cAMP is generated as a second messenger through adenylyl cyclase activation following ligation of GPCRs by different ligands such as hormones, prostaglandins, autacoids, and pharmacologic agents. The receptor-mediated activation of adenylyl cyclase catalyzes the conversion of AMP to cAMP. Activated adenylyl cyclase converts ATP to cAMP by removing two phosphate groups forming a cyclic structure. cAMP acts as a second messenger by binding to and activating PKA. Once bound, the activated PKA is ready to phosphorylate the target proteins.