Regulation of Glycolysis/Gluconeogenesis

1
Regulation of Glycolysis/Gluconeogenesis Citric
Acid Cycle / TCA cycle / Krebs cycle Electron
Transport Chain (Oxidative Phosphorylation)
2
3 major points of regulation of glycolysis
Hexokinase Phosphofructokinase-1 Pyruvate kinase
-32.9
-24.5
Each regulatory reaction is held far from
equilibrium (large DG) Each regulatory step
in glycolysis is coordinately regulated
with gluconeogenesis Substrate Cycle


-26.4
3
Each regulatory step in glycolysis is
coordinately regulated with gluconeogenesis
Futile/Substrate
cycle
Pi
ATP
ADP
H2O
Glycolysis
Gluconeogenesis
Sum ATP H20 ? ADP Pi
? If allowed to occur at the same time
utilization of ATP without useful metabolic
work being done ? Cycles provide important
means of regulation

4
Hexokinase
Muscle High affinity for glucose Usually
saturated and working at maximal
rate Inhibited by glucose 6-phosphate (product
inhibition) Liver much lower affinity
(higher Km) for glucose. Allows liver to
process high levels of glucose Not inhibited by
glucose 6-phosphate
Muscle
Liver
5
Coordinate Regulation of PFK-1 and FBPase-1
(regulation by energy state of cell)
Commitment step of glucose into glycolysis
PFK-1 inhibited by signals of adequate energy
supplies ATP Citrate Activated by signals of
low energy supplies ADP AMP
FBPase-1 Inhibited by signals of low energy
supplies AMP
6
Hormonal regulation of glycolysis and
gluconeogenesis mediated by fructose 2,6
bisphosphate
F26BP activates PFK-1 (binds to allosteric site
and increases its affinity for its substrate
(F6-phosphate)
F26BP inactivates FBPase 1 (decreases its
affinity for its substrate (F1,6 BP))
7
Where does F2,6 Bisphosphate come from? Hormonal
regulation of PFK-2 / FBPase-1
Formation of F26BP
Breakdown of F26BP
1 bifunctional NZ with 2 separate activities
regulated by insulin and glucagon Regulation
of NZ is by phosphorylation
Glucagon stimulates phosphorylation of
PFK-2/FBPase 2 -Activation of FBPase-2 activity
(phosphatase) -Reduction of F26BP -
Glycolysis Gluconeogenesis
?
?
Insulin stimulates dephosphorylation of
PFK-2/FBPase 2 -Activation of PFK2 activity
(Kinase) -Increase in F26BP - Glycolysis
Gluconeogenesis
?
?
8
Enzymes of glycogen metabolism are regulated by
allosteric and hormonal mechanisms
Allosteric Regulation
Glycogen Phosphorylase
Glycogen Synthase
Activation G6P AMP Inhibition
ADP, Pi ATP, G6P, Glucose
Signals of high energy supply (G6P, ATP)
stimulate glycogen synthesis and inhibit glycogen
breakdown. Signals of low energy supply (ADP,
AMP) stimulate glycogen breakdown and inhibit
glycogen synthesis
9
Hormonal Regulation of glycogen synthesis and
breakdown Covalent Modification
Glycogen breakdown
Inhibition of Glycogen breakdown
Glucagon
Activation of phosphoprotein phosphatase -1 DeP
of Glycogen phosphorylase Phosphorylase kinase
Activation of PKA (via cAMP)
(IA)
P of phosphorylase kinase P of Glycogen
phosphorylase And P of Glycogen synthase
(A)
(IA)
Stimulation of glyc breakdown Inhibition of
glyc synthesis
(A)
(IA)
10
Fed State - Insulin

• Glucose enters hepatocytes through transporter
• 2) Synthesis of glycolytic enzymes

