Cyclic AMP (cAMP) or cyclic adenosine monophosphate acts as a secondary messenger and is used in cell signalling. It can be deactivated by phosphodiesterase (PDE) into AMP. cAMP can activate protein kinase A (PKA) by binding to it's regulatory subunits to produce catalytic subunits. These catalytic subunits can then go on to phosphorylate other proteins, creating a singalling cascade. ATP is converted to cAMP by adenylyl cyclase, cAMP is then used in a range of signalling pathways.