The Second Messenger Model

• One mechanism of hormone action is known as the second messenger model. This mechanism is used by two hormones involved in the regulation of blood glucose concentration, adrenaline and glucagon.
• In the mechanism for adrenaline, firstly the adrenaline binds to a transmembrane protein receptor within the cell-surface membrane of a liver cell. This binding causes the protein to change shape on the inside of the membrane.
• The change in protein shape leads to the activation of an enzyme called adenyl cyclase. The activated adenyl cyclase converts ATP into cyclic AMP (cAMP), which then acts as a 'second messenger' that binds to protein kinase enzyme, changing its shape and therefore activating it.
• Once the protein kinase enzyme has been activated, it will catalyse the conversion of glycogen to glucose, which moves out of the liver cell by facilitated diffusion and into the blood, through channel proteins.