Which statement explains how the same second-messenger molecule, cAMP, can be involved in different pathways in different cells?

Question

BlackTom AI · Accepted Answer

Second messengers like cAMP operate by activating specific effector proteins (for example, Protein Kinase A (PKA), EPAC, and cyclic nucleotide-gated channels), and those effectors differ among cell types.

1) Relevant knowledge points and concepts
- Signal transduction often uses second messengers (cAMP) generated from ATP by adenylyl cyclase after a ligand-bound GPCR activates Gs.
- cAMP does not itself specify the final response; it modulates downstream effector proteins.
- Cells express different repertoires of effector proteins, isoforms, scaffolding proteins (AKAPs), and substrates, so identical cAMP signals can yield divergent outcomes.

2) Step-by-step reasoning
- A ligand binds a receptor → receptor activates G protein (Gs) → Gs activates adenylyl cyclase → ATP converted to cAMP.
- cAMP binds regulatory subunits of PKA, causing release of active catalytic subunits; or cAMP activates EPAC or ion channels.
- Active effectors (PKA catalytic subunits, EPAC, etc.) phosphorylate or interact with cell-specific target proteins.
- Because each cell type has a unique set of downstream targets, scaffolds, and isoforms, the biochemical changes produced by those effectors differ.

3) Intermediate mechanism steps (example)
- ATP --(adenylyl cyclase)--> cAMP.
- cAMP + PKA regulatory subunits → conformational change → release of PKA catalytic subunits.
- PKA catalytic subunits phosphorylate substrates (transcription factors, metabolic enzymes) specific to that cell.

4) Why the correct answer is correct
The correct choice states that cells contain different proteins that carry out different functions but respond to the same small molecule. That captures the core explanation: the same cAMP signal engages distinct effectors and substrates in different cells, producing different physiological responses.

5) Why other options are incorrect
- Option B: If different pathways had the same target protein, responses would tend to be similar, not different. It fails to account for cell-specific effectors/substrates.
- Option C: Temporal separation alone doesn’t explain different outcomes originating from the same messenger in different cell types.
- Option D: Vague and misleading—while DNA encodes protein expression, the mechanism of differing responses is the specific proteins and pathways present, not a generic “program” to ignore signals.

6) Conclusion
Therefore, the correct explanation is: "The cells contain different proteins that carry out different functions but respond to the same small molecule." This matches the provided answer.

Which statement explains how the same second-messenger molecule, cAMP, can be involved in different pathways in different cells? 单项选择题

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