Lecture (8) Carbohydrates biochemistry .ppt

Biochemistry

1. Gluconeogenesis

2. Gluconeogenesis is making a new glucose
from non-carbohydrate precursors
In other words:
Create new glucose from the products of its
breakdown
Its main function is to supply blood glucose in
cases of carbohydrate deficiency
(fasting, starvation and low carbohydrate diet).
What is the Gluconeogenesis?

3. Sites for gluconeogenesis
Cytoplasm and mitochondria of liver and
kidney tissues due to presence of glucose-6-
phosphatase and fructose-1,6-biphosphatase.
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.

4. Glycolysis pathway

5. The reactions of gluconeogenesis:
Synthesis of glucose from pyruvate utilizes many of
the same enzymes as glycolysis.
Three Glycolytic reactions are essentially irreversible
Hexokinase (or Glucokinase)
Phosphofructokinase
Pyruvate Kinase.
These steps must be by passed in Gluconeogenesis.

6. First Bypass Reaction
Convervsion of Pyruvate to Phosphoenolpyruvate
•Enzymes involved:
- Pyruvate carboxylase
- PEP carboxykinase

C
C
CH
2
O O

OPO
3
2
C
C
CH
3
O O

O
ATP ADP + P
i C
CH
2
C
C
O
O O

O

O
HCO
3

GTP GDP
CO
2
pyruvate oxaloacetate PEP
Pyruvate Carboxylase PEP Carboxykinase

7. In Mitochondria
•Pyruvate carboxylase (PC) exists in the mitochondria of
liver and kidney but absent in muscle
•ATP, biotin, Mn
++
and CO
2 are required.

8. Transport of Oxaloacetate into cytosol as
Malate

9. In cytosol

10. Summary of the first bypass

11. 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 which is Fructose 1,6-bisphosphatase.
•Fructose 1,6-bisphosphatase presents in liver and kidney.
•This reaction is also irreversible.
Fructose 1,6-bisphosphate + H
2O fructose 6-
phosphate + Pi

12. Phosphofructokinase (In Glycolysis):
fructose-6-P + ATP  fructose-1,6-bisP + ADP
Fructose-1,6-bisphosphatase (In Gluconeogenesis):
fructose-1,6-bisP + H
2
O  fructose-6-P + P
i

fructose-6-phosphate fructose-1,6-bisphosphate
Phosphofructokinase 
CH
2OPO
3
2
OH
CH
2OH
H
OH
H
H HO
O
6
5
4 3
2
1 CH
2OPO
3
2
OH
CH
2OPO
3
2
H
OH
H
H HO
O
6
5
4 3
2
1
ATP ADP
P
i H
2O
 Fructose-1,6-biosphosphatase

13. Third bypass reaction
Glucose 6-phosphate to Glucose
•Because the hexokinase reaction is irreversible, the
final reaction of gluconeogenesis is catalyzed by
Glucose 6-phosphatase.
Glucose 6-phosphate + H
2
O 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.

14. Hexokinase or Glucokinase (In Glycolysis):
glucose + ATP  glucose-6-phosphate + ADP
Glucose-6-Phosphatase (In Gluconeogenesis):
glucose-6-phosphate + H
2
O  glucose + P
i

H
O
OH
H
OHH
OH
CH
2OH
H
OH
HH
O
OH
H
OHH
OH
CH
2OPO
3
2
H
OH
H
H
2O
1
6
5
4
3 2
+ P
i
glucose-6-phosphate glucose
Glucose-6-phosphatase

15. Glyceraldehyde-3-phosphate
Dehydrogenase
Phosphoglycerate Kinase
Enolase
PEP Carboxykinase
glyceraldehyde-3-phosphate
NAD
+
+ Pi
NADH + H
+

1,3-bisphosphoglycerate
ADP
ATP
3-phosphoglycerate
Phosphoglycerate Mutase
2-phosphoglycerate

H2O
phosphoenolpyruvate
CO2 + GDP
GTP
oxaloacetate
Pi + ADP
HCO3

+ ATP
pyruvate
Pyruvate Carboxylase
Gluconeogenesis
Summary of
Gluconeogenesis
Pathway:
Gluconeogenesis
enzyme names in
red.
Glycolysis enzyme
names in blue.

16. Glucose-6-phosphatase
Fructose-1,6-bisphosphatase
glucose Gluconeogenesis
Pi

H2O
glucose-6-phosphate
Phosphoglucose Isomerase
fructose-6-phosphate
Pi
H2O
fructose-1,6-bisphosphate
Aldolase
glyceraldehyde-3-phosphate + dihydroxyacetone-phosphate
Triosephosphate
Isomerase
(continued)

17. 1- 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 (Cori cycle).
Substrates for gluconeogenesis

18. Lactate produced from pyruvate passes via the blood to
the liver, where it may be converted to glucose.
The glucose travels back to the muscle to fuel Glycolysis.

19. 2- Glucogenic amino acids:
•Amino acids by deamination can be converted
into keto acids as pyruvic, 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.
Substrates for gluconeogenesis

20. Carbon sources for gluconeogenesis
Glucose-alanine cycle:
Glucose oxidation produces pyruvate which can undergo
transamination to alanine in liver. This reaction is catalyzed by
alanine transaminase (ALT).
Additionally, during fasting, skeletal muscle protein is degraded
yielding high amount of alanine which then enters the blood
stream and is transported to the liver.
In liver, alanine is converted back to pyruvate which is then a
source of carbon atoms for gluconeogenesis. The newly formed
glucose can then enter the blood for delivery back to the muscle.

23. 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 + CO
2
NAD
+
NADH+H
+

24. 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.