Gluconeogenesis is the formation of glucose from non-carbohydrate sources and occurs mainly in the liver and to a limited extent in the kidney and small intestine. It utilizes many of the same enzymes as glycolysis but must bypass three essentially irreversible steps. This is accomplished through hydrolysis reactions catalyzed by glucose-6-phosphatase and fructose-1,6-bisphosphatase. The bypass of pyruvate kinase involves two enzymes, pyruvate carboxylase and PEP carboxykinase, which convert pyruvate to oxaloacetate and then to PEP using ATP and GTP as energy sources.

1. Gluconeogenesis
Formation of glucose from
non-carbohydrate source

2. • occurs mainly in liver and
• to a limited extent in kidney & small intestine
• Synthesis of glucose from pyruvate utilizes many of the
same enzymes as Glycolysis
• Three Glycolysis reactions have such a large negative ∆G
that they are essentially irreversible
 Hexokinase (or Glucokinase)
 Phosphofructokinase
 Pyruvate Kinase
These steps must be bypassed in Gluconeogenesis.
The first two of the bypass reactions involve simple
hydrolysis reactions

3. Glucose-6-phosphatase
6 CH OPO 2− CH2OH
2 3
5 O O
H H H H
H H2O H
4
OH H 1
OH H + Pi
OH OH OH OH
3 2
H OH H OH
glucose-6-phosphate glucose
Glucose-6-Phosphatase (Gluconeogenesis) catalyzes:
glucose-6-phosphate + H2O  glucose + Pi
This enzyme is embedded in the endoplasmic
reticulum (ER) membrane in liver cells

4. Phosphofructokinase →
6 CH OPO 2− 1CH2OH 6 CH OPO 2− 1CH2OPO32−
2 3 2 3
O ATP ADP O
5 H HO 2 5 H HO 2
H 4 3 OH H 4 3 OH
Pi H2O
OH H OH H
fructose-6-phosphate fructose-1,6-bisphosphate
←
Fructose-1,6-bisphosphatase
Fructose-1,6-biosphosphatase
Fructose-1,6-bisphosphatase (Gluconeogenesis) catalyzes:
fructose-1,6-bisP + H2O  fructose-6-P + Pi

5. Bypass of Pyruvate Kinase:
Pyruvate Kinase (last step of Glycolysis) catalyzes:
phosphoenolpyruvate + ADP  pyruvate + ATP
For bypass of the Pyruvate Kinase reaction, 2 ~P bonds is
required.

6. Pyruvate Carboxylase PEP Carboxykinase
O O−
O− C O−
O O
C ATP ADP + Pi C O GTP GDP C
C O CH2 C OPO32−
HCO3− C CO2
CH3 CH2
O O−
pyruvate oxaloacetate PEP
Bypass of Pyruvate Kinase (2 enzymes):
Pyruvate Carboxylase (Gluconeogenesis) catalyzes:
pyruvate + HCO3− + ATP  oxaloacetate + ADP + Pi
PEP Carboxykinase (Gluconeogenesis) catalyzes:
oxaloacetate + GTP  PEP + GDP + CO2