What is the function of cAMP in cellular signaling?

The correct answer is that cAMP acts as a second messenger in cellular signaling. In the context of physiological processes, a second messenger is a molecule that transmits signals received by a receptor from outside a cell to target molecules within the cell, thereby initiating a physiological response.

cAMP (cyclic adenosine monophosphate) is synthesized from ATP by the enzyme adenylate cyclase, which is activated by various receptors when a ligand, such as a hormone, binds to them. Once produced, cAMP activates protein kinase A (PKA), leading to the phosphorylation of various target proteins that mediate the cellular response to the original signal. This is a critical mechanism in many cellular processes, including metabolic regulation, gene expression, and cell growth.

In contrast, a receptor is a cellular structure that receives a signal but does not act as a messenger itself. A first messenger is typically the extracellular signaling molecule (like a hormone) that initially binds to the receptor, while feedback inhibition refers to a regulatory mechanism where the end product of a process inhibits an upstream process to prevent overactivity. These concepts provide necessary context for understanding the specific and essential role cAMP plays as a second messenger in cellular signaling pathways.