Final answer:
The enzyme responsible for the synthesis of cAMP is adenylyl cyclase. It is activated by a G-protein following the reception of an extracellular signal, and it converts ATP into cAMP which acts as a secondary messenger to regulate metabolic pathways through the activation of protein kinases.
Step-by-step explanation:
The enzyme that catalyzes the synthesis of cyclic AMP (cAMP) is adenylyl cyclase. This membrane-bound enzyme converts ATP to cAMP as a result of G-protein activation. The process begins when an extracellular signal molecule (such as a hormone) binds to a cell surface receptor. This binding initiates a conformational change that activates a G-protein inside the cell. The activated G-protein then interacts with and activates adenylyl cyclase. Upon activation, adenylyl cyclase catalyzes the conversion of ATP to cAMP, which is the secondary messenger in this pathway. cAMP plays a crucial role in cell signaling by activating cAMP-dependent protein kinase (A-kinase), which in turn regulates various metabolic pathways via the phosphorylation of target proteins. Therefore, the correct answer to the question is D) adenylate cyclase.