3. Glycogen synthesis and degradation

Last updated on November 19, 2018 at 17:16

Summary

  • Glycogen is stored in virtually all tissues, but mainly muscle and liver
  • Is converted to glucose 1-phosphate
  • Can be converted to G6P for glycolysis in liver and muscle, or into glucose in the liver
  • Is a large molecule composed of up to 50 000 glucose units bound by α1 -> 4 bonds, with α1 -> 6 bonds every 8-12 residues to create branches.
  • Has one reducing end and many non-reducing ends
  • The reducing end is attached to a protein called glycogenin.

Synthesis

The pathway of synthesis goes as follows: glucose -> G6P -> G1P -> UDP-glucose. The glucose unit of UDP-glucose is then attached to a non-reducing end of glycogen by glycogen synthase.

Degradation of glycogen.

Glycogen synthase can only catalyse the creation of α1 -> 4 bonds. For the creation of the branches in the glycogen molecule, glycogen branching enzyme is needed. The advantage of these branches is that the number of non-reducing ends are increased from 1 to many. Glycogen synthase and glycogen phosphorylase, which breaks down glycogen, can only work on non-reducing ends. By increasing the number of these ends, the enzymes can work at many ends simultaneously and massively increase the speed of degradation and synthesis.

Degradation

Glycogen is degraded by glycogen phosphorylase and debranching enzyme. The former converts the glucose units into glucose 1-phosphate by breaking of α1 -> 4 bonds. Debranching enzymes has two activities, transferase activity and glucosidase activity. When glycogen phosphorylase has reached the last 4 glucose units of a branch, the transferase activity of debranching enzyme takes the outermost 3 glucose units and puts them on the “main chain”, while leaving a branch of just 1 glucose. The glucosidase activity of debranching enzyme converts the last glucose on the branch into glucose 1-phosphate.

Glucose 1-phosphate can be converted into G6P, which can be further converted into glucose (in liver and kidney only), or go into the glycolysis.

Regulation

Regulation is covered in the next chapter.


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2. Gluconeogenesis

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4. Regulation of glycogen synthesis and degradation

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