Which of the following is considered a second messenger in cell signaling?

• A. ATP
• B. cAMP
• C. G protein
• D. Receptor protein

Answer: (B)

The correct response identifies cAMP, or cyclic adenosine monophosphate, as a second messenger in cell signaling. Second messengers are molecules that transmit signals received by cell surface receptors to target proteins inside the cell, leading to a physiological response.

In the case of cAMP, it is produced from ATP by the enzyme adenylyl cyclase in response to various signals, such as hormones or neurotransmitters binding to G protein-coupled receptors. Once formed, cAMP activates protein kinase A (PKA), which then phosphorylates various target proteins to modulate their activity and bring about changes in cellular function.

The other options do not fit the definition of a second messenger. ATP is primarily an energy carrier and not involved directly as a signaling intermediary. G proteins are molecular switches that relay signals from receptors to intracellular targets but do not serve as second messengers themselves. Receptor proteins are essential for initially receiving signals but do not act downstream in the signaling cascade as second messengers do. Therefore, cAMP plays a pivotal role in the relay of information within the cell following initial receptor activation.