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Carbohydrate Metabolism 2: Glycogen degradation, glycogen synthesis, reciprocal regulation of glycog


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Title: Carbohydrate Metabolism 2: Glycogen degradation, glycogen synthesis, reciprocal regulation of glycog

Carbohydrate Metabolism 2Glycogen degradation,
glycogen synthesis, reciprocal regulation of
glycogen metabolism
Bioc 460 Spring 2008 - Lecture 34 (Miesfeld)
Glycogen phosphorylase enzyme is a dimer that is
regulated by both phosphorylation and allostery
Carbohydrates in pasta are a good way to
replenish muscle glycogen stores
Gerty Cori won the 1947 Nobel Prize for her work
on glycogen metabolism
Key Concepts in Glycogen Metabolism
  • Glycogen is a highly-branched polymer of glucose
    that can be quickly degraded to yield glucose-1P
    which is isomerized to glucose-6P for use in
    glycolysis by muscle cells, or it is
    dephosphorylated in liver cells and exported.
  • Glycogen phosphorylase removes one glucose at a
    time from the nonreducing ends using inorganic
    phosphate (Pi) which makes glucose release a
    free reaction. Glycogen phophorylase is
    activated by phosphorylation in response to
    glucagon and epinephrine, and allosterically-regul
    ated by energy charge.
  • Glycogen synthase adds glucose residues to
    nonreducing ends in a reaction involving
    UDP-glucose, a nucleotide charged form of
    glucose. Since the UDP that is released
    following glucose addition needs to be
    phosphorylated to regenerate UTP, the cost of
    glycogen synthesis is 1 ATP/glucose residue.
  • Glycogen phosphorylase and glycogen sythase are
    reciprocally-regulated by phosphorylation. Net
    phosphorylation leads to glycogen degradation,
    whereas, net dephosphoryation, results in
    glycogen synthesis.

Overview of Glycogen Metabolism
The storage form of glucose in most eukaryotic
cells (except plants) is glycogen, a large highly
branched polysaccharide consisting of glucose
units joined by ?-1,4 and ?-1,6 glycosidic bonds.
Most glycogen in animals is stored in the muscle
and liver tissues. Glycogen degradation and
synthesis occurs in the cytosol and the substrate
for these reactions is the free ends of the
branched polymer (nonreducing ends).
The large number of branch points in glycogen
results in the generation of multiple nonreducing
ends that provide a highly efficient mechanism to
quickly release and store glucose.
The reducing and nonreducing ends of glycogen
  • The nonreducing end of glycogen molecules refers
    to the carbon that is opposite to the reducing
    end in the ring structure. The reducing end of a
    linear glucose molecule can be oxidized by Cu2
    by definition.

Reducing end
Nonreducing end
Nonreducing end

Reducing end
Glycogen Core Complexes
  • Glycogen core complexes consist of glycogenin
    protein and 50,000 glucose molecules with a-1,6
    branches about every 10 residues creating 2,000
    nonreducing ends.
  • Twenty to forty glycogen core complexes associate
    inside liver and muscle cells to form glycogen
    particles containing over a million glucose

These glycogen particles can be visualized by
electron microscopy and account for up to 10 by
weight of liver tissue.
Pathway Questions
  • Liver glycogen is used as a short term energy
    source for the organism by providing a means to
    store and release glucose in response to blood
    glucose levels liver cells do not use this
    glucose for their own energy needs.
  • Muscle glycogen provides a readily available
    source of glucose during exercise to support
    anaerobic and aerobic energy conversion pathways
    within muscle cells muscle cells lack the enzyme
    glucose-6-phosphatase and therefore cannot
    release glucose into the blood.

Pathway Questions
  • 2. What are the net reactions of glycogen
    degradation and synthesis?
  • Glycogen Degradation
  • Glycogenn units of glucose Pi ? Glycogenn-1
    units of glucose glucose-6-phosphate
  • Glycogen Synthesis
  • Glycogenn units of glucose glucose-6-phosphate
    ATP H2O ? Glycogenn1 units of glucose
    ADP 2Pi

Pathway Questions
  • 3. What are the key enzymes in glycogen
  • Glycogen phosphorylase enzyme catalyzing the
    phosphorylysis reaction that uses Pi to remove
    one glucose at a time from nonreducing ends of
    glycogen resulting in the formation of
    glucose-1P. Liver and muscle glycogen
    phosphorylase are isozymes (two different genes)
    that are both activated by phosphorylation but
    have distinct responses to allosteric
    effectors.Glycogen synthase - enzyme catalyzing
    the addition of glucose residues to nonreducing
    ends of glycogen using UDP-glucose as the glucose
    donor. Glycogen synthase activity is inhibited by
    phosphorylation binding of the allosteric
    activators glucose or glucose-6P promotes
    dephosphorylation and enzyme activation.Branchin
    g and debranching enzymes - these two enzymes are
    responsible for adding (branching) and removing
    (debranching) glucose residues to the glycogen
    complex through the cleavage and formation of
    a-1,6 glycosidic bonds.

