Regulation of Glycogen Metabolism

1
Regulation of Glycogen Metabolism

• Protein Kinases
• Protein Phosphatases
• cAMP
• G proteins
• Calcitonin
• Insulin, glucagon, and epinephrine

2
Biochemical Definitions
1. Equilibrium (Kinetics) When the rate of the
forward reaction matches the rate of the reverse
reaction. Does not specify quantity at
equilibrium.
2. Equilibrium constant (Thermodynamics) The
quantity B/A at equilibrium. Does not specify
rate.
3. Flux Net carbon flow in one direction
4. Steady-state A dynamic condition that allows
flux without changing the concentration of
components in the pathway.
3
Connecting Kinetics with Thermodynamics
Chemistry
?Go ?RT ln Keq
Biochemistry
4
Practically Speaking..
1. A negative ?Go means P gt S at
equilbrium.
2 -1.8
10 -5.9
100 -11.9
1,000 -17.8
10,000 -23.7
100,000 -29.7
5
Steady-State vs Equilibrium Reactions
Rule An equilibrium reaction occurs in a closed
system whereby two components, a reactant and a
product, achieve a constancy of concentration
based on chemical potentials
Rule A steady-state reaction occurs in an open
system whereby two or more components achieve a
constancy of concentration based on similar rates
of components entering and leaving the system
See Strategies p 163-167
Z
Y
6
Representing a Steady-State
A
A
A
7
Basics of Metabolic Homeostasis
Rule A shift away from a dynamic steady-state
evokes factors to restore the steady-state.
Rule Restoring steady-state requires modulating
the activity of a rate-controlling enzyme(s) in
the pathway
Enzyme activity can be modulated by
1. Covalent modification
2. Changes in pathway S or enzyme cofactors
3. Allosterism (Vmax or Km)
4. Hormonal intervention
5. Enzyme turnover
8
Regulation of Carbon Flux

• Net carbon flux through an individual step in a
  pathway is defined as the difference between the
  forward and reverse reaction velocities
• J VF - VR (these are rates of change)
• At equilibrium VF VR J 0 i.e., there is
  no net flux even though VF and VR could be large
• When VF gtgt VR, J VF
• At steady state, J k (constant)
• At steady state, J depends on the rate
  determining (slowest) step in the pathway

Textbook p591
9
Meaning of Flux
Glucose
J 0
Rate-controlling Step
VF gt VR
Lactate
10
Flux varies with equilibrium position
A

• If A 100mM B 10mM, an increase in A
  by 10 mM
• would increase flux would by 10. (from
  101 to 111)

• If A 20mM B 10mM, an increase in A by
  10 mM
• would increase flux by 33 . (21 to 31)

• If A 10mM B 10mM, an increase in A by
  10 mM
• would increase flux by 100 (11 to 21).

11
Rule Regulators with the lowest steady-state
concentration have the greatest impact on
regulation
AMP (0.1 mM)
ATP (5 mM)
ADP (1 mM)
Adenylate Kinase
2ADP AMP ATP
ATP X
ATP XP ADP
5
0.5
4.5
0.5
0.5
ATP AMP
2ADP
0.5
0.5
0.5
Final Talley
Before
After
ATP
5 mM
5 mM
No change
ADP
1 mM
1 mM
No change
AMP
0.1 mM
0.6 mM
6 times
12
Why are there 2 enzymes in glycogen metabolism?
How can glycogen synthesis and degradation be
tuned to the needs of the cell? Only glycogen or
glucose 1-PO4 concentration can affect VF or VR
Because separate enzymes control synthesis and
degradation, VF and VR can vary depending on
the enzyme that controls the direction. This
opens the way to allosteric and covalent control
of glycogen
Stimulating or inhibiting the enzyme cannot
control direction of flux
13
cAMP as a Regulator of Glycogen
Pkinase
Protein Kinase targets of cAPK
cAMP Protein kinase (cAPK)
Adenyl cyclase
Glucagon Epinephrine
Textbook p586
14
cAMP Protein kinase (cAPK)
Now its ready to phosphorylate phosphorylase b
Example would be phosphorylase b kinase
15
Inactivation of Glycogen Synthase
Inactive
Casein kinase II (primer)
Glycogen synthase kinase 3 (GSK3)
16
Phosphoprotein Phosphatase-1 (PP1)
Remember, this is the stripper enzyme. It
takes off phosphate groups on proteins
17
Insulin stimulates glycogen synthesis in liver
and muscle by blocking the action of GSK3 and
activating PP1
Insulin
3 ATP
3 ATP
GSK3
CKII
ATP
Active
Inactive
3Pi
PP1
Insulin
Glucose
Text p586
Glucagon epinephrine
Glucose- 6-phosphate
18
Phosphoprotein Phosphatase-1 (Muscle)
Insulin
(site 1 phosphorylation)
Insulin-stimulated protein kinase
more active
Glycogen synthesis stimulated, breakdown blocked
K1
Epinephrine, glucagon
(site 2 phosphorylation)
K2
Glycogen Breakdown stimulated, synthesis
blocked
cAPK
Less active
K2
(site 1 and 2 phosphorylation)
inactive
Text p588
19
Insulin regulation of GSK3
Shut down of GSK3 by PKB
Text p 587
20
Summary of Catabolic Hormonal Effects on Glycogen

• Glucagon and Epinephrine stimulate synthesis of
  cAMP
• cAMP activates phosphorylase b kinase that
  converts phosphorylase b to phosphorylase a
• Phosphorylase a breaks down glycogen at an
  accelerated rate
• cAMP also inactivates PP1, prolonging the action
  of phosphorylase a

21
Summary of Anabolic Effects on Glycogen

• Insulin activates glycogen synthase by
  stimulating phosphorylation of GDK3
• GDK3 inactivates itself allowing phosphoprotein
  phosphatase (PP1) to remove phosphate and
  activate glycogen synthase
• PPI also removes phosphate from phosphorylase a
  thereby shutting off glycogen breakdown

22
See Tutorial on cAMP-dependent Protein Kinases
available on the web
23
Liver
Text p446-447
R State
T State
Glucose
Phosphorylase a
High Glucose
Low Glucose
Phosphorylase b

Binds weakly to b form
Active
24
Summary of Phosphoprotein Phosphatase-1
(Liver)
1. The Phosphatase binds to phosphorylase a (T
or R form)
(It does not bind to glycogen or a G protein)
2. The T form has a serine exposed to allow
hydrolysis of -PO4
3. The R form of phosphorylase a -PO4 cannot be
hydrolyzed
4. Glucose converts R to T form which causes
hydrolysis of -PO4 and dissociate the
Phosphatase.
(glucose is an allosteric effector of
phosphorylase in liver)
5. The liberated Phosphatase is free to activate
glycogen synthase, thereby stimulating glycogen
synthesis