Takusagawa’s Note©
1
Chapter-15
Chapter-15: Glycogen Metabolism
-
Glycogen is the animal storage form of branched poly(glucose).
-
Processes of glycogen breakdown is:
(glucose)n → glucose-1-phosphate + (glucose)n-1
-
Processes of glycogen synthesis is:
(glucose)n-1 + UDP-glucose → (glucose)n
-
Regulation of glycogen breakdown and synthesis:
is controlled by two key enzyme (glycogen phosphorylase and glycogen synthase) activities
which are activated/inactivated by allosteric regulation and phosphorylation /
dephosphorylation.
1. Glycogen breakdown
Glycogen structure
- Glucose molecules in the main chains are connected by α(1→4) glycosidic bonds.
- The branches are attached by α(1→6) glycosidic linkages.
- A glucose unit on the non-reducing ends is cleaved or attached one by one.
CH2OH
CH2OH
O
CH2OH
O
O
OH
OH
OH
O
HO
O
OH
CH2OH
CH2OH
O
CH2
O
OH
CH2OH
O
OH
O
OH
CH2OH
O
OH
HO
Reducing end
HO O α(1Æ6) linkage
OH
O
OH
O
OH
O
OH
OH
Non-reducing
ends
Branch
point
O
OH
OH
OH
α(1Æ4) linkage
Different pathways of glycogen breakdown
- In muscle: Glycogen → glucose-6-phosphate (G6P) → glycolysis
- In liver: Glycogen → G6P → glucose → bloodstream → various cells → glycolysis
- Because the muscle cells mainly consume glucose molecules whereas the liver cells mainly
store the glucose molecules.
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Takusagawa’s Note©
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Chapter-15
Glycogen breakdown requires three enzymes
1. Glycogen phosphorylase (simply call it phosphorylase)
(Glycogen)n + Pi ↔ (glycogen)n-1 + G1P
(n residues)
(n-1 residues)
This enzyme releases a glucose unit one by one until it reaches ~ five units (limit branch)
from a branch point.
- The enzyme has a crevice where 4-5 units of a left-handed helical glycogen can fit, but it is
too narrow to fit a branch point.
O
-
-
Enzyme-catalyzed modification/demodification process yields two forms of phosphorylase
Phosphorylase a --- E-O-PO32- (attachment at Ser-14).
Phosphorylase b --- No phosphate attachment.
Both forms of enzyme are allosterically activated or inactivated.
Allosteric inhibitors: ATP, G6P and glucose.
Allosteric activator: AMP.
Pyridoxal phosphate (PLP) is an essential cofactor for phosphorylase
- PLP covalently bound to phosphorylase via a Schiff base to Lys-679.
- PLP’s phosphate group probably functions as an acid-base catalyst.
Phosphorylase
(CH2)4
-
O
H
H
C
O P O C
-
O
H
N
O
-
O
OH
CH
O P O C
-
+
N
H
O
CH3
H
OH
+
N
CH3
H
H
Pyridoxal phosphate (PLP)
PLP bound to Lys 679
via Shiff base
2
Takusagawa’s Note©
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Chapter-15
Reaction mechanism
1. Formation of an [E-PL · Pi · glycogen] ternary complex.
2. Cleavage of the glycosidic bond by an acid catalysis (the Pi donates H to the bridge O
because the Pi receives H from PL), and formation of oxonium ion intermediate (half-chair
conformation).
3. Reaction of Pi with the oxonium ion to form G1P.
Half-chair oxonium ion
CH2OH
HO
O
HO
OH
H
CH2OH
+
O
HO
HO
O
CH2OH
H
HO
OH
-
O
Acid catalysis
H
HO
O
O
O
P
-
O
H
O
O
PL
HO
O
OH O
HO
OH O
-
O
-
O
P
-
O
O
CH2OH
HO
H
O
P
O
(Glu)n-2
CH2OH
O
H
O
OH O
O
(Glu)n-2
P
-
O
O
PL
E
E
Glycogen + Pi + Enzyme
Ternary complex
3
H
-
O
O
O
O
P
-
O
O
PL
E
P
-
O
Takusagawa’s Note©
4
Chapter-15
2. Glycogen debranching enzyme
- Removes branches so that glycogen phosphorylase can complete reaction.
