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Page 1: Glycogen Metabolism - PiratePanelcore.ecu.edu/.../5800pdf/GlycogenMetabolism.pdf · Allosteric Control of Glycogen Metabolism: ... Hormonal Regulation of Glycogen MetabolismHormonal

Glycogen MetabolismGlycogen Metabolism

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Higher organisms ensure a continuous supply of glucose by storing it as glucose polysaccharides.

Higher organisms ensure a continuous supply of glucose by storing it as glucose polysaccharides.

plants: starch

animals: glycogen

plants: starch

animals: glycogen

Glycogen granules prominent in liver and muscle cells.Glycogen granules prominent in liver and muscle cells.

Glycogen granules contain enzymes that synthesize and degrade glycogen and the regulatory enzymes for these processes.

Glycogen granules contain enzymes that synthesize and degrade glycogen and the regulatory enzymes for these processes.

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Glycogen is degraded by the sequential removal of glucose units from the non-reducing end (GLYCOGENOLYSIS).

Glycogen is degraded by the sequential removal of glucose units from the non-reducing end (GLYCOGENOLYSIS).

Glycogenolysis has slightly different roles in muscle vs. liver.Glycogenolysis has slightly different roles in muscle vs. liver.

Muscle: Glycogen breakdown leads to formation of glucose-6-phosphate that is metabolized via glycolysis and TCA cycle to form ATP for contraction.

Muscle: Glycogen breakdown leads to formation of glucose-6-phosphate that is metabolized via glycolysis and TCA cycle to form ATP for contraction.

Liver: End product of glycogenolysis is free glucose that enters the bloodstream and is delivered to other cells.Liver: End product of glycogenolysis is free glucose that enters the bloodstream and is delivered to other cells.

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Rate-limiting reaction: Glycogen PhosphorylaseRate-limiting reaction: Glycogen Phosphorylase

Catalyzes the removal of the terminal glucose residue of glycogen when the bond is an α-1,4 linkage. Bond cleavage is by substitution of a phosphoryl group (phosphorolysis).

Catalyzes the removal of the terminal glucose residue of glycogen when the bond is an α-1,4 linkage. Bond cleavage is by substitution of a phosphoryl group (phosphorolysis).

glycogen (n) + Pi glycogen (n-1) + glucose-1-Pglycogen (n) + Pi glycogen (n-1) + glucose-1-P

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α−1,6α−1,6

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α−1,6α−1,6

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α−1,6α−1,6

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α−1,6α−1,6

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α−1,6α−1,6

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α−1,6α−1,6

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α−1,6α−1,6

Glycogen phosphorylase stops four glucose residues from a branch point, producing a limit dextrin.

Glycogen phosphorylase stops four glucose residues from a branch point, producing a limit dextrin.

The limit dextrin is degraded by glycogen debranching enzyme, which has two separate activities.The limit dextrin is degraded by glycogen debranching enzyme, which has two separate activities.

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1) the 4-glucanotransferase activity transfers 3 glucose units from a branch to a free 4’ end of the glycogen

molecule.

1) the 4-glucanotransferase activity transfers 3 glucose units from a branch to a free 4’ end of the glycogen

molecule.

α−1,6α−1,6

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1) the 4-glucanotransferase activity transfers 3 glucose units from a branch to a free 4’ end of the glycogen

molecule.

1) the 4-glucanotransferase activity transfers 3 glucose units from a branch to a free 4’ end of the glycogen

molecule.

α−1,6α−1,6

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2) the amylo-1,6-glucosidase activity hydrolyzes the remaining α-1,6 linked glucose.

2) the amylo-1,6-glucosidase activity hydrolyzes the remaining α-1,6 linked glucose.

α−1,6α−1,6

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2) the amylo-1,6-glucosidase activity hydrolyzes the remaining α-1,6 linked glucose.

2) the amylo-1,6-glucosidase activity hydrolyzes the remaining α-1,6 linked glucose.

α−1,6α−1,6

This is a hydrolysis reaction; not phosphorolysis, so the product is glucose, not glucose-1-P.This is a hydrolysis reaction; not phosphorolysis, so the product is glucose, not glucose-1-P.

glucoseglucose

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In cells, glucose-1-P is rapidly interconverted with glucose-6-P in a near-equilibrium reaction catalyzed by

phosphoglucomutase.

In cells, glucose-1-P is rapidly interconverted with glucose-6-P in a near-equilibrium reaction catalyzed by

phosphoglucomutase.

In liver, glucose-6-P is hydrolyzed to glucose by glucose-6-phosphatase.In liver, glucose-6-P is hydrolyzed to glucose by glucose-6-phosphatase.