3) inactivation of GSK3 and activation of PP1
Activate glycogen synthase and inactivate
glycogen
phosphorylase ?Glycolysis
?Glycogen synthase ?Glycogen
breakdown
Fasting State - glucagon
Activation of PKA
through cAMP 1) Activates phosphorylase
kinase ? ?glycogen
phosphorylase
2) Inactivates glycogen synthase 3)
Phosphorylates PFK-2/FBPase-2 ? in F2,6BP
4) Inactivates pyruvate kinase
?Glycolysis ?Glycogen synthase
?Glycogen breakdown
11
Overview of Steps
Glycolysis
Glucose Pyruvate
Cytoplasm
Pyruvate
Transition/Prep Phase
Mitochondria
Acetyl - CoA
NADH
FADH2
Citric Acid Cycle
C1
C3
ATP synthase
C2
C4
ATP
Electron Transport
12
Citric Acid Cycle
2 C acetyl group combines with 4 C oxaloacetate
to yield 6 C citrate
1-
1 Glucose
2 pyruvates
2- Carbons lost in pathway as C02 -These
Cs are not from acetyl-coA 3- Oxaloacetate
consumed in the 1st step is regenerated in
the last. 4- Energy produced is transferred as
energy-rich electrons to NAD to yield
NADH or to FAD to yield FADH2 5- 2 rounds
of Cycle yields 4 CO2, 6 NADH, 2 FADH2, and
2 ATP 6- CAC is amphibolic has a role in
oxidation of carbohydrates and provides
precursors for other pathways (ex AA
metabolism)
Pyruvate Dehydrogenase
2 Acetyl-CoA
Citrate (C6)
Oxaloacetate (C4)
CoASH
Isocitrate (C6)
C02
NADH
NADH
Malate
a-ketoglutarate (C5)
CoASH
Fumarate (C4)
C02
FADH2
NADH
Succinate (C4)
CoASH
Succinyl co-A (C4)
GTP / ATP
Substrate level phosphorylation
13
Regulation of Citric Acid Cycle
A- Substrate Availability (oxaloacetate acety
l-CoA)
B- Product Inhibition
1- PD
C- Competitive Feedback Inhibition
NADH Citrate
Regulated enzymes of CAC
4-CS
1- Pyruvate Dehydrogenase (irreversible)
Product/Feedback inhibition
Covalent Modification (Phosphorylation

inactive)
2- ID

2- Isocitrate Dehydrogenase

Product/ Feedback Inhibition

3- a-Ketoglutarate Dehydrogenase

3-KD

4- Citrate synthase

All function far from equilibrium (- ?G)
NADH and Acetyl CoA compete with NAD and CoA
for binding sites competetive feedback
inhibition
14
Electron Transport Chain
15
2H
4H
4H
Cyt. C
Intermembrane space
Complex I
Membrane Soluble Coenzyme Q
Cyts Cu
FMN FeS
Cyts FeS
Complex II
Complex IV
Complex III
FAD FeS
Matrix
NADH
NAD
½ O2 2H
FAD
FADH2
Succinate
H20
Fumarate
1- Electrons from NADH and FADH2 are passed
through redox centers of 4 membrane
bound complexes and 2 membrane soluble electron
shuttles 2-During electron transfer, protons
are translocated from the matrix into the
intermembrane space (Complex 1, Complex III, and
Complex IV) 3- Final reduction is of molecular
O2 4- Electrons from NADH yield 2.5 ATP
Electrons from FADH2 yield 1.5 ATP
16
Proton Motive Force - Chemiosmotic theory
Storage of energy as a proton/voltage gradient
across a membrane
1- In ETC, the transport of H from low H to
high H requires energy (ENDERGONIC) 2-
Discharge of proton is EXERGONIC Free energy
of discharge of proton gradient is harnessed by
ATP synthase
Outer membrane
Intermembrane space
Low pH
H
H
H
Fo


lll
lV
V
l
Inner membrane
---------
---------
H
ATP
High pH
F1
Oxidative phosphorylation
17
Electron transport and ATP synthesis are
coupled ATP synthesis requires discharge of
proton gradient Proton gradient cannot be
discharged without synthesis of ATP Proton
gradient is established by electron transporting
complexes.
No drug
uncoupler added
Inhibitor added
DNP
DNP
DNP
ATP Synthesis
02 Consumption
Time
Time
Time
Uncoupler of ETC Destroys proton gradient No
stored energy for ATP DOES NOT stop electron
flow Oxygen still being consumed
Electrons flowing through ETC O2 is being
consumed ATP is being synthesized
Inhibitor of ETC Stops flow of electrons No
proton pumping No proton motive force No
longer consuming O2 No longer making ATP
18
(No Transcript)