Reaction mechanism
1. Three glucose units of the branch are cleaved and reattached to the non-reducing end of the
main chain.
2. The branch point, the glucose attached to the main chain by the α(1-6) glycosidic bond is
hydrolyzed.
G1P
G1P
G1P
G1P
Limit branch
G1P
G1P
G1P
G1P
G1P
Free glucose
G1P
G1P
G1P
G1P
G1P
G1P
G
G1P
4
Takusagawa’s Note©
5
Chapter-15
3. Phosphoglucomutase
- G1P produced from the glycogen breakdown must be convert to G6P in order to enter
glycolysis or to produce glucose in liver.
- Phosphoglucomutase catalyzes the conversion of G1P to G6P.
- The Ser of the enzyme is phosphorylated.
Reaction mechanism
1. The O6 of G1P attacks the phosphoenzyme to form a dephosphoenzyme-G1,6P intermediate.
2. The Ser-OH group on the dephosphoenzyme attacks the phosphoryl group at C1 to
regenerate the phosphoenzyme, and the product G6P is released.
Enzyme
Ser
H2C O
2PO3
Enzyme
Enzyme
Ser
Ser
H2C O PO3
2-
CH2OH
2-
CH2OPO3
O
CH2OPO3
O
OH
HO
2-
H2C O H
O
OH
2-
OPO3
OH
G1P
HO
OH
2-
OPO3
OH
HO
G1,6P
OH
OH
G6P
2. GLYCOGEN SYNTHESIS
- Biosynthetic and degradative pathways of metabolism are almost always different.
Glycogen synthesis
Glucose
Glycogen
Glycogen breakdown
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Chapter-15
6
Takusagawa’s Note©
A. UDP-glucose formation by UDP-glucose pyrophosphorylase
- In the glycogen synthesis pathway, at first, the uridine diphosphate (UDP) is attached to
glucose.
- This reaction is catalyzed by UDP-glucose pyrophosphorylase.
Reaction mechanism
1. The phosphoryl oxygen of G1P attacks the α-phosphorus atom of UTP to form UDP-glucose
(ΔG°’ ≈ 0). Since the ΔG°’ ≈ 0, this reaction is not proceeded without some input energy.
2. The released PPi is rapidly hydrolyzed by inorganic pyrophosphatase (ΔG°’ < 0).
Since this hydrolysis reaction is exergonic, the first phosphorylation reaction is pulled.
ΔG°’ (kJ/mol)
G1P + UTP ↔ UDP-glucose + PPi
~0.0
H2O + PPi → 2Pi
-33.5
Overall G1P + UTP → UDP-glucose + 2Pi
-33.5
6
Chapter-15
7
Takusagawa’s Note©
B. Glycogen synthesis by glycogen synthase
Reaction mechanism
1. The glycosidic bond between glucose and UDP in UDP-glucose is hydrolyzed. The cleaved
glucose ion takes the oxonium ion intermediate (half-chair conformation), which is stabilized
by the enzyme.
2. The glucose unit of UDP-glucose is transferred to the C4-OH group on one of glycogen’s
non-reducing ends to form an α (1→4) glycosidic bond.
-
UDP is recycled by conversion to UTP with ATP.
UDP + ATP ↔ UTP + ADP
Thermodynamics
- ΔG of glycogen breakdown by glycogen phosphorylase is < 0 (ΔG ≈ -5 to -8 kJ/mol).
(glucose)n + Pi ⎯→ G1P + (glucose)n-1
- ΔG of glycogen synthesis by glycogen synthase is < 0 (ΔG ≈ -14 kJ/mol).
(glucose)n-1 + UDP-glucose ⎯→ (glucose)n
- Thus both reactions takes place spontaneous under the same physiological condition.
- However, glycogen synthesis uses one ATP hydrolysis per G1P for energy source.