Pathway Questions
  • 4. What are examples of glycogen metabolism in
    real life?
  • The performance of elite endurance athletes can
    benefit from a diet regimen of carbohydrate
    "loading" prior to competition. Carbohydrate
    loading regimens can result in a build-up of
    stored muscle glycogen that is sometimes higher
    than what can be obtained by simply following a
    high carbohydrate diet.

Key is to deplete glycogen before carbo loading
to get 2x glycogen level.
Function of Glycogen Phosphorylase
  • Glycogen degradation is initiated by glycogen
    phosphorylase, a homodimer that catalyzes a
    phosphorolysis cleavage reaction of the a-1,4
    glycosidic bond at the nonreducing ends of the
    glycogen molecule. Inorganic phosphate (Pi)
    attacks the glycosidic oxygen using an acid
    catalysis mechanism that releases glucose-1P as
    the product.

Although the standard free energy change for this
phosphorylysis reaction is positive (?Gº' 3.1
kJ/mol), making the reaction unfavorable, the
actual change in free energy is favorable (?G'
-6 kJ/mol) due to the high concentration of Pi
relative to glucose-1P inside the cell (ratio of
close to 100).
Structure of Glycogen Phosphorylase
  • Exists as a dimer and has binding sites for
    glycogen and catalytic sites that contain
    pyridoxal phosphate (derived from vitamin B6).
    The critical Pi substrate is bound to the active
    site by interactions with pyridoxal phosphate and
    active site amino acids.

Function of Phosphoglucomutase
  • The the next reaction in the glycogen degradation
    pathway is the conversion of glucose-1P to
    glucose-6P by the enzyme phosphoglucomutase. The
    enzyme first donates a phosphate group to the
    substrate to generate an intermediate
    bisphosphate compound, and then the bisphosphate
    compound is dephosphorylated to regenerate the
    phosphoenzyme and release the product.
  • Have you seen this type of reaction before, was
    it in glycolysis?

Glycogen Debranching Enzyme
  • The glycogen debranching enzyme (also called
    a-1,6-glucosidase) recognizes the partially
    degraded branch structure and remodels the
    substrate in a two step reaction. 1) the
    debranching enzyme transfers three glucose units
    to the nearest nonreducing end to generate a new
    substrate for glycogen phosphorylase. 2) the
    bifunctional debranching enzyme cleaves the a-1,6
    glycosidic bond to release free glucose.
  • Since a-1,6 branch points occur about once every
    10 glucose residues in glycogen, complete
    degradation releases 90 glucose-1P and 10
    glucose molecules.

Is there a difference in the amount of energy
that can be recovered from glucose-1P and glucose?
Regulation of Glycogen Phosphorylase Activity
  • Activity is regulated by both covalent
    modification (phosphorylation) and by allosteric
    control (energy charge).
  • Glycogen phosphorylase is found in cells in two
  • active conformation, R form
  • inactive conformation, T form
  • Phosphorylation of serine 14 (Ser 14) shifts the
    equilibrium in favor of the active R state.
  • This phosphorylated form of glycogen
    phosphorylase is called phosphorylase a, and the
    unphosphorylated form is called phosphorylase b.

Regulation of Glycogen Phosphorylase Activity
  • The enzyme responsible for phosphorylating
    glycogen phosphorylase b to activate it, is
    phosphorylase kinase which is a downstream target
    of glucagon and epinephrine signaling.

Regulation of Glycogen Phosphorylase Activity
  • Insulin stimulates the activity of protein
    phosphatase-1 (PP-1), leading to inactivation of
    glycogen phosphorylase. PP-1 is the same enzyme
    that dephosphorylates PFK-2/FBPase-2, the enzyme
    responsible for controlling fructose-2,6-BP

Tissue-specific isozymes of glycogen phosphorylase
  • The activity of glycogen phosphorylase can also
    be controlled by allosteric regulators, which
    bind to the enzyme and shift the equilibrium.
  • Liver and muscle isozymes of glycogen
    phosphorylase are allosterically-regulated in
    different ways, which reflects the unique
    functions glycogen in these two tissues.

Tissue-specific isozymes of glycogen phosphorylase
  • Liver glycogen phosphorylase a, but not muscle
    glycogen phosphorylase a is subject to allosteric
    inhibition by glucose binding which shifts the
    equilibrium from the R to T state.
  • When liver glycogen phosphorylase a
    (phosphorylated form) is shifted to the T state,
    it is a better substrate for dephosphorylation by
    PP-1 than is the R state.
  • Why does it make sense that muscle glycogen
    phosphorylase b, but not liver glycogen
    phosphorylase b, would be allosterically
    activated by AMP in the absence of hormone

Glycogen synthase catalyzes glycogen synthesis
  • The addition of glucose units to the nonreducing
    ends of glycogen by the enzyme glycogen synthase
    requires the synthesis of an activated form of
    glucose called uridine diphosphate glucose
  • Glucose-6P is first converted to glucose-1P by
    phosphoglucomutase, and then the enzyme
    UDP-glucose pyrophosphorylase catalyzes a
    reaction involving the attack of a phosphoryl
    oxygen from glucose-1P on the gamma phosphate of
    uridine triphosphate (UTP).
  • The rapid hydrolysis of PPi by the abundant
    cellular enzyme pyrophosphatase results in a
    highly favorable coupled reaction.