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Glycogen SynthesisGlycogen Synthesis

Four enzymes required to incorporate a molecule of glucose into a growing chain:Four enzymes required to incorporate a molecule of glucose into a growing chain:

1) Phosphoglucomutase converts glucose-6-P to glucose-1-P.1) Phosphoglucomutase converts glucose-6-P to glucose-1-P.

2) Glucose-1-P is activated by reaction with UTP:

UDP-glucose pyrophosphorylase

g-1-P + UTP UDP-glucose + PPi.

2) Glucose-1-P is activated by reaction with UTP:

UDP-glucose pyrophosphorylase

g-1-P + UTP UDP-glucose + PPi.

(Subsequent hydrolysis of PPi contributes to the metabolic irreversibility of this reaction.)

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3. Glycogen synthase catalyzes the addition of a glucose residue from UDP-glu to the non-reducing end of glycogen:

3. Glycogen synthase catalyzes the addition of a glucose residue from UDP-glu to the non-reducing end of glycogen:

UDP-glu + glycogen (n) UDP + glycogen (n + 1)UDP-glu + glycogen (n) UDP + glycogen (n + 1)

Glycogen synthase requires a preexisting primer of at least 4 glucose residues linked α-1,4 and attached to the 1’OH of a tyrosine residue of the protein glycogenin.

Glycogen synthase requires a preexisting primer of at least 4 glucose residues linked α-1,4 and attached to the 1’OH of a tyrosine residue of the protein glycogenin.

Glycogen synthase catalyzes the rate-limiting step in glycogen synthesis.Glycogen synthase catalyzes the rate-limiting step in glycogen synthesis.

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Branch formation:Branch formation:

4. Amylo-(1,4 1,6)-transglycosylase (branching enzyme) removes at least 6 residues from the non-reducing end of an elongated chain and attaches it in an α-1,6 to a glucose residue of the same or another chain.

4. Amylo-(1,4 1,6)-transglycosylase (branching enzyme) removes at least 6 residues from the non-reducing end of an elongated chain and attaches it in an α-1,6 to a glucose residue of the same or another chain.

UTP regenerated by:

nucleoside diphosphate kinase

UDP + ATP UTP + ADP

UTP regenerated by:

nucleoside diphosphate kinase

UDP + ATP UTP + ADP

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Allosteric Control of Glycogen Metabolism:Allosteric Control of Glycogen Metabolism:

Muscle glycogen phosphorylase is activated by AMP and inhibited by ATP, glucose-6-P and caffeine.Muscle glycogen phosphorylase is activated by AMP and inhibited by ATP, glucose-6-P and caffeine.

Glycogen synthase is activated by glucose-6-P.Glycogen synthase is activated by glucose-6-P.

High demand for ATP (low [ATP], low [g-6-P], high [AMP]) stimulates phosphorylase and hence glycogen breakdown. When [ATP] and [g-6-P] are high, the reverse is true and glycogen synthesis is favored.

High demand for ATP (low [ATP], low [g-6-P], high [AMP]) stimulates phosphorylase and hence glycogen breakdown. When [ATP] and [g-6-P] are high, the reverse is true and glycogen synthesis is favored.

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Second Messengers and Signal

Transduction

Second Messengers and Signal

Transduction

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Binding of a hormone (first messenger) to its receptor on the cell surface causes an increase in the intracellular concentration of the second messenger.

Binding of a hormone (first messenger) to its receptor on the cell surface causes an increase in the intracellular concentration of the second messenger.

The second messenger binds to and activates target enzymes (many of which are protein kinases). In the case of c-AMP, the target enzyme is the c-AMP dependent protein kinase (PK-A).

The second messenger binds to and activates target enzymes (many of which are protein kinases). In the case of c-AMP, the target enzyme is the c-AMP dependent protein kinase (PK-A).

c-AMP kinase can then phosphorylate a number of cellular proteins, including those involved in glycogen metabolism.c-AMP kinase can then phosphorylate a number of cellular proteins, including those involved in glycogen metabolism.

A key feature of second messenger cascades is their enormous amplification potential.A key feature of second messenger cascades is their enormous amplification potential.