1. G1P + UTP ⎯→ UDP-glucose + 2Pi
2. UDP-glucose + (glucose)n-1 ⎯→ (glucose)n + UDP
3. UDP + ATP ⎯→ UTP + ADP
_____________________________________________________________________________________
G1P + (glucose)n-1 + ATP ⎯→ (glucose)n + ADP + 2Pi
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Chapter-15
8
Takusagawa’s Note©
C. Glycogen Branching
- About 7 units of the non-reducing end of α-amylose chain are removed at α(1→4) linkage,
and reattached to the C6 of other α-amylose chain by α(1→6) linkage.
- This transfer is carried out by amylo-(1,4→1,6)-transglycosylase (branching enzyme).
Note:
- ΔG of hydrolysis of α(1→4) glycosidic bond is -15.5 kJ/mol.
- ΔG of hydrolysis of α(1→6) glycosidic bond is -7.1 kJ/mol.
- In the branching process, hydrolysis of the α(1→4) glycosidic bond (ΔG = -15.5 kJ/mol)
derives the formation of the α(1→6) glycosidic bond (ΔG = +7.1 kJ/mol).
1. α(1→4) glycosidic bond hydrolysis ΔG = -15.5 kJ/mol
2. α(1→6) glycosidic bond formation ΔG = +7.1 kJ/mol
ΔG = -8.4 kJ/mol
- Since the reverse reaction (debranching) is endergonic (ΔG > 0), the debranching pathway
must be different.
- In the debranching process, the breaking and reforming the α(1→4) glycosidic bond are
energetically canceling each other, and the α(1→6) bond hydrolysis (ΔG = -7.1 kJ/mol)
drives the debranching.
1. α(1→4) glycosidic bond hydrolysis ΔG = -15.5 kJ/mol
2. α(1→4) glycosidic bond formation ΔG = +15.5 kJ/mol
3. α(1→6) glycosidic bond hydrolysis ΔG = -7.1 kJ/mol
ΔG = -7.1 kJ/mol
8
Chapter-15
9
Takusagawa’s Note©
3. CONTROL OF GLYCOGEN METABOLISM
A. Allosteric control of glycogen phosphorylase and glycogen synthase
Glycogen phosphorylase
- has two forms (a and b), and each form is activated allosterically from T-form to R-form.
- Phosphorylase b is the non-phosphorylated enzyme and its R-form is less active, but its
response is fast since its activation signals (activators) are come from the inside of cell.
- Phosphorylase a is the Ser-14 phosphorylated enzyme and its R-form is the most active
enzyme, but its response is slow since its activation signals (effectors) are come from the
outside of cell.
- The enzyme is phosphorylated and dephosphorylated by phosphorylase kinase and
phosphoprotein phosphatase, respectively. The phosphorylation is called “covalent
modification”.
- Phosphorylase kinase and phosphoprotein phosphatase are enzyme modificator and
demodificator, respectively.
- Unmodified form (phosphorylase b) is mostly T-form, whereas modified form
(phosphorylase a) is mostly in the R-form.
Allosteric
regulators
Highly active
form
-
Glycogen synthase has the same forms of phosphorylase, i.e., glycogen synthase a and
b, and their T- and R-forms.
The allosteric activators and inhibitors of phosphorylase and synthase are listed below.
Enzyme
Activators
Inhibitors
Glycogen phosphorylase
AMP
ATP, G6P
Glycogen synthase
G6P
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Takusagawa’s Note©
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Chapter-15
Let us consider how these enzymes are activated or deactivated.
- At high demand of ATP, i.e., low [ATP], low [G6P] and high [AMP]: Glycogen
phosphorylase is stimulated and glycogen synthase is inhibited, so flux through this pathway
favors the glycogen breakdown.
- At high [ATP] and [G6P]: Glycogen synthesis is favored.
B. Covalent modification of enzymes by cyclic cascades: Effector “Signal” amplification
- As described above, two enzymes (phosphorylase kinase and phosphoprotein phosphatase)
are involved in the modification of phosphorylase.
- The phosphorylation is the most effective activation process of phosphorylase.
