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Metabolism of Glycogen in Animals
synthesis and breakdown of glycogen
Undergraduate 3

Additional Biochemistry Flashcards




Storing Glucose
  • if the amount of glucose stored in the cell was dissolved freely then it would change the osmotic properties of the cell---.4M
  • storing glucose as a poylmer instead reduces the concentation to .01uM
  • glycogen in muscle provides a source of quick energy and glycogen is exhausetd in less than an hour during activity
  • liver glycogen is reservoir for tissues like the brain, which can't use fatty acids as fuel- depleted in 12-25hr
  • fat stores more energy than glycogen but can't be converted to glucose
Glycogen Breakdown is Catalyzed by Glycogen Phosphorylase
  • In muscle an liver, glycogen suplies glycolytic pathway with glucose through the action of three enzymes
  • glycogen phosphorylase catalyzes the reaction in which alpha 1-4 glycosidic bonds between 2 glucose residues at non reducing ends are attacked by Pi, removing a terminal glucose as Glucose 1-phosphate
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  • in the phosphorylysis reaction some energy of the glycosidic bond is preserved in forming a phosphate bond
  • pyridoxal phosphate is an essential cofactor here. its phosphate group acts as a a general acid catalyst, promoting attack by Pi on the glycosidic bond
  • glycogen phosphorylase acts until its gets four residues away from an alpha 1-6 branch point
  • at this point a debranching enzyme catalyzes two reactions that transfer branches
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Glucose 1-Phosphate Enters Glyolysis or, in liver, replenishes Blood Glucose
  • glucose 1-phosphate interconversion to Glucose 6-Phosphate by phosphoglucomutase
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  • 1)enzyme is phosphorylated on a ser residue
  • 2)enzyme donates its phosphoryl group to glucose 1-phosphate...=glucose 1,6 bisphosphate
  • 3)phosphoryl group at C-1 of glucose 16 BP is transferred back to the enzyme


  • G6P formed in skeletal muscle can enter glycolysis and serve as an energy source
  • in liver, glycogen breakdown releases glucose into the blood when glucose lvl drops. requires glucose 6 phosphatase and various glucose transporters. muscle doesnt have this enzyme  and therefore cannot convert G6P to glucose and get it into teh blood
Sugar Nucleotide UDP-Glucose Donates Glucose for Glycogen Synthesis
  • many reactions in which hexoses are transformed involve sugar nucleotides
  • these are compounds in which anomeric carbon of sugar is activated by attachment to a nucleotide thru phosphate ester linkage
  • used as substrated for polymerization into many types of saccharides (mono, di, poly, starch, cellulose etc)
Properties of Sugar Nucleotides which Make them Good for Biosynthesis
  1. their formation is irreversible, making their pathways irreversible. The free energy of the reactions is always large and negative, the products are removed quickly and synthetic reactions are pushed
  2. the nucleotide moiety of sugar nucleotides can undergo many noncovalent interactions with enzymes, allowing for additional free energy of binding to contribute to reaction favorability
  3. the nucleotide group(UMP, ATP) is a good leaving group, facilitating nucleophilic attack by activating sugar carbon to which it is attached
  4. cells can sequester hexoses with nucleotide groups on them from each other based on the group and fate of the molecule
Other Paths to Glycogen
  • Glycogen synthesis happens in almost all animal tissues, but mostly in liver and skeletal muscles
  • starting point of glycoge synthesis is G6P
  • catalyzed by hexokinaseI and II in muscle and IV in liver: D-Glucose+ATP-> D-Glucose 6-Phosphate+ADP
  • some glucose is first taken up by erythrocytes and converted to lactate via glycolysis
  • lactate then goes to liver and is converted to G6P by gluconeogenesis
  • to initiate glycogen synthesis, G6P is converted to G1P using phosphoglucomutase
  • G1P + UTP ->UDP-glucose + PPi
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  • UDP glucose -  donor of glucose residues in the reaction catalyzed by glycogen synthase.. promotes transfer of glucose residue from UDP glucose to nonreducing end of branched glycogen molecule
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