Glycogen degradation I

Summary

The video introduces the enzymes and steps involved in glycogen breakdown. It focuses on how glycogen’s structure determines which enzymes can act on it; regulation is left for a later video.

Glycogen and its role

• Glycogen is a highly branched polymer made entirely of glucose.
• Its glucose units are joined mainly by α-1,4 linkages, with α-1,6 linkages at branch points.
• Glycogen is stored in granules. Typical granules are described as roughly 10–40 nm in diameter and containing around 55,000 glucose units.
• The largest glycogen stores are in the liver and muscles, but they serve different purposes:
  • Liver glycogen can be broken down to help maintain blood glucose, supplying tissues such as the brain and red blood cells.
  • Muscle glycogen is used locally to provide energy for muscle activity.
• The protein glycogenin is located at the center of a glycogen granule and acts as a primer during glycogen synthesis.

How glycogen is broken down

1. Glycogen phosphorylase removes glucose from nonreducing ends.

   • It breaks α-1,4 linkages by adding inorganic phosphate, releasing glucose-1-phosphate (G1P).
   • This process is phosphorolysis rather than ordinary hydrolysis.
   • The enzyme acts on straight-chain sections but stops when it is four glucose residues from a branch point.

2. Phosphoglucomutase converts G1P to glucose-6-phosphate (G6P).

   • G6P can enter pathways such as glycolysis or the pentose phosphate pathway.
   • The enzyme rearranges the position of the phosphate. A phosphorylated serine residue in its active site is part of this process.

3. The glycogen branch is remodeled so phosphorylase can continue.

   • A transferase moves a segment of glucose residues from the branch to another chain, extending a straight section.
   • An α-1,6-glucosidase then hydrolyzes the remaining α-1,6-linked glucose at the branch point, releasing free glucose.
   • These two activities are components of the glycogen debranching enzyme.
   • Once the branch is removed, glycogen phosphorylase can continue degrading the newly extended chain.

4. The free glucose released at a branch can be phosphorylated.

   • The lecture says hexokinase can convert it to G6P so it can enter metabolic pathways.
   • Whether glucose is instead released from the cell depends on the tissue. The liver can export glucose, while muscle generally uses glucose within the cell.

Liver-specific step

• The liver contains glucose-6-phosphatase, which removes the phosphate from G6P and produces free glucose.
• This enzyme is absent from muscle. As a result, the liver can release glucose into the bloodstream, while muscle retains its glycogen-derived fuel for local use.

Speakers and sources featured

• Speaker: One instructor or narrator explains the material alongside diagrams and a metabolic map.
• Other sources: No additional speakers or named external sources are identified. Background music is audible.