- Let us assume that one molecule of phosphorylase kinase can phosphorylate many
phosphorylase molecules (say 106 molecules). Then each phosphorylated phosphorylase
molecule catalyzes the glycogen breakdown by 106 times.
- Thus the one activation of the kinase molecule is translated into 1012 glycogen breakdown
reactions, i.e., production of 1012 glucose molecules.
- These processes are called enzyme cascade.
- There are several cycles of enzyme cascades. Monocycle enzyme cascade is shown below.
Monocycle cascade
Less
active F
+
e1
K1
F e1 More
active
ADP
ATP
P
OH
Less
active Eb
More
Ea active
H2O
Pi
Less
active R
-
+
R e2 More
active
e2
K2
F and R are kinase and phosphatase (hydrolase), respectively.
e1 and e2 are allosteric effectors of F and R, respectively, and those quantities are very little.
Activated F·e1 and R·e2 can catalyze the phosphorylation and dephosphorylation reactions in
many times.
Thus, the effector’s effects are amplified very much.
Thus, amplification potential of a “signal” of e1 or e2 is enormous.
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Takusagawa’s Note©
11
Chapter-15
Bicyclic enzyme cascade
Activation process:
1. Effector e1 activates F1 by forming a F1·e1 complex.
2. The activated F1·e1 phosphorylates F2b, and converts it to an active F2a.
3. The activated F2a phosphorylates Eb, and converts it to an active Ea.
4. The activated Ea catalyzes the reaction such as glycogen breakdown.
Deactivation process:
1. Effector e2 activates R1 by forming an R1·e2 complex.
2. The activated R1·e2 hydrolyzes (dephosphorylates) the active F2a, and converts it to an
inactive F2b.
3. Similarly effector e3 activates R2 by forming an R2·e3 complex.
4. The activated R2·e3 hydrolyzes (dephosphorylates) the active Ea, and converts it an inactive Eb.
F1
+
e1
K1
F1 e1
ADP
ATP
P
OH
F 2b
F 2a
ADP
ATP
H2O
Pi
P
OH
R1
+
e2
K2
R1 e2
Eb
Ea
H2O
Pi
R2 +
e3
K3
R2 e3
In the glycogen metabolism (bicycle enzyme cascade), the corresponding symbols are:
P = PO32-; e1 = cAMP; F1 = cAMP-dependent protein kinase; F2 = Phosphorylase kinase;
E = Glycogen phosphorylase; [R1·e2 and R2·e3 = Phosphoprotein phosphatase-1].
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Takusagawa’s Note©
12
Chapter-15
C.
-
Glycogen phosphorylase bicyclic cascade
Epinephrine binds its receptor on the cell surface. Its signal is transferred to a G-protein.
The activated G-protein (Gα) activates the adenylate cyclase (AC).
The activated AC catalyzes the cAMP formation from ATP.
cAMP (effector e1) activates cAMP-dependent protein kinase (cAPK).
-
Insulin activates an insulin-stimulated protein kinase.
The activated insulin-stimulated protein kinase activates phosphoprotein phosphatase-1 (PP1).
PP-1 dephosphorylates the phosphorylated proteins to inactivate them except for glycogen synthase
which is activated by PP-1.
-
ATP
R 2C2
cAMP-dependent
protein kinase
(inactive)
Adenylate cyclase
2C
cAMP-dependent
protein kinase
(active)
+ 4cAMP
ATP
ADP
H 2O
ATP
ADP
ATP
Ser 14-CH 2O-P
Glycogen
phosphorylase a
Glycogen
phosphorylase b
Pi
Phosphoprotein
phosphatase-1
(inactive)
H2O
P
Phosphoprotein
phosphatase
inhibitor-1 a
H 2O
Phosphoprotein
phosphatase
inhibitor-1 b
ATP
Glycogen-P
synthase b
Pi
Phosphoprotein
phosphatase-1
(active)
ADP
Glycogen
synthase a
H 2O
Pi
insulin-stimulated
protein kinase
Other
kinases
Ca2+
Ser 14-CH2OH
Insulin
+ R 2(cAMP)4
PP
(α β γ δ)4
Phosphorylase
kinase a
(α β γ δ)4
Phosphorylase
kinase b
Pi
Epinephrin
P
Phosphoprotein
phosphatase
inhibitor-1 a
ADP
Glycogen breakdown favor pathway
Glycogen synthesis favor pathway
12
Note: Phosphorylation
activates phosphorylase
but inactivates synthase
Takusagawa’s Note©
13
Chapter-15
Adenosine-3’,5’-cyclic monophosphate (cAMP)
- The primary intracellular signal e1 is adenosine-3’,5’-cyclic monophosphate (cAMP) in both
glycogen phosphorylase and glycogen synthase cascades.
