Glycogen Metabolism

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Glycogen Metabolism

• OUTLINE
• Glycogen breakdownrequires the interplay of
  several enzymes
• Phosphorylase is regulated by allosteric
  interactions and reversible phosphorylation
• Epinephrine and glucagon signal the need for
  glycogen breakdown
• Glycogen is synthesized and degraded by different
  pathways
• Glycogen breakdown and synthesis are reciprocally
  regulated
• Glycogen synthesis glycogenesis
• Degradation of glycogen glycogenolysis.

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Glycogen

• Glycogen is a highly branched, very large polymer
  of glyc mols linked ? 1? 4
• Branches arise by ? 1? 6 at about every 8-10th
  residue
• It is found in the cytosol.
• It is the storage form of Glc.
• Liver and muscle are the major sites for the
  storage of glycogen.

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Degradation of Glycogen

• Is not a simple reversal of the synthetic pathway
• Other enzymes involved
• Glycogen ? G-1-P
• Shortening of chains
• Glycogen phosphorylase (?-1 ? 4)
• It is an exoglucosidase
• Degrades the gly. Chains at their non-reducing
  ends until four glucosyl units remain on each
  chain before the branch point
• The resulting structure ? a limit dextrin
• Phosphorylase cannot grade this any further!

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Degradation of Glycogen Continued

• Removal of branches
• Branches are removed through two enzymatic
  activities of the debranching enzyme
• a. Glucosyl 44 transferase removes the outer 3
  of 4 glucosyl residues
• b. Single glucose residue attached in an ? 1?6
  cinkage is then removed by the ?-amylo (16)
  glucosidase activity of the debranching enzyme,
  releasing free glucose

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Glycogen degradation
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Regulation phosphorylase

• Regulation of glycogen metabolism is different in
  muscle and liver.
• In muscle the end served by glycolysis is ATP
  production, and the rate of glycolysis increases
  as muscle works more, demanding more ATP.
• The liver has a different role in whole-body
  metabolism and glucose metabolism in the liver is
  different. The liver makes sure that glucose
  level is constant in the blood. Producing and
  exporting Glc.

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Regulation of Glycogen Breakdown

• Glycogen represents the most immediately
  available large-scale source of metabolic energy.
  Therefore it is important that animals be able
  to activate glycogen mobilization very rapidly.
• Glycogen breakdown is the hormone-controlled
  process.
• Structure of glycogen phosphorylase
• Dimer exists in two forms.
• Active phosphorylase a
• Inactive phosphorylase b
• Activation ? by phosphorylase kinase
• Deactivation ? phosphorylase phosphatase
• Control of phosphorylase activity
• Phosphorylase kinase is activated by c-AMP
  protein kinase

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Muscle phosphorylase

• In muscle, phosphorylase has 2 forms.
• Phosphorylase a active form
• Phosphorylase b inactive form (in especially
  resting muscle)
• The rate of glycogen breakdown is due to the a/b
  which is controlled by hormones especially by
  epinephrine.
• Phosphorylase a 2 subunits, in each Ser residue
  at position 14 is Plated (Phosphorylase kinase
  does this).
• Phosphorylase b structurally identical except
  that Ser residues are not Plated. It is active
  when AMP is high! It is inactive when ATP and Glc
  6-P are high!So, muscle phosphorylase b is active
  only when the energy charge of the muscle is low.
• Resting muscle all enzyme is its ianctive form.
• Phosphorylase a--------gt phosphorylase b by
  dephosphorylation catalyzed by phosphorylase a
  phosphatase.

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Phosphorylase b
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Muscle phosphorylase
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Control of Glycogen Phosphorylase in Muscle
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Muscle phosphorylase regulation

• Both phosphorylase b and phosphorylase a exist as
  equilibria between an active R state and a less
  active T state. Phosphorylase b is usually
  inactive because the equilibrium favors the T
  state. Phosphorylase a is usually active because
  the equilibrium favors the R state. Regulatory
  structures are shown in blue and green.

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AMP dependency of phosphorylase b

• Phosphorylase a is AMP-independent
• Phosphorylase b is AMP-dependent.
• The stimulation of phosphorylase b by AMP can be
  prevented by high ATP concentrations. AMP binds
  its allosteric site and stabilizes the
  conformation of phosphyrylase b in the R state
  (figure 21.11). ATP acts as a negative allosteric
  modulator by competing with AMP and so favors the
  T state.
• Intensive exercise---------gt AMP/ATP (?)
  Phosphorylase b (active)
• In resting muscle------gt AMP/ATP (?)
  Phosphorylase b (inactive)
• Exercise will also result in hormone release
  (epinephrine) that generates the phosphorylated a
  form of the enzyme.

