Biochemistry I, Spring Term
Lecture 30
April 5, 2013
Lecture 30: Gluconeogenesis, Pathway Regulation
Gluconeogenesis:
Regulation:
1. Location: cytosol
1. General Properties & Mechanisms
2. Input: pyruvate
2. Local pathway sensing.
3. Output: glucose
3. Inter-pathway communication.
4. Energetics: 6 ATP, 2 NADH required
4. Energy Sensing.
5. Key Reg. Step: F 1-6 phosphatase.
Gluconeogenesis: The formation of glucose is essential for the maintenance of constant blood glucose levels.
The liver, and to a lesser extent the kidneys, are the only organs that carry out this process. All of our other
tissues and organs (especially the brain) require this newly-synthesized glucose during periods of fasting, i.e.
between meals and during sleep. This process is particularly important during strenuous exercise, where the
lactic acid produced during anaerobic metabolism is returned to the liver and converted back to glucose.
Steps in the forward
Gibbs Energy:
pathway that have large
Glucose
negative Gibbs energy are
Pyr
usually accomplished in the
reverse pathway by
PEP
G-6-P F-6-P
F-1,6-P
performing the reaction in a
F-6-P G-6-P
different way with a different
F-1,6-P
PEP
enzyme. The allows the
Gluconeogenesis
Glucose
Glycolysis
reverse pathway to operate
Pyr
with a negative Gibbs energy
at each step. The use of different
CH OH
O
enzymes in each pathway also provides a
OH
OH
OH
mechanism for coordinated regulation.
PO [glucose-6OH
Glucose
There are three such steps in glycolysis /
phosphatase]
ATP
HO
gluconeogenesis:
[hexokinase]
O P-O-CH
ADP
O
i) Glucose  Glucose-6-P
OH
Glucose-6-P
OH
CH
-OPO
CH
OH
ii) Fructose-6-P  Fructose 1,6 P
OH
O
OH
HO
Fructose-6-P
iii) PEP  Pyruvate
OH
2
4
2
3
2
These energetically favorable reactions in
glycolysis are reversed in
gluconeogenesis by:
iii) Use of ATP and GTP (=ATP) to
convert Pyr to PEP.
ii) & i) Spontaneous hydrolysis of the
phosphate from the sugar.
General properties of Catabolic and
Anabolic pathways:
Catabolic
Anabolic
Energy Produce
Consume
Input
Complex→ Simple→
→out
Simple
Complex
*
Redox Oxidizing : Reducing:
NADH/
NADH/
FADH2
FADH2
produced
required
(electron
(electron
donors)
acceptors)
*
lose electrons → oxidation
1
3
[phosphofructokinase] CH2-OPO3
2
2
ATP
OH
O
CH2-O-PO3
PO4
ADP
H2O
Fructose-1-6-P
[fructose-1,6bisphosphatase]
HO
OH
[aldolase]
OH
glyceraldehyde-3-P dihydroxyacetone phos.
NAD+
NAD+
[glyceraldehyde-3-P
dehydrogenase]
NADH
NADH
1,3 bisphosphoglycerate
ADP
[phosphoglycerate kinase]
ATP
ATP
3-phosphoglycerate
ADP
[phosphoglycerate mutase]
2-phosphoglycerate
H2O
[enolase]
O
H2O
O H
H
P O
O
OH
O
H
H
P
O
O phosphoenol pyruvate O
O
O
ADP
GTP
ATP
OH
O
[pyruvate kinase]
2 X Pyruvate
H3 C
HO
O
O
ATP
OH
HCO3
[PEP
carboxylase]
CO2
O
C
H2
O
oxaloacetate
OH
[pyruvate
carboxylase]
Biochemistry I, Spring Term
Lecture 30
April 5, 2013
Regulation of Biochemical Pathways:
General Properties of Regulation:
 Step below a convergence point is usually regulated
 Step that has a high energy change (Go, G) is
usually regulated (e.g. PFK).
 Opposing pathways are coordinately regulated,
usually at the same step. (e.g. glucose synthesis
/degradation, glycogen synthesis/degradation).
Mechanisms of Regulation (from slow →fast)
 Change in levels of enzymes by regulation of the
synthesis/degradation.

Change in the activity of enzymes by covalent
modification ( phosphorylation) of the enzyme*.
*Indirectly regulates glycolysis/gluconeogenesis.
lactose
Glu
[hexose ATP
kinase]
ADP
ATP
G-6P
F6P
ADP
F16P
ATP(2)
[Pyruvate
kinase]
2

Feedback regulation (FB) - change in the activity
of enzymes due an allosteric inhibition or
activation by a chemical that is near the end of the
pathway (e.g. ATP & PFK), or in another pathway
(e.g. citrate & PFK)

Product inhibition (PI). e.g. hexose kinase via G-6P

Substrate availability (all enzymes, KM≈[S]in vivo).
glycogen
ATP(2)
PEP
Pyr
Biochemistry I, Spring Term
Lecture 30
ATP Balance in Cells:




April 5, 2013
ATP
ATP is hydrolyzed to produce energy for
cellular activities, ADP + inorganic phosphate
(Pi)
ATP is re-synthesized from ADP in oxidative
phosphorylation.
ADP is also converted to ATP by the enzyme
adenylate kinase, producing AMP as well.
ATP levels are kept relatively constant; however
AMP and ADP levels can change dramatically.
ADP + Pi
ATP + AMP
Adenylate
kinase
2 ADP
ATP
Conc
ADP, AMP
Exercise
Regulation of Glycolysis/Gluconeogenesis by Energy Sensing:
A cell has HIGH energy reserves when:
ATP
Glycolysis (GlucoseATP)
Gluconeogenesis (Glucose)
Glycolysis (GlucoseATP)
Gluconeogenesis (Glucose)
AMP, ADP
A cell has LOW energy reserves when:
ATP
AMP, ADP
Allosteric control of – PFK:
T-state stabilized by R stabilized by
ATP
ADP, AMP
Citrate (TCA)
F2,6P (Hormone)
Glucose
O
P O
O
O
H2
C
fructose-6
phosphate
OH
H2C
O
HO
Glycolysis
OH
OH
AMP
ADP
[PFK-1]
[Fructose-bis
phosphatase-1]
ATP
Citrate
F 2,6 P
Allosteric control of Fructose bis-phosphatase-1:
T-state stabilized by R stabilized by
nothing
AMP
F2,6P (hormone)
O
O
O
P O
O
H2
C
H2C
O
HO
P
O O
OH
OH
Pyruvate
3
O
Gluconeogenesis
fructose 1,6
bis phosphate