NH2
N
N
H
PPi
H
ATP
Adenylate
cyclase
H
H
N
N
AMP
O
phosphodiesterase
C
O
H2O
O
P O
OH
O-
-
cAMP is absolutely required for the activity of cAMP-dependent protein kinase (cAPK).
cAPK phosphorylates specific Ser and/or Thr residues of numerous cellular proteins.
These protein have cAPK’s consensus recognition sequence, Arg-Arg-X-Ser/Thr-Y (X:
small residue; Y: Large hydrophobic residue).
cAPK is tetramer composed of two regulatory and two catalytic subunits, R2C2.
cAMP binds to the regulatory subunits → dissociate active catalytic monomers (2C).
Protein kinases play key roles in the signaling pathways by hormones, growth factors,
neurotransmitters and toxins.
Phosphorylase kinase
- is activated by Ca2+ (as low as 10-7 M) and by covalent modification.
- is (αβγδ)4.
- The γ subunit has full catalytic activity (ability to convert phosphorylase b to phosphorylase a).
- The α, β, and δ subunits are inhibitors of the catalytic reaction.
- The phosphorylation of α and β-subunits reduces the inhibitory activities of α and βsubunits.
- The δ subunit is calmodulin (CaM), which is a ubiquitous eukaryotic Ca2+-binding protein.
- When Ca2+ binds to CaM’s 4 Ca2+-binding sites, CaM undergoes an extensive
conformational change that activates phosphorylase kinase. Actually,
- Ca2+ binds to the Ca2+-binding domain → conformational change exposes a hydrophobic
patch → CaM binds to the CaM-binding domain of phosphorylase kinase γ subunit at the
hydrophobic patch section → binding of CaM activates phosphorylase kinase (also other
protein kinases).
P Hydrophobic
P
patch
Ca2+
δ
δ
α
α
δ
α
cAPK
γ
γ
γ
β
β
β
ATP ADP
Inactive
P
hydrophobic section
P
Inactive
13
Active
Chapter-15
14
Takusagawa’s Note©
X-ray and NMR studies on the structure of CaM indicate that CaM changes its structure
significantly when it binds to the specific oligopeptide of the γ-subunit of phosphorylase
kinase.
14
Chapter-15
15
Takusagawa’s Note©
Muscle contraction processes
Initial muscle contraction by nerve impulses → release Ca2+ from reservoir to cytosol → Ca2+
bind to CaM → activate phosphorylase kinase → phosphorylate (activate) glycogen
phosphorylase → glycogen breakdown → ATP synthesis → ATP is used for contraction of
muscle.
Phosphoprotein phosphatase-1 (PP-1)
- catalyzes hydrolytic dephosphorylation (remove the phosphate group by hydrolysis).
- is only active when it is bound to glycogen through its glycogen-binding G-subunit.
- has two phosphorylation sites.
1. Activation site (P1) by insulin-stimulated protein kinase.
2. Deactivation site (P2) by cAPK. When the P2 site is phosphorylated by cAPK, PP-1 is
released from G-subunit. The PP-1 itself is inactive.
Insulin and epinephrine have antagonistic effects on glycogen metabolism.
-
-
PP-1 is also inhibited by phosphoprotein phosphatase inhibitor 1 (protein).
- Phosphoprotein phosphatase inhibitor 1 is an active inhibitor protein when it is
phosphorylated by cAPK.
