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

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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
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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.
• Glycogen phosphorylase removes one glucose at a
  time from the nonreducing ends using inorganic
  phosphate (Pi).
• Glycogen synthase adds glucose residues to
  nonreducing ends in a reaction involving
  UDP-glucose the cost of glycogen synthesis is 1
  ATP/glucose residue.
• Net phosphorylation leads to glycogen
  degradation, whereas, net dephosphoryation,
  results in glycogen synthesis.

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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.

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.
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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
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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. Glycogen is stored primarily in
  liver and skeletal muscle cells.

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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.

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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

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Pathway Questions

• 3. What are the key enzymes in glycogen
  metabolism?
• 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..Glycogen synthase - enzyme
  catalyzing the addition of glucose residues to
  nonreducing ends of glycogen using UDP-glucose as
  the glucose donor.
• Branching and debranching enzymes - these two
  enzymes are responsible for adding (branching)
  and removing (debranching) glucose residues.

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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.
• Key is to deplete glycogen before carbo loading
  to get 2x glycogen level.

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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).
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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.

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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.
• Where have you seen this type of reaction before
  (a mutase rxn)?

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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.
• 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?
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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
  conformations
• 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 (active),
  and the unphosphorylated form is called
  phosphorylase b. It is the same polypeptide, just
  a different name.

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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, as well
  as, insulin signaling.

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Tissue-specific isozymes of glycogen phosphorylase

• The activity of glycogen phosphorylase can also
  be controlled by allosteric regulators, which
  binds to the T state and shifts the equilibrium
  to the R state.
• Liver and muscle isozymes of glycogen
  phosphorylase are allosterically-regulated in
  different ways, which reflects the unique
  functions glycogen in these two tissues.

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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
  signaling?
• Hint what does the liver do with the glucose-6P
  that is produced?

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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
  (UDP-glucose).
• 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?
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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
  chain.
• The UDP moiety is released and UTP is regenerated
  in a reaction involving ATP and the enzyme
  nucleoside diphosphate kinase.

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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.

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Growing Glycogen Tree - Starting with Glycogenin
Protein
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Regulation of Glycogen Synthase Activity

• The activity of glycogen synthase is also
  primarily controlled by reversible
  phosphorylation.
• Dephosphorylation activates glycogen synthase,
  whereas, glycogen phosphorylase is activated by
  phosphorylation.
• In this case, the active glycogen synthase a
  (active) 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.

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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.

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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.
What is the metabolic logic of glucose inhibition
of glycogen phosphorylase activity and activation
of glycogen synthase?
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Hormone signaling in liver cells

• Net phosphorylation drives glycogen degradation,
  and net dephosphorylation drives glycogen
  synthesis.

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Glucagon signaling

• cAMP triggers two types of phosphorylation
  circuits in muscle cells one that stimulates
  glycogen degradation and a second that inhibits
  glycogen synthesis.

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Insulin signaling

• Insulin signaling results in dephosphorylation of
  glycogen metabolizing enzymes and elevated rates
  of glycogen synthesis.

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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.
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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.