Which statement about second messengers in GPCR signaling is correct?

• A. They are not involved in GPCR signaling.
• B. They are enzymes that catalyze reactions.
• C. They are small molecules like cAMP, IP3, and DAG that relay signals and amplify responses.
• D. They are transcription factors.

Answer: (C)

In GPCR signaling, second messengers are small, diffusible molecules that carry the signal from the activated receptor deeper into the cell and amplify the response. Examples like cyclic AMP, IP3, and DAG are produced when a GPCR activates G proteins, which then stimulate enzymes such as adenylyl cyclase or phospholipase C. cAMP activates PKA, IP3 triggers Ca2+ release, and DAG along with Ca2+ activates PKC, creating multiple downstream effects. A key feature is amplification: one receptor activation can generate many second-messenger molecules, leading to a large cellular response from a single signal. These messengers are not enzymes themselves, nor transcription factors, which is why they fit the description of relaying and amplifying signals rather than performing catalytic reactions or directly regulating transcription.