Thus, cAMP concentration controls not only the phosphorylation activity (stimulation) but
also dephosphorylation activity (suppression).
15
16
Chapter-15
Takusagawa’s Note©
D. Glycogen synthase
- is inactivated by the bicycle enzyme cascade.
1. cAMP activates cAMP-dependent protein kinase.
2. cAMP-dependent protein kinase phosphorylates phosphorylase kinase a.
3. Phosphorylase kinase a phosphorylates glycogen synthase a. The phosphorylation on
glycogen synthase inactivates it, glycogen synthase b is inactive phosphorylated form.
- is activated by phosphoprotein phosphatase-1 which dephosphorylates the glycogen synthase
(b → a).
E. Integration of glycogen metabolism control mechanisms
- The rates of glycogen breakdown and synthesis largely depend on the rate of the
phosphorylation and dephosphorylation reactions of the two bicyclic cascades.
- Phosphorylation and dephosphorylation are controlled by hormones.
Hormones trigger glycogen metabolism through the intermediacy of second messengers.
In liver: Hormone glucagon controls glycogen metabolism (stimulate glycogen breakdown).
- Glucagon is a 29 amino acid polypeptide.
In muscle and various tissues: Insulin and epinephrine (adrenaline) and norepinephrine
(noradrenaline) control glycogen metabolism.
- Insulin is a polypeptide hormone. Inactive
form, proinsulin is a single peptide of 84
amino acid residues, while the active mature
hormone is two chains (30 and 21 amino
acid residues) connected by two S-S bonds.
HO
OH
HO
H
C C
H
H
H
N H
X
X = CH3 Epinephrin
X=H
Norepinephrin
16
17
Chapter-15
-
Takusagawa’s Note©
Hormonal stimulation at their plasma membranes occurs through the mediation of
transmembrane protein, receptor (review Chapter 21b).
Hormone
Receptor
Cell membrane
Cytosol
-
-
Second messenger
When a hormone binds at the receptor, the second messenger is released into cytosol.
cAMP, Ca2+, inositol-1,4,5-triphosphate (IP3), diacylglycerol (DG), and NO are second
messengers.
Hormonal stimulation by glucagon or epinephrine increases the intracellular [cAMP] →
increase cAPK activity → increases the rate of phosphorylation of many proteins and
decreases the rate of dephosphorylation.
Cyclic cascades amplify the small change of [cAMP] to a large change in the fraction of
enzymes in their phosphorylated forms.
Hormonal stimulation by insulin increases the phosphoprotein phosphatase-1 (PP-1) activity.
PP-1 dephosphorylates various proteins.
Thus epinephrine and insulin are antagonistic relation.
F. Maintenance of blood glucose levels
- Need to maintain glucose concentration at ~5 mM in blood.
When the [glucose] in blood is low:
1. Glucagon is secreted from pancreas into bloodstream.
2. Glucagon binds at glucagon receptor on the liver cells and activates adenylate cyclase,
thereby increasing intracellular [cAMP].
3. The [cAMP] increase triggers an increase in the rate of glycogen breakdown, leading to
increase intracellular [G6P].
4. Since G6P cannot pass through the cell membrane and is not major energy source of liver,
G6P is hydrolyzed to glucose by glucose-6-phosphatase. The resulting glucose enters the
blood stream.
When blood glucose level is high (immediate after meal)
1. Glucagon level decrease.
2. Insulin is secreted from pancreatic β cells.
- In muscle,
- Insulin stimulates the glucose transporter on the muscle cells. Thus, glucose in bloodstream
enters into the cells.
- Insulin stimulates insulin-stimulated protein kinase which phosphorylates G-subunit of
phosphoprotein phosphatase-1 (PP-1).
- The phosphorylated PP-1 dephosphorylates glycogen synthase b to increase glycogen
synthesis, and also dephosphorylates glycogen phosphorylase a to decrease glycogen
breakdown.
17
-
Takusagawa’s Note©
18
Chapter-15
In liver,
Glucose in blood enters freely into the liver cells when the [glucose] is high in bloodstream.