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GDPGDPRR

G proteinsG proteins

c-AMP Amplification Cascade:c-AMP Amplification Cascade:

adenylate cyclaseadenylate cyclase

1 hormone molecule1 hormone molecule

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adenylate cyclaseadenylate cyclaseRR

G proteinsG proteins

GTPGTP

1 hormone molecule1 hormone molecule

c-AMP Amplification Cascade:c-AMP Amplification Cascade:

ATPATPc-AMPc-AMP (40 molecules)(40 molecules)

inactive c-AMPkinase

inactive c-AMPkinase

(10 molecules)(10 molecules)

active c-AMPkinase

active c-AMPkinase

inactive target enzyme

inactive target enzyme

active target enzyme

active target enzyme (100 molecules)(100 molecules)

inactive target enzyme

inactive target enzyme

active target enzyme

active target enzyme

(1000 molecules)(1000 molecules)

c-AMP cascade discovered by Earl Sutherland who won a Nobel Prize in 1972 for this work.

c-AMP cascade discovered by Earl Sutherland who won a Nobel Prize in 1972 for this work.

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Hormonal Regulation of Glycogen MetabolismHormonal Regulation of Glycogen Metabolism

1938: Carl and Gerti Cori found that glycogen phosphorylase exists in two forms: a (active w/o AMP) and b (required AMP).

1938: Carl and Gerti Cori found that glycogen phosphorylase exists in two forms: a (active w/o AMP) and b (required AMP).

1959: Ed Krebs and Ed Fischer demonstrated that phosphorylase a (the active form) was phosphorylated on serine 14 and phosphorylase b was the dephosphorylated (and less active) form (Nobel Prize 1992).

1959: Ed Krebs and Ed Fischer demonstrated that phosphorylase a (the active form) was phosphorylated on serine 14 and phosphorylase b was the dephosphorylated (and less active) form (Nobel Prize 1992).

Three enzymes are involved in the hormonal control of glycogen phosphorylase (and glycogen synthase as well).Three enzymes are involved in the hormonal control of glycogen phosphorylase (and glycogen synthase as well).

Page 26: Glycogen Metabolism - PiratePanelcore.ecu.edu/.../5800pdf/GlycogenMetabolism.pdf · Allosteric Control of Glycogen Metabolism: ... Hormonal Regulation of Glycogen MetabolismHormonal

1. Phosphorylase kinase: phosphorylates glycogen phosphorylase on serine 14.1. Phosphorylase kinase: phosphorylates glycogen phosphorylase on serine 14.

2. c-AMP kinase: phosphorylates and thereby activates phosphorylase kinase.2. c-AMP kinase: phosphorylates and thereby activates phosphorylase kinase.

3. Phosphoprotein phosphatase-1: dephosphorylates and thereby deactivates both glycogen phosphorylase a and phosphorylase kinase.

3. Phosphoprotein phosphatase-1: dephosphorylates and thereby deactivates both glycogen phosphorylase a and phosphorylase kinase.

Phosphorylase b is the form subject to allosteric control by AMP, ATP and glucose-6-P. Conversion to phosphorylase a removes the allosteric controls. In resting cells, the levels of ATP and g-6-p are high enough to inhibit phos. b; thus the level of phosphorylase activity is controlled by the amount of enzyme in the a (active) form.

Phosphorylase b is the form subject to allosteric control by AMP, ATP and glucose-6-P. Conversion to phosphorylase a removes the allosteric controls. In resting cells, the levels of ATP and g-6-p are high enough to inhibit phos. b; thus the level of phosphorylase activity is controlled by the amount of enzyme in the a (active) form.

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Phosphorylase kinase (converts phosphorylase b to a) is activated by Ca2+ and phosphorylation.Phosphorylase kinase (converts phosphorylase b to a) is activated by Ca2+ and phosphorylation.

Phos. kinase is a tetramer (αβγδ): Phos. kinase is a tetramer (αβγδ):

γ: catalytic subunit

α & β: contain phosphorylation sites

δ: is a small protein, calmodulin, which binds Ca2+

γ: catalytic subunit

α & β: contain phosphorylation sites

δ: is a small protein, calmodulin, which binds Ca2+

Phos. kinase must be phosphorylated on two sites and bind 4 molecules of free calcium in order to be fully activated.

Phos. kinase must be phosphorylated on two sites and bind 4 molecules of free calcium in order to be fully activated.

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Phosphoprotein phosphatase-1 removes phosphoryl groups from:Phosphoprotein phosphatase-1 removes phosphoryl groups from:

phosphorylase aα & β subunits of phosphorylase kinasephosphorylase aα & β subunits of phosphorylase kinase

Phosphoprotein phosphatase-1 is inhibited by binding to protein phosphatase inhibitor. The inhibitor is activated by phosphorylation by c-AMP kinase.

Phosphoprotein phosphatase-1 is inhibited by binding to protein phosphatase inhibitor. The inhibitor is activated by phosphorylation by c-AMP kinase.

