Liver cell ①pE Adenylyl G protein/② ATP. Activates Receptor GTP kinase-A FIGURE 53.7 The action of epinephrine on a liver cell.(D) Epinephrine binds to specific receptor proteins on the cell surface. ()Acting through intermediary G proteins, the hormone-bound receptor activates the enzyme adenylyl cyclase, which converts ATP into cyclic AMP (cAMP). ) Cyclic AMP performs as a second messenger and activates protein kinase-A, an enzyme that was previously present in an inactive form. (4)Protein kinase-A phosphorylates and thereby activates the enzyme phosphorylase. which catalyzes the hydrolysis of glycogen into glucose