Glucose itself inhibits glycogen phosphorylase a (R state) and shifts equilibrium to glycogen
phosphorylase b (T state).
[Phosphorylase A → Phosphorylase B]
Phosphoprotein phosphatase inhibitor-1 is dephosphorylated and is released from PP-1.
PP-1 activates glycogen synthase and deactivates glycogen phosphorylase a. Therefore, the
liver can store the excess of glucose as glycogen.
Enzyme activity in mouse liver
Glycogen synthase a
Phosphorylase a
0
2
4
6
8
Time after glucose infusion (min)
How does liver work as a buffer of blood [glucose]?
- Liver does not have hexokinase, but glucokinase.
- Hexokinase (that is in tissues other than in liver) obeys Michaelis-Menten kinetics, and is
inhibited by product G6P.
- Glucokinase (that is in liver) displays sigmoidal kinetics, and is not inhibited by G6P at
physiological concentration.
- The KM of hexokinase is much smaller than K0.5 of glucokinase, indicating that hexokinase
has stronger glucose affinity than glucokinase, i.e., hexokinase is good for glycolysis.
KM (Hexokinase) << K0.5 (Glucokinase), i.e., 0.2 mM << 5 mM.
[Glucose] in blood < 5 mM: Liver releases glucose (G6P→ Glucose)
[Glucose] in blood > 5 mM: Liver stores glucose (Glucose → G6P)
18
Takusagawa’s Note©
19
Chapter-15
Liver cells are freely permeable to glucose, but other cells are not
- Thus, at low [glucose] in bloodstream, liver releases glucose whereas at high [glucose] in
bloodstream, liver cells receive glucose and convert it to glycogen since glucose freely
permeable to liver.
Other [glucose] regulator, β-D-fructose-2,6-bisphosphate (F2,6P)
- F2,6P is an extremely potent allosteric activator of PFK and inhibitor of FBPase.
- Thus, the [glucose] is regulated by the [F2,6P].
-2
O3P
2-
O CH2
O
O PO3
HO
CH2OH
OH
β-D-Fructose-2,6-bisphosphate
(F2,6P)
-
F2,6P is formed from fructose-6-phosphate (F6P) catalyzed by phosphofructokinase-2 (PFK-2)
and is hydrolyzed to F6P by fructose bisphosphatase-2 (FBPase-2).
Phosphofructokinase-2 (PFK-2) and Fructose bisphosphatase-2 (FBPase-2)
- The 100kD homodimeric protein carries both PFK-2 and FBPase-2 activities which are
located on different domains.
Phosphorylation sites
FBPase-2
PFK-2
-
Isozymes in liver and heart muscle have the phosphorylation site, whereas the isozyme in
skeletal muscle does not have the phosphorylation site.
19
Takusagawa’s Note©
20
Chapter-15
Liver isozyme
- Phosphorylation by cAPK effects:
⎬ F2,6P is hydrolyzed → [F2,6P]↓
- Inhibition of PFK-2 activity.
⎭
- Activation of FBPase-2 activity.
- Thus, increase FBPase activity and decrease PFK since the allosteric activator F2,6P is
not available to activate PFK and to inhibit FBPase.
- Therefore, the glycolysis is inversed, and G6P is produced. Then G6P hydrolysis
produces free glucose that is secreted into bloodstream.
PFK
[F1,6P]↓
PFK-2
[F6P]↑
FBPase
FBPase-2
[F2,6P]↓
Glycogen synthesis pathway
m
a
e
r
t
s
d
o
o
l
B
Glucose
G6P
G1P
Glycogen
Heart muscle isozyme
- Phosphorylation by cAPK effects:
⎬ F2,6P is produced → [F2,6P]↑
- Activation of PFK-2 activity
⎭
- Inhibition of FBPase-2 activity
- Thus, increase PFK activity and decrease FBPase since F2,6P activates PFK and inhibits
FBPase.
- Therefore, the glycolysis pathway is activated.
