Glycogen Degradation

Low blood [glucose] signals release of glucagon & epinephrine

Glucagon and Epinephrine bind to a 7TM receptor

A G-protein dissociates from the receptor and exchanges GDP for GTP

The G-protein and GTP complex activates Adenylyl Cyclase

Adenylyl Cyclase begins converting ATP to cAMP

As [cAMP] increases, cAMP-dependent Protein Kinase A is activated

PKA phosphorylates Phosphorylase Kinase

Separately, a nerve impulse, muscle contraction, or hormone releases Ca2+

Phosphorylase Kinase is activated and phosphorylates Glycogen Phosphorylase B, turning it into A

Glycogen Phosphorylase begins cleaving alpha-1,4 bonds and releasing glucose from glycogen

Signal dies down, hormone is no longer present

G-protein GTPase activity hydrolyzes GTP, the complex detaches from adenylyl cyclase, deactivitying it.

Phosphodiesterases work to convert cAMP to AMP

Phosphoprotein Phosphatase 1 dephosphorylates Phosphorylase Kinase and Glycogen Phosphorylase