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G-Protein Coupled Receptors Quiz
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Activation of which G-protein alpha subunit most directly leads to an increase in intracellular cyclic AMP (cAMP)?
- A G-alpha s subunit activating adenylyl cyclase You picked ✓
- B G-alpha 12/13 subunit activating RhoGEF You picked ✕
- C G-alpha t subunit activating cyclic GMP phosphodiesterase You picked ✕
- D G-alpha q subunit activating phospholipase C You picked ✕
- E G-alpha i subunit inhibiting adenylyl cyclase You picked ✕
Correct!
Incorrect
The answer is A: G-alpha s subunit activating adenylyl cyclase
Explanation
Why A is correct
See Signal Transduction. Option A is correct because the Gs (stimulatory) alpha subunit activates the enