Regulation

Glycogen phosphorylase is the key enzyme in glycogenolysis as it is rate-limiting; the remaining enzymes in the pathway are near equilibrium and non-saturated. Consequently, agents which alter glycogen phosphorylase activity alter the rate of glycogenolysis.

The agents are best considered in terms of site as there is slight variation:

• the liver, where glycogenolysis is tailored to plasma blood glucose concentration:
  • hormones:
    • glucagon and adrenaline increase glycogenolysis
    • insulin decreases glycogenolysis
  • local factors:
  • the intracellular glucose concentration feedsback to inhibit glycogen phosphorylase, so providing fine-control over glycogenolysis
    • calcium increases glycogenolysis
    • low energy state within the cell increases glycogenolysis
• in skeletal muscle:
  • hormones: as above, but glucagon has minimal effect
  • local factors: as above, but calcium release during muscular contraction has a more significant role in rapid activation of glycogen phosphorylase