Why does rapid conversion of PPi --gt 2 Pi result
in a more favorable reaction?
Glycogen Synthase Reaction
  • Glycogen synthase transfers the glucose unit of
    UDP-glucose to the C-4 carbon of the terminal
    glucose at the nonreducing end of a glycogen
  • The UDP moiety is released and UTP is regenerated
    in a reaction involving ATP and the enzyme
    nucleoside diphosphate kinase.

Glycogen Branching Enzyme
  • Once the chain reaches a length of 11 glucose
    residues, the glycogen branching enzyme transfers
    seven glucose units from the end of the chain to
    an internal position using a a-1,6 branchpoint.
  • This new branchpoint must be at least four
    glucose residues away from the nearest branch.
    Not the exact reverse of the debranching enzymes

Growing Glycogen Tree - Starting with Glycogenin
Regulation of Glycogen Synthase Activity
  • The activity of glycogen synthase is also
    primarily controlled by reversible
  • The effect of phosphorylation on the activity of
    glycogen synthase is however the reverse of what
    we saw with glycogen phosphorylase.
  • Dephosphorylation activates glycogen synthase,
    whereas, glycogen phosphorylase is activated by
  • In this case, the active glycogen synthase a form
    is dephosphorylated and favors the R state,
    whereas, the inactive glycogen synthase b form is
    phosphorylated and favors the T state.
  • The a form is always the active form glycogen
    phosphorylase a is phosphorylated, whereas,
    glycogen synthase a is dephosphorylated.

Regulation of Glycogen Synthase Activity
  • Hormone activation of glycogen synthase activity
    is mediated by insulin, which promotes the
    activation of glycogen synthase by stimulating
    PP-1 activity. Epinephrine and glucagon signaling
    leads to inactivation of glycogen synthase.

Reciprocal regulation of glycogen metabolism
Since glycogen phosphorylase and glycogen
synthase have opposing effects on glycogen
metabolism, it is critical that their activities
be reciprocally regulated to avoid futile cycling
and to efficiently control glucose-6P
concentrations within the cell.
  • Infusion of glucose into mouse liver results in a
    rapid decrease in glycogen phosphorylase activity
    within 1 minute, followed by a dramatic increase
    in glycogen synthase activity by 4 minutes.

What is the metabolic logic of glucose inhibition
of glycogen phosphorylase activity and activation
of glycogen synthase?
Hormone signaling in liver cells
  • insulin signaling on glycogen metabolism in liver
    cells where it can be seen that glucagon
    stimulates glucose efflux and insulin stimulates
    glucose influx through the GLUT2 glucose
    transporter protein.
  • Net phosphorylation drives glycogen degradation,
    and net dephosphorylation drives glycogen

Glucagon signaling
  • Glucagon signaling in liver cells results in
    Gsa-mediated stimulation of adenylate cyclase
    activity leading to the production of the second
    messenger cyclic AMP.
  • Activation of protein kinase A (PKA) by cAMP
    triggers two types of phosphorylation circuits in
    muscle cells one that stimulates glycogen
    degradation and a second that inhibits glycogen

Insulin signaling
  • Insulin signaling results in dephosphorylation of
    glycogen metabolizing enzymes and elevated rates
    of glycogen synthesis.
  • Activation of insulin receptor tyrosine kinase
    activates the phosphoinositide-3-kinase (PI-3K)
    pathway which phosphorylates AKT kinase. In
    turn, Akt phosphorylates glycogen synthase kinase
    3 (GSK3) which is inactivated by phosphorylation.
  • Without active GSK3 around to maintain glycogen
    synthase in the inactive phosphorylated state,
    the level of active dephosphorylated glycogen
    synthase increases.

Human glycogen storage diseases
A number of human diseases have been identified
that affect glycogen metabolism. Disease symptoms
in many cases include liver dysfunction due to
excess glycogen fasting-induced hypoglycemia (low
blood glucose levels) in the most severe
diseases, death at an early age.
Human glycogen storage diseases
von Gierke's disease is due to a deficiency in
the enzyme glucose-6-phosphatase which causes a
build-up of glycogen in the liver because
glucose-6P accumulates and activates glycogen
synthase. McArdle's disease harbor defects in
muscle glycogen phosphorylase. These individuals
suffer from exercise-induced muscle pain due to
their inability to degrade muscle glycogen.