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Flashcards in Glycogen Metabolism Deck (16)
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describe the location of glycogen in the body

  • the largest amount of glycogen is found in skeletal muscle (400g)
  • the highest concentration of glycogen is found in the liver (100g)
  • glycogen storage diseases can affect both


what activates glycogen synthesis?

  • insulin activates glycogen synthesis in both liver and muscle
  • glycogen synthase is the regulated enzyme and is active when it is dephosphorylated
  • insulin activates protein phosphatase 1 which cleaves phosphates that were added by PKA


what inhibits glycogen synthesis in the liver and muscle?

  • glucagon and epinephrine activate PKA which phosphorylates key regulatory enzymes
  • glycogen synthase is inhibited when it is phosphorylated


describe the molecules involved in glycogen synthesis


describe the enzyme involved in converting glucose 6-P and glucose 1-P 

  • phosphoglucomutase catalyzes the change in position of the phosphate; the direction of the reversible rxn is dependent on the concentrations


describe UDP glucose

  • UDP glucose is the highly activated form of glucose which is used for synthesis of glycogen as well as other glycosylations
  • UDP-glucose is synthesized from G1P and UTP (UDP-glucose pyrophosphorylase)


explain why glycogen synthase needs a primer

  • glycogen synthase cannot link UDP-glucose to each other without a primer
    • in most cases the primer is in the form of branched glycogen chains that have not been degraded
  • the protein glycogenin is needed for de-novo synthesis after all glycogen chains are degraded and will form a primer that can be used by glycogen synthase
  • glycogenin stays inside the newly formed glycogen granule


describe the steps of glycogen synthesis

  • glycogenin is a self-glucosylating enzyme that uses UDP-glucose and adds approx. 8 glucose units in tandem to its own tyrosine residue
  • glycogen synthase recognizes the precursor and is able to form a1,4 bonds at the nonreducing ends
  • the branching enzyme (4:6 transferase) removes a chain of 6-8 glucosyl residues from the nonreducing end by cleavage of an a1-4 bond and attaches it to a non-terminal glucosyl residue by an a1-6 linkage


contrast glycogen degradation in liver vs muscle

  • liver:
    • glycogen degradation in hepatocytes needs hormonal activation by glucagon and epinephrine which occurs only during fasting and stress
    • the main purpose is to increase blood glucose
  • muscle:
    • glycogen degradation in skeletal muscle and is activated by Ca during contraction
    • in flight and fight situations: epinephrine accelerates muscle glycogen degradation
    • the main purpose is to produce ATP


describe the steps of glycogen degradation


describe the regulation of phosphorylation 


summarize the covalent modifications of enzymes and activation

  • glycogen synthase is activated by dephosphorylation
    • protein kinase 1, insulin
  • glycogen phosphorylase is activated by phosphorylation
    • glycogen phosphorylase kinase
  • glycogen phosphorylase kinase is activated by phosphorylation
    • PKA, epinephrine
    • glucagon only in liver


describe the allosteric regulation of glycogen metabolism in liver and muscle

  • Glucose 6-P activates glycogen synthase and inhibits glycogen phosphorylase in liver and muscle
  • ATP inhibits phosphorylase in liver and muscle
  • glucose inhibits hepatic glycogen phosphorylase


  • AMP activates muscle glycogen phosphorylase


describe the steps of glycogen degredation and the coenzyme needed

  • glycogen phosphorylase performs phosphorolytical cleavage of glycogen to G1P
  • inorganic phosphate is used instead of water
  • PLP is needed for this reaction
    • PLP is a coenzyme which is formed from vit. B6


describe the function of the debranching enzyme


  • the 4:4 transferase activity forms longer branches
  • the 1:6 glucosidase activity cleaves the a(1,6) bond and forms 1 molecule of free glucose
  • longer branches with only a(1,4) linkages are again available for glycogen phosphorylase


summarize the function of PKA

glucagon and epinephrine initiate PKA activity