ABSTRACT
Glycogen degradation and glycogen synthesis are controlled both by allosteric regulation and by hormonal control. Phosphorylase exists in two interchangeable forms; active phosphorylase a and a normally inactive phosphorylase b. Glycogen metabolism is tightly controlled by hormones. When epinephrine and glucagon levels in the bloodstream fall again, the hormone dissociates from the receptor, no more cyclic Amp (cAMP) is made and existing cAMP is converted to 5' AMP by cAMP phosphodiesterase, an enzyme that is constantly active in the cell. Insulin is released into the bloodstream by the ß cells of the pancreas when blood glucose levels are high after feeding, and stimulates glycogen synthesis to store excess glucose as glycogen. Insulin binds to its receptor in the plasma membrane and activates it. This receptor has tyrosine kinase activity. Its activation leads to the activation of an insulin-responsive protein kinase that then phosphorylates protein phosphatase I, thus activating it.
