1. GLUCONEOGENESIS Synthesis of glucose from noncarbohydrate precursors
Learning objectives:
List gluconeogenic precursors
List the enzymes and intermediates involved in gluconeogenesis
List the irreversible and regulated steps of gluconeogenesis
Discuss regulation of gluconeogenesis

2. Gluconeogenic precursors
18 amino acids
(diet and degradation of protein)
Lactate
(anaerobic glycolysis)
Glycerol
(hydrolysis of triacylglycerols)
Notable exception: Fatty acids

3. Gluconeogenic precursors
Lactate and some amino acids can be converted to pyruvate
Some amino acids can be converted to oxaloacetate
Glycerol can be converted to dihydroxyacetone phosphate

4. Gluconeogenic pathway describes conversion of pyruvate to glucose
Main organs producing glucose via the
gluconeogenic pathway are liver and kidney

5. Gluconeogenic pathway is NOT a simple
reversal of glycolysis
The 3 irreversible steps of glycolysis
Hexokinase/Glucokinase
Phosphofructokinase
Pyruvate kinase
must be circumvented
The 7 reversible steps of glycolysis are
part of gluconeogenesis

6. All the intermediates of glycolysis are part of gluconeogenesis
In addition, gluconeogenesis involves
oxaloacetate and (indirectly) malate
O O- C
C O
CH2
O O- C
H C OH
CH2
Oxaloacate
Malate

7. Stoichiometry 2 Pyruvate + 4 ATP + 2 GTP + 2 NADH + 2 H+ + 6 H2O →
1 Glucose + 4 ADP + 2 GDP + 6 Pi + 2 NAD+

8. This reaction occurs in the mitochondria It is a regulated step
Pyruvate
carboxylase
Pyruvate + CO2 + ATP + H2O Oxaloacetate + ADP + Pi + 2 H+
This reaction occurs in the mitochondria
It is a regulated step
Biotin is a coenzyme for the reaction

9. Pyruvate is transported from the cytoplasm to the mitochondria.
In the mitochondria, pyruvate is converted to oxaloacetate by pyruvate carboxylase
Oxaloacetate can not be transported to the cytoplasm.
Oxaloacetate is reduced in the mitochondria to malate:
Malate dehydrogenase
Oxaloacetate + NADH + H Malate + NAD+
Malate is transported to the cytoplasm and reoxidized back to oxaloacetate:
Malate + NAD Oxaloacetate + NADH + H+

10. It is a regulated step Phosphoenolpyruvate carboxykinase (PEPCK)
Oxaloacetate + GTP Phosphoenolpyruvate + GDP + CO2
It is a regulated step

11. Irreversible Regulated step Fructose-1,6- bisphosphatase
Fructose 1,6-bisphosphate + H2O Fructose 6-phosphate + Pi
Irreversible
Regulated step

12. Only present in large amount in liver and kidney
Glucose-6-phosphatase
Glucose 6-phosphate H2O Glucose + Pi
Irreversible
Regulated step
Only present in large amount in liver and kidney
Reaction occurs in the endoplasmic reticulum

14. Glycerol kinase
Glycerol + ATP Glycerol phosphate + ADP + H+
Glycerol
phosphate
dehydrogenase
Glycerol phosphate + NAD Dihydroxyacetone phosphate + NADH + H+

15. LIVER IN THE FASTED STATE
Pyruvate
carboxylase
Pyruvate + CO2 + ATP + H2O Oxaloacetate + ADP + Pi + 2 H+ +
Acetyl CoA
(High energy signal)
LIVER IN THE FASTED STATE
Energy is derived mostly from fatty acids
Pyruvate
dehydrogenase
Acetyl-CoA
Fatty acids -
Pyruvate +
Oxaloacetate
Pyruvate
carboxylase

16. + - Phosphoenolpyruvate carboxykinase (PEPCK)
Oxaloacetate + GTP Phosphoenolpyruvate + GDP + CO2
Phosphoenolpyruvate carboxykinase is regulated at the level of
gene transcription +
Glucagon, glucocorticoids (fasted state)
Insulin (fed state) -

17. - - + Fructose-1,6- bisphosphatase
Fructose 1,6-bisphosphate + H2O Fructose 6-phosphate + Pi -
AMP (low-energy state)
Fructose 2,6-bisphosphate (fed state, high insulin/glucagon ratio)
ATP (high-energy state) - +

18. + - + Glucose-6-phosphatase Glucose 6-phosphate + H2O Glucose + Pi
The catalytic subunit of glucose-6-phosphatase is regulated
at the level of gene transcription +
Glucagon, glucocorticoids (fasted state)
Insulin (fed state)
Glucose (fed state) - “Paradoxical regulation” - +

19. Glycolysis Gluconeogenesis
Glucokinase
Glucose
Insulin
Glucose
Glucose-6-phosphatase -
Insulin
Glucagon
Glucose + + +
Glucose-6-phosphate +

20. Fructose 1,6-bisphosphate
Glycolysis Gluconeogenesis
Phosphofructokinase
Fructose 6-phosphate
Fructose-1,6-
bisphosphatase -
ATP
Citrate H+
Fructose 2,6-bisphosphate
AMP +
ATP
Fructose 2,6-bisphosphate
AMP - -
Fructose 1,6-bisphosphate + - + -

21. Glycolysis Gluconeogenesis
Pyruvate kinase
PEPCK
Phosphoenolpyruate -
Glucagon
ATP
Alanine
Fructose 1,6-bisphosphate
Glucose +
Glucagon
Insulin
Acetyl-CoA - -
Pyruvate carboxylase - + + +
Pyruvate

22. Allosteric regulator of glycolysis and gluconeogenesis
Fructose 2,6-bisphosphate
Phosphofructokinase 2
Fructose 6-phosphate + ATP Fructose 2,6-bisphosphate + ADP
Fructose bisphosphatase 2
Fructose 2,6-bisphosphate + H2O Fructose 6-phosphate + Pi

23. G-6-P
F-6-P
PFK 2
FBPase 2
F-2,6-P2 + -
PFK 1
FBPase 1 F
-1,6-P 2 + PK
The PFK2 and FBPase 2 activities are located in a single protein:
The bifunctional enzyme

24. Fasted state:
High Glucagon -> High cAMP -> activation of PKA -> phosphorylation of bifunctional enzyme -> inhibition of PFK2, activation of FBPase2 -> decrease in fructose 2,6-bisphosphate -> no stimulation of glycolysis, no inhibition of gluconeogenesis -> Gluconeogenesis prevails!
Fed state:
Low Glucagon -> No/Low cAMP -> no activation of PKA -> dephosphorylation of bifunctional enzyme prevails -> activation of PFK2, inhibition of FBPase2 -> increase in fructose 2,6-bisphosphate -> stimulation of glycolysis, inhibition of gluconeogenesis -> Glycolysis prevails!

25. The Cori cycle Blood- stream LIVER MUSCLE Glucose Glucose
Lactate
Glucose
Lactate
Gluconeogenesis
Glycolysis