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Regulation of Glycogen Synthase:Regulation of Glycogen Synthase:

Like phosphorylase, exists in two forms:Like phosphorylase, exists in two forms:

1) phosphorylated form (b or “D” form) which is inactive1) phosphorylated form (b or “D” form) which is inactive

2) dephosphorylated form (a or “I” form) which is active2) dephosphorylated form (a or “I” form) which is active

Both c-AMP kinase and phosphorylase kinase and other kinases as well phosphorylate glycogen synthase a to convert it to glycogen synthase b.

Both c-AMP kinase and phosphorylase kinase and other kinases as well phosphorylate glycogen synthase a to convert it to glycogen synthase b.

At least 9 different serine residues can be phosphorylated.At least 9 different serine residues can be phosphorylated.

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Phosphoprotein phosphatase-1 can remove the phosphates from glycogen synthase, and thus activate it.Phosphoprotein phosphatase-1 can remove the phosphates from glycogen synthase, and thus activate it.

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Integration of Glycogen Metabolism Control Mechanisms:Integration of Glycogen Metabolism Control Mechanisms:

When hormonal stimulation by glucagon or epinephrine increases c-AMP concentration, the c-AMP kinase activity increases.

When hormonal stimulation by glucagon or epinephrine increases c-AMP concentration, the c-AMP kinase activity increases.

The net effect is to maintain glycogen phosphorylase in the “a” or active form since phosphorylase kinase is stimulated and the phosphatase is inhibited. Thus, GLYCOGEN BREAKDOWN PROCEEDS!!

The net effect is to maintain glycogen phosphorylase in the “a” or active form since phosphorylase kinase is stimulated and the phosphatase is inhibited. Thus, GLYCOGEN BREAKDOWN PROCEEDS!!

The net effect is to maintain glycogen synthase in the “b” or inactive form by phosphorylation. Thus, GLYCOGEN SYNTHESIS IS INHIBITED!!

The net effect is to maintain glycogen synthase in the “b” or inactive form by phosphorylation. Thus, GLYCOGEN SYNTHESIS IS INHIBITED!!

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To reverse the effects of the c-AMP cascade, the intracellular concentration of c-AMP must be lowered to resting levels (i.e., those which existed before stimulation of the cell by the hormone).

To reverse the effects of the c-AMP cascade, the intracellular concentration of c-AMP must be lowered to resting levels (i.e., those which existed before stimulation of the cell by the hormone).

c-AMP is broken down by the enzyme cyclic nucleotide phosphodiesterase.c-AMP is broken down by the enzyme cyclic nucleotide phosphodiesterase.

Caffeine is an inhibitor of phosphodiesterase.Caffeine is an inhibitor of phosphodiesterase.

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Epinephrine (fight or flight hormone) prepares an organism for mobilization of energy in both muscle and throughout the body. It acts on both muscle and liver cells:

Epinephrine (fight or flight hormone) prepares an organism for mobilization of energy in both muscle and throughout the body. It acts on both muscle and liver cells:

promotes breakdown of glycogen via enzyme cascadepromotes breakdown of glycogen via enzyme cascade

inhibition of glycogen synthesisinhibition of glycogen synthesis

stimulation of glycolysis (2000-fold increase in rate)stimulation of glycolysis (2000-fold increase in rate)

Glucagon acts primarily in liver to maintain steady-state levels of glucose in blood and other tissues. Muscle cells do not contain glucagon receptors.

Glucagon acts primarily in liver to maintain steady-state levels of glucose in blood and other tissues. Muscle cells do not contain glucagon receptors.

promotes breakdown of glycogen via enzyme cascadepromotes breakdown of glycogen via enzyme cascade

activates gluconeogenesisactivates gluconeogenesis

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glucagon or epinephrineglucagon or epinephrine

inactive adenylate cyclase

inactive adenylate cyclase

active adenylate cyclase

active adenylate cyclase

ATPATP c-AMPc-AMP

inactive c-AMP kinase

inactive c-AMP kinase

active c-AMP kinase

active c-AMP kinase

inactive phosphorylase kinase

inactive phosphorylase kinase

active phosphorylase kinase

active phosphorylase kinase

ATPATPADPADP

inactive glycogen phosphorylase(b)

inactive glycogen phosphorylase(b)

active glycogen phosphorylase (a)

active glycogen phosphorylase (a)

ADPADPATPATPactive

glycogen synthase (I)

active glycogen synthase (I)

inactive glycogen synthase (D)

inactive glycogen synthase (D)

ATPATPADPADP PP

PP

PP

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Isn’t this more fun than any human being should ever have?

Isn’t this more fun than any human being should ever have?


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