Glycogen Metabolism

• Glucose


  A six carbon molecule that's used to make energy in the form of adenosine triphosphate or ATP
• Glucose is such an important energy source that our body stores excess glucose in skeletal muscle cells and liver cells in the form of glycogen
• Glycogen


  • An enormous molecule or polymer that's made up of glucose molecules linked together by glycosidic bonds
  • Has a main chain with multiple branches sprouting off
  • Allows glycogen to be compact and capable of rapid addition and removal of glucoses
• Glycogen synthesis


  1. Attaching a uridine diphosphate or UDP molecule to glucose

  2. Attaching the glucose part of the UDP glucose molecule to a glycogen primer called glycogenin forming a short linear glycogen chain

  3. Adding more glucose molecules to the primer

  4. Adding branches to the glycogen molecule
• Phosphoglucomutase


  Enzyme that moves the phosphate from the sixth carbon of glucose 6-phosphate to the first carbon creating glucose 1
• UDP-glucose pyrophosphorylase


  Enzyme that cuts phosphate molecules off of UTP which gives the energy necessary to complete the reaction to create UDP-glucose
• Glycogen synthase


  Enzyme that catalyzes the attachment of the glucose part of UDP-glucose to another glucose residue at the end of the glycogen branch forming an alpha 1-4 glycosidic bond
• Glycogenin


  Protein that catalyzes the attachment of four glucoses to itself creating a short chain connected with alpha 1-4 glycosidic bonds, allowing glycogen synthase to elongate this short chain
• Branching enzyme


  Enzyme that goes to the end of the glycogen chain, cuts off a chain of about six to eight glucose residues in length, and attaches that chain to the side of the linear glycogen strand by creating an alpha 1-6 glycosidic bond
• Glycogen breakdown


  1. Glycogen phosphorylase cleaves the alpha 1-4 bonds between individual glucose residues and catalyzes the transfer of a phosphate group to the free glucose

  2. De-branching enzyme cuts off glycogen branches, transferring three out of the four glucose molecules off of the branch and reattaching them to the linear glycogen chain instead

  3. Alpha 1-6 glucosidase component of the de-branching enzyme cleaves the alpha-1-6 glycosidic bond and releases a free glucose
• Glucose 6-phosphatase


  Enzyme in liver cells that removes the phosphate off of the sixth carbon, releasing free glucose into the bloodstream
• Insulin


  Activates protein phosphatase which removes phosphates from glycogen synthase (making it active) and glycogen phosphorylase (making it inactive), promoting glycogen synthesis and decreasing its breakdown
• Glucagon


  Activates adenylyl cyclase which converts ATP to cAMP, activating protein kinase A which adds phosphates to glycogen phosphorylase kinase (activating it) and glycogen synthase (deactivating it), promoting glycogen breakdown and decreasing its synthesis
• Glycogen


  • Multi-branched compact structure
  • Made of alpha-1-4 glycosidic bonds between glucose molecules and alpha-1-6 bonds at branching points
  • Major form of glucose storage in the body, primarily in liver and skeletal muscle cells
• After a meal, high insulin levels promote glycogen synthesis, whereas during fasting, high glucagon and epinephrine levels promote glycogen breakdown