PFK
[F1,6P]↑
PFK-2
[F6P]↓
FBPase
[F2,6P]↑
FBPase-2
Glycolysis pathway
s
i
s
y
l
o
c
y
l
G
G6P
-
G1P
Glycogen
The phosphorylation of PFK-2/FBPase-2 in heart muscle activates PFK-2 rather than inhibits
PFK-2 as seen in liver.
Increase glycogen breakdown effects:
- In liver: increase glucose secretion into bloodstream, and decrease glycolysis.
- In muscle: increase glycolysis.
In skeletal muscle isozyme, there is no phosphorylation site. Thus, there is no subject to cAMPdependent phosphorylation control.
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Chapter-15
21
Takusagawa’s Note©
G. Response to stress
- Epinephrine and norepinephrine are released into the bloodstream by adrenal glands in
response to stress.
- also stimulate the pancreatic α cells to secrete glucagon.
- These hormones bind to:
- β-adrenergic receptors, stimulates adenylate cyclase, and produces cAMP.
- α-adrenergic receptors, stimulates phospholipase C, and produces inositol-1,4,5-triphosphate
(IP3), diacylglycerol (DG), and Ca2+. These second messengers act as reinforce the cells’
response to cAMP.
Muscle cells do not have
glucagons receptors.
← Liver Cell
21
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Chapter-15
Takusagawa’s Note©
4. GLYCOGEN STORAGE DISEASE
Enzyme deficiency
Glucose-6-phosphatase
α-1,4-Glucosidase
Amylo-1,6-glucosidase
Reaction
G6P → glucose
maltose → 2 glucose
branch glycogen → glucose
Amylo-(1,4→1,6)transglycosylase
Glycogen phosphorylase
(muscle):
Glycogen phosphorylase
(liver):
Phosphofructokinase
glucose → branch glycogen
Symptoms
[G6P]↑ in liver, [glucose]↓ in blood
Large accumulation of glycogen
Accumulation of abnormal short
chain glycogen
Very long unbranched glycogen
glycogen → G6P
Inability of glycogen breakdown
glycogen → G6P
Inability of glycogen breakdown
F6P → FBP (F1,6P)
Accumulation of G6P and F6P
Phosphorylase kinase
glycogen breakdown
Glycogen synthase
UDP-glucose → glycogen
Inability to convert phosphorylase b
to a
Inability to synthesize glycogen in
liver
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Chapter-15
D.
-
23
Takusagawa’s Note©
Mechanism of passive-mediated glucose transport
Erythrocyte glucose transporter is a 55 KD protein, which is composed of 12 transmembrane helices.
The helices form a bundle whose interior is hydrophilic and exterior is hydrophobic.
Glucose is transport through the channel.
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Chapter-15
24
Takusagawa’s Note©
Glucose transport occurs via a gated pore mechanism (Erythrocytes, Liver cells)
- When glucose transports across the erythrocyte membrane, no other molecule or ion are accompanied.
- Simply glucose moves from high concentration side to low concentration side.
Insulin-sensitive exocytosis/endocytosis controls the glucose transport (Fat and muscle cells)
- Cellular glucose uptake is regulated through the insulinsensitive exocytosis/endocytosis of the vesicles
containing the glucose transports.
- When the [glucose] in blood is high, insulin is secreted
from pancreas.
- Insulin stimulates the exocytosis activity of the vesicle
that contains the glucose transporters.
- The glucose transporters are moved on the membrane.
- Glucose in blood is taken up and stored in cells.
-
When the [glucose] in blood is low, the glucose
transporters are moved into the cell by endocytosis so that
glucose in cells cannot flow out.
-
Thus glucose uptake is regulated by population of glucose
transporters.
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Chapter-15
25
Takusagawa’s Note©
A. Na+-Glucose symport (Small intestine)
- Glucose is absorbed from lumen of small intestine into brush border cells by Na+ concentration gradient.
- Actually, the Na+ is pumped out by (Na+-K+)-ATPase from the brush border cell to create the Na+ gradient.
-
Na+-glucose symport system is a random Bi Bi kinetic mechanism.
Note: most antiport systems are ordered sequential kinetic mechanism.
25