Describe the regulation of glycogen metabolismSolutionThe im

Describe the regulation of glycogen metabolism.

Solution

The important enzymes of glycogen metabolism are – glycogen phosphorylase and glycogen synthase.

Glycogen phosphorylase which breaks down glycogen into glucose molecules (makes ATP), becomes active in low energy states, while glycogen synthase which synthesizes glycogen from glucose molecules becomes active in high energy states of the body (thereby stores energy).

The glycogen metabolism is regulated both allosterically and covalently.

Covalent modification: Covalent modification is controlled by hormones insulin, glucagon and epinephrine.

Insulin responds to high sugar levels in blood and acts to lower the blood sugar levels by activating glycogen synthase and by deactivating enzyme glycogen phosphorylase. While glucagon responds to low blood sugar levels and acts to increase it by activating enzyme glycogen phosphorylase and by deactivating enzyme glycogen synthase. Epinephrine responds in emergency situations and like glucagon, it also increases blood sugar levels.

Glucagon and epinephrine stimulates an enzyme known as phosphorylase kinase which phosphorylates the inactive form of glycogen phosphorylase (T form) to an active form (R form). Insulin stimulates another enzyme phosphor protein phosphatase which converts the active form to the4 inactive form of glycogen phosphorylase.

Allosteric regulation: In allosteric regulation, low energy state as indicated by high levels of AMP, which activates T form of glycogen phosphorylase to R form. High energy as indicated by high levels of ATP or Glucose-6-phosphate allosterically inhibit glycogen phosphorylase such that R form gets converted to T form. Also when the concentration of glucose is more in a cell, glycogen phosphorylase becomes less active than when glucose concentration is limited.

Glucagon and epinephrine stimulate phosphorylation to activate glycogen phosphorylase and inhibit glycogen synthase thereby increasing blood glucose levels by breaking down glycogen.

Insulin stimulates dephosphorylation to deactivate glycogen phosphorylase and activate glycogen synthase thereby reducing blood glucose levels and initiating glycogen formation. Glucose – 6- phosphate activates dephosphorylation to activate glycogen synthase, even when there is no insulin for covalent modification.