Epinephrine Pathway (Basic)

Epinephrine Pathway (Basic)

Binding of epinephrine to a G Protein Coupled Receptor (GPCR) causes exchange of GDP for GTP on the alpha subunit of the G protein. Upon exchange, the G protein is activated and binds to an adenylyl cyclase. Adenylyl cyclase thus utilizes ATP for the formation of cAMP. Such cAMP binds to the regulatory subunits of a cAMP dependent protein kinase (PKA). PKA is activated and releases its catalytic subunits.

The catalytic subunits phosphorylate, using ATP hydrolysis, phosphorylase kinase. Once phosphorylated, the phosphorylase kinase is now activated and finds glycogen phosphorylase. Glycogen phosphorylase is then phosphorylated by phosphorylase kinase and now activated.

Now that glycogen phosphorylase is activated, it can change the glycogen stored in the liver into glucose. Such pathway is amplified by 100x via each step. In other words, one molecule of epinephrine (ligand) causes the activation of 100 PKA, which activates 10,000 phosphorylase kinase, which activates 1,000,000 glycogen phosphorylase!

Such pathway usually is quick and brief. However, it does give an extremely high boost of energy when needed (such as running away from a predator). Once the need for escape is over, the GTP on the G Protein is hydrolyzed back to GDP, which causes it to reform with its other subunits and return to the receptor, ready for its next activation by a ligand.