Final answer:
When activated by a GPCR, adenylyl cyclase converts ATP to cAMP, which is part of the signal transduction pathway leading to cellular responses.
Step-by-step explanation:
When activated by a GPCR (G-protein-coupled receptor), adenylyl cyclase converts ATP to cAMP. Adenylyl cyclase is a membrane-bound enzyme that is activated by a G-protein upon a signal from a GPCR. This enzyme plays a key role in the signal transduction pathway by catalyzing the conversion of ATP (adenosine triphosphate) to cAMP (cyclic adenosine monophosphate), which is a secondary messenger. cAMP then goes on to activate a group of proteins known as protein kinases. These kinases are responsible for the phosphorylation of substrate molecules, which causes a structural change and activates the substrate, thereby mediating changes in cellular processes.