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Liver phosphorylase

• Liver phosphorylase and muscle phosphorylase are
  90 identical in amino acid sequence.
• Liver phosphorylase a but not b has the most
  responsive T-to-R transition.
• The binding of Glc shifts the allosteric
  equilibrium of the a form from the R to the T
  state, deactivating the enzyme (Figure 21.12).
• Why would Glc function as a negative regulator of
  liver phosphorylase a?
• When there is plenty of Glc, no need to breakdown
  liver glycogen!

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Liver Glycogen phosphorylase is regulated by
hormones and blood glucose.

• Liver glycogen has a different role in our
  system, it is the reservoir which releases
  glucose whenever it is needed
• When blood glucose is low (lower than 4-5 mM)
• Glycogen --------gt Glc-1-P---------gt
  Glc-6-P-------------gt Glc
• So, when blood glucose low, glucose is released
  into the blood stream and carried to the needy
  tissues.
• Glycogen phosphorylase of liver is under hormonal
  control, glucagon is the hormone.
• When glucose is low------------------gt glucagon
  is released.
• Liver phosphorylase is allosterically regulated
  by Glc not AMP.
• When Glc is high in blood, it enters hepatocytes,
  binds regulatory sites of the enzyme, causing
  conformational changes (favoring the T state).
  Therefore glycogen phosphorylase is a glucose
  sensor. When Glc is high, it stops its own
  FORMATION., efficiency..

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Phosphorylase kinase is activated by
phosphorylation and calcium ions

• Phosphorylation of phosphorylase enzyme is done
  by phosphorylase kinase enzyme. This enzyme also
  is found in two forms 1) fully active and 2)
  inactive form.
• 1200 kd
• 4 subunits (abgd)
• g catalytic activity, the other subunits are
  regulatory subunits.
• It is under dual control, it is regulated by
  phosphorylation (b subunit is phosphorylated by
  cAMP dependent PKA).
• Phosphorylase kinase can also be partly activated
  by calcium levels of the order of 1 mM. Its d
  subunit is calmodulin, a calsium sensor that
  stimulates many enzymes.
• Phosphorylase kinase has the highest activity
  only after both phosphorylation of the b subunit
  and activation of the d subunit by Ca binding.

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Epi and Glucagon signal the need for glycogen
breakdown

• PKA---gt activates phosphorylase kinase
• Glycogen phosphorylase activated
• Glc 1-P is made
• What activates PKA then?
• HORMONES
• Glucagon
• Epinephrine
• Signal transduction
• Epi
• GTP-bound G proteins
• Increased cAMP
• PKA increases
• cAMP amplifies the effects of hormones

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What shuts off glycogen breakdown?

• This signal transduction pathway is shut down by
  the same pathway.
• How?
• GTP is deactivated by its inherent GTPase
  activity
• cAMP is converted to AMP (not a second messenger)
  by phosphodiesterase enzyme.

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Steps in Glycogen Synthesis

• UDPG synthesis
• G-6-P
• G-1-P UTP ? UDPG PPi
• A primer is required for glycogen synthesis
• If there is glycogen ? it serves as primer
• If not, a specific protein glycogenin
• Elongation of glycogen chains
• UDP-G transfer ? to the non reducing end of the
  growing chain.
• New glycosidic bond
• C-1 and C-4
• Enzyme glycogen synthase
• If no other enzyme acted on the chain ? ? 1-4,
  amylose

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Steps in Glycogen Synthesis Continued

• UDP is released
• UDP ATP ? UTP ADP
• Creating branches in glycogen
• Amylose ? unbranched
• Glygogen ? branches (8)
• - the branching enzyme (glucosyl 46
  transferase)
• - amylo ? 1, 4 - ? 1,6 transglycosylase)
• This enzyme transfers 5-8 from the non-reducing
  ends to another residue by ? 1,6 link.
• Further elongation
• Branches have two important functions
• - increase solutions of glycogen.
• - increase number of non-reducing ends
• - therefore increase the rate of glycogen
  synthesis

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Initiation of Glycogenesis By Glycogenin
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Elongation
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Branching
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Summary of the synthesis

• UDP-Glucose synthesis UDP-glucose phosphorylase
• A primer is required for glycogen synthesis
  (glycogenin or a fragment of glycogen)
• Glc units are added to the either the existing
  glycogen chains or glycogenin (enzyme glycogen
  synthase).
• C-4 is the nonreducing end of glycogen chaain,
  new glucose molecules are always added to this
  nonreducing terminus.
• Elongation of glucose chains
• Creating branches in glycogen (enzyme
  transferase)
• Branches have 2 functions
• 1. An increase in solubility of glycogen molecule
• 2. Increase the rate of glycogen synthesis

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How Is Glycogen Synthesis Regulated?

