17.3.1. Regulation of glycogenolysis is by the phosphorylation cascades
The conversion of inactive glycogen phosphorylase ‘b’ to active glycogen phosphorylase ‘a’ is dependent upon the phosphorylation cascade through cyclic AMP-dependant Protein Kinase A (PKA) which is a Ser/Thr kinase which in turn is activated by cyclic AMP.
Both hormones Adrenaline (epinephrine, which is released in response to a threat or stress - the ‘fight-or-flight’ response) and glucagon, (which is released by pancreatic alpha cells in response to low blood glucose levels), stimulate glycogenolysis by binding to their respective receptors (both of which are G-protein coupled receptors) which activates membrane localized protein adenyl cyclase. Adenyl cyclase forms cAMP from ATP then activates PKA. Which stimulates glycogen phosphorylase and inhibits glycogen synthase.
Insulin have counter effect on glycogenolysis. It inhibits glycogenolysis by activating protein phosphatase 1 (PP1) and the enzyme phosphodiesterase which both contribute to the inactivation of glycogen phosphorylase.
Calcium ions or cyclic AMP (cAMP) act as secondary messengers which is an example of negative control. The calcium ions activate phosphorylase kinase which activates glycogen phosphorylase and inhibits glycogen synthase.