the right (i.e., in the
direction of biosynthesis).
Second Bypass Reaction: Conversion of Fructose
1,6- bisphosphate to Fructose 6-phosphate
• The second glycolytic reaction (phosphorylation of
fructose 6-phosphate by PFK) is irreversible.
• Hence, for gluconeogenesis fructose 6-phosphate must be
generated from fructose 1,6-bisphosphate by a different
enzyme: Fructose 1,6-bisphosphatase.
• Fructose 1,6-bisphosphatse presents in liver and kidney.
• This reaction is also irreversible.
Fructose 1,6-bisphosphate + H2O
fructose 6phosphate + Pi
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 (In Glycolysis):
fructose-6-P + ATP  fructose-1,6-bisP + ADP
Fructose-1,6-bisphosphatase (In Gluconeogenesis):
fructose-1,6-bisP + H2O  fructose-6-P + Pi
Third Bypass Reaction:
Glucose 6-phosphate to Glucose
• Because the hexokinase reaction is irreversible, the final
reaction of gluconeogenesis is catalyzed by Glucose 6phosphatase.
Glucose 6-phosphate + H2O
glucose + Pi
• Glucose 6-phosphatase is present in the liver, kidney and
small intestine but absent in brain and muscle. Thus,
glucose produced by gluconeogenesis in the liver, is
delivered by the bloodstream to brain and muscle.
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 (In Glycolysis):
glucose + ATP  glucose-6-phosphate + ADP
Glucose-6-Phosphatase (In Gluconeogenesis):
glucose-6-phosphate + H2O  glucose + Pi
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
PEP Carboxykinase
GTP
oxaloacetate
Pi + ADP
HCO3 + ATP
pyruvate
Pyruvate Carboxylase
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)
Carbon sources for gluconeogenesis
• Glycerol: is released during hydrolysis of
triacylglycerols in adipose tissue and is
delivered by the blood to the liver.
Glycerol kinase
• Glycerol
Glycerol 3-P
G3P dehydrogenase
• Glycerol 3-P
DHAP
• DHAP is converted into glyceraldehyde 3-P
Carbon sources for gluconeogenesis
• Lactate (Lactic acid): In vigorous skeletal
muscle activity, large amount of lactic acid
produced
pass to liver through blood
stream
converted into pyruvic and lastly
to glucose
reach muscle again through
blood stream to provide energy. (This called
Cori cycle).
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.
Carbon sources for gluconeogenesis
• Propionic acid: product of odd number
fatty acid degradation.
• It is converted into succinyl CoA which
converted into oxaloactic acid that can
form glucose.
Carbon sources for gluconeogenesis
• Glucogenic amino acids: amino acids by
deamination can be converted into keto
acids as pyruvic, α-ketoglutaric and
oxaloacetic acid.
• Proteins are considered as one of the main
sources of blood glucose especially after 18
hr due to depletion of liver glycogen.
Carbon sources for gluconeogenesis
• Glucose-alanine cycle: During starvation,
alanine is formed from protein catabolism.
• Alanine is converted into pyruvic acid in liver
which can give glucose.
Acetyl CoA can not produce
glucose
• Acetyl CoA cannot give rise to a net synthesis
of glucose. This is due to the irreversible nature
of the pyruvate dehydrogenase reaction, which
converts pyruvate to acetyl CoA.
Pyruvate dehydrogenase
• Pyruvate
acetyl CoA + CO2
NAD+
NADH+H+
Importance of gluconeogenesis
1- Maintenance of blood glucose during
starvation, fasting and prolonged exercise.
2- Removal of lactic acid.
3- Removal of glycerol produced by lipolysis.
Regulation of gluconeogenesis
• In liver: ATP
PK
PC glycolysis is
inhibited and gluconeogenesis is activited
• During starvation, the priority is to conserve
blood glucose for the brain and muscle. Thus,
under these conditions, PK in the liver is
switched off. This occurs because the hormone
glucagon is secreted into the bloodstream and
activates a cAMP cascade that leads to the
phosphorylation and inhibition of this enzyme.
Regulation of gluconeogenesis
• In liver: AMP
PK PC glycolysis
is activated and gluconeogenesis is
stopped.
• In case of Acetyl CoA PK PC
When
acetyl
CoA
is
gluconeogenesis is activated
abundant,