• Glucagon and Epi promote glycogenolysis, at the
  same time they inhibit glycogen synthesis.
• Both effects are mediated by cAMP and cAMP
  dependent protein kinase.
• Regulated enzyme glycogen synthase
• PKA and other kinases phosphorylate the enzyme.
• Plated form is inactive (b).
• Protein kinase (Ser phosphorylated)
• a form active (no phosphorylation)
• b form inactive (phosphorylated)
• Steps after the binding of the hormones
• Epi ? liver cell recep.
• Adenylate cyclase activity
• cAMP
• cAMP ? Pkinase which phosphorylates and
  inactivates glycogen synthase

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Coordinate Control of Glycogen Breakdown and
Synthesis by cAMP Cascades
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Breakdown and synthesis are reciprocally regulated

• Hormone -triggered cAMP cascade acting through
  PKA
• Glycogen breakdown and synthesis are
  reciprocally regulated.
• A. Glycogen degradation
• B. Glycogen synthesis
• Inactive forms are shown in red, active forms are
  shown in green.
• Phosphorylase kinase also inactivates glycogen
  synthase.
• PP1(protein phosphatase 1) reverses the
  regulatory effects of glycogen metabolism.
• PKA action is reversed by phosphatases
• PP1 inactivates phosphorylase kinase and
  phosphorylase a by dephosphorylating these
  enzymes.
• PP1 also removes P groups from glycogen synthase
  b to a form (more active)

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• PP1 has 3 components
• P1
• Rgl
• I
• How is phosphatase activity of PP1 regulated?
• Rgl phosphorylation by PKA prevents its binding
  to PP1, therefore activation of cAMP cascade
  leads to the inactivation of PP1 because it
  cannot bind to its substrate.
• Phosphorylation of inhibitor 1 by protein kinase
  A blocks catalysis by PP1.
• Thus, Epi increases glycogen breakdown by making
  phosphorylase a and decreases glycogen synthesis
  by making inactive phosphatase.

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Insulin stimulates glycogen synthesis by
activating protein phosphatase 1

• When blood glucose is high, insulin is
  stimulated.
• Figure 21.20
• Activated insulin-sensitive protein kinase makes
  activated protein phosphatase
• The consequent dephosphorylation of glycogen
  synthase, phosphorylase kinase and phosphorylase
  promotes glycogen synthesis and blocks its
  degradation!

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Phosphorylation of the enzymes is regulated by
hormones

• Phosphorylated groups can be removed by
  phosphatases, therefore the action of
  phosphatases always opposes kinases. If kinases
  activity is greater than activity of phosphatase
  enzyme is in the phosphorylated mode.
• Insulin, Glucagon, Epi are three important
  hormones which affect glycogen metabolism!

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Glycogen metabolism in the liver regulates the
blood-glucose level

• After a carbohydrate-rich meal blood glucose
  increases.
• Insulin is the primary signal for glycogen
  synthesis.
• Blood glucose level 80-120 mg/dL (4.4-6.7 mM)
• The liver senses the concentration of blood
  glucose either release or takes up glucose.
• Glucose infusion changes the enzymes involved in
  glycogen metabolism

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Events Followed by Glucose Infusion
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Glucose regulation of glycogen metabolism

• Phosphorylase a is the glucose sensor in liver
  cells
• Glucose is high
• Binding of Glc converts R----gtT
• PP1 is released
• Inactivation of glycogen breakdown and the
  activation of glycogen synthesis take place..

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Glucose regulation of glycogen metabolism
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Glucose regulation of glycogen metabolism
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Reciprocal Regulation of Glycogen Synthase and
Glycogen Phosphorylase
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Summary of the Regulation of Glycogen Synthesis
and Degradation

• Synthesis and degradation are regulated by the
  same hormonal signals!
• Increase insulin ? Increase glycogen synthesis
• Increase glucagon and Epi ? glycogen degradation
• cAMP increase ? in response to Epi and glucogon
  release
• cAMP decrease ? in the presence of insulin!
• Key enzymes are phophorylated by a family of
  kinases, some of which are cAMP dependent.
• Phosphorylation of an enzyme causes 3D change
  that affects the active site. It may either
  increase or decrease its activity depending on
  the type of enzyme.

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Glycogen storage diseases

• Glycogen metabolism is a finely controlled
  system.
• It is not surprising that genetically determined
  enzyme deficiencies result in disease state.
• Genetic diseases are in fact valuable research
  tools for us.
• There are VIII glycogen storage diseases
• Type I and Type V will be covered
• Type I Von Gierke Disease, Glc 6-phosphatase is
  missing.
• Type V McArdle Disease, Phosphorylase is
  missing.

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