Gluconeogenesis
Synthesis of "new glucose" from common
metabolites
•Humans use ~160 g of glucose per day
•75% is used by the brain
•Body fluids contain only 20 g of glucose
•Glycogen stores yield 180-200 g of glucose
•So the body must be able to make its own glucose
• 90% of gluconeogenesis occurs in the liver and
kidneys
Figure 18.1 The Glycolysis Pathway
Figure 18.1 The Glycolysis Pathway
Why is gluconeogenesis not just the
reverse of glycolysis?
The reverse of glycolysis is
2 Pyruvate + 2ATP + 2 NADH + 2H+ + 2H20 a
glucose +2ADP +2Pi + 2 NAD + (DG = +74 kJ/mol)
This is thermodynamically unfavorable, so energetically unfavorable steps
in the reverse glyolysis reaction are replaced and energy is added in the
form of GTP and ATP to give:
The actual equation for gluconeogenesis of
a
2Pyruvate + 4ATP +2GTP+ 2NADH + 2H+ + 6H20
glucose +4ADP +2GDP +6Pi + 2 NAD + (DG = -38 kJ/mol)
Notice the extra ATPs and GTPs drive the process
Glycolysis vs Gluconeogenesis
Glycolosis
Gluconeogenesis
Glucose (6C) to 2 pyruvates (3C)
Creates energy 2ATP
Reduces 2 NAD+ to 2 NADH
Active when energy in cell low
10 steps from glucose to pyruvate
Pyruvate to AcCoA before Krebs
2 pyruvates (3C) to Glucose (6C)
Consumes energy 4ATP+2GTP
Oxidizes 2NADH to 2 NAD+
Active when energy in cell high
11 steps from pyruvate to glucose
AcCoA isn’t used in gluconeogenesis
Gluconeogenesis uses 7 of the 10 enzymatic reactions
of glycolysis but in the reverse direction. The 3 not used
are the ones requiring ATP in glycolysis.
First Reaction of Gluconeogenesis
- recall that pyruvate is the final product of glycolysis.
The pyruvate carboxylase reaction.
(Simplified)
Biotin is an essential cofactor in
most carboxylation reactions.
It is an essential vitamin in the
human diet, but deficiencies are
rare.
Avidin, a protein found in egg
white binds tightly to biotin and
excessive consumption of raw egg
white can lead to biotin deficiency.
ATP
Carbonyl phosphate
oxaloacetate
Pyruvate is converted to oxaloacetate
in the mitochondria
Oxaloacetate cannot be transported
directly across the mitochondrial
membrane so it is converted to
malate, then transported, then
oxidized back to oxaloacetate.
The PEP carboxykinase reaction.
Nucleotide diphosphate kinases
Both glycolysis and Oxidative phosphorylation produce
ATP with its high energy phoshoanhydride bonds: How
does GTP get made from GDP?
Directly from a single step in the Krebs cycle AND from
the following reaction
GDP + ATP → GTP + ADP
This is carried out in the cell by an enzyme called
Nucleotide diphosphate kinase which carries out the
general reaction
NDP + ATP → NTP + ADP (where N is T, G, or C)
Enolase Reaction
glycolysis
gluconeogenesis
Fig. 18-26, p. 595
The Phosphoglycerate Mutase Reaction
glycolysis
gluconeogenesis
Fig. 18-23, p. 594
Isomerase: An enzyme that catalyzes the
transformation of compounds into their positional
isomers. In the case of sugars this usually
involves the interconversion of an aldose into a
ketose, or vice versa.
Kinase: An enzyme that catalyzes the
phosphorylation (or dephosphorylation) of a
molecule using ATP (or ADP).
Mutase: An enzyme that catalyzes the
transposition of functional groups, such as
phosphates, sulfates, etc.
glycolysis
gluconeogenesis
Phospoglycerate kinase
Fig. 18-20, p. 593
The glyceraldehyde-3-phosphate dehydrogenase reaction
glycolysis
gluconeogenesis
glycolysis
gluconeogenesis
Triose phosphate isomerase
Fig. 18-14, p. 589
Aldolase
4th reaction of glycolysis (7th reaction of gluconeogenesis).
Reversible reaction also used in gluconeogenesis.
An aldol cleavage reaction (the reverse of an aldol condensation).
glycolysis
gluconeogenesis
Fig. 18-4, p. 584
Glucose-6-phosphatase
-enzyme unique to liver and kidney allowing them to supply glucose
to other tissues. Found in ER
The Cori Cycle
Regulation of Gluconeogenesis
Glucose-6-phosphatase is subject to substrate level
control.
- at higher G6P concentrations reaction rate increases
- recall, this happens in the liver. Other tissues do not
hydrolyze their G6P, thereby trapping it in the cells.
Glycolysis and gluconeogenesis are reciprocally regulated.
- regulatory molecules that inhibit gluconeogenesis often
activate glycolysis, and vise versa.
A potent allosteric regulatory molecule.
- activates phosphofructokinase.
- inhibits fructose-1,6-bisphosphatase.
- its synthesis and degradation are catalyzed by the same
bifunctional enzyme.
Fructose-2,6-bisphosphate activates
glycolysis and inhibits gluconeogenesis, so
its level is very important.
F2,6 BP
PFK-1
F2,6 BP
ATP
Pi
ADP
PFK-2
F2,6 BP
6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase
6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase
Low glucose
High glucagon
P
Increased
phosphorylation
Phosphorylation of the enzyme results in the inactivation of the
phosphofructokinase-2 activity and activation of the fructose-2,6bisphosphatase activity. This results in a down regulation of
glycolysis and increased gluconeogenesis.
Substrates for gluconeogenesis:
Pyruvate
Lactate
TCA cycle intermediates
Most amino acids
Not substrates for gluconeogenesis:
Acetyl-CoA
Fatty acids
Lysine
Leucine
Plants and bacteria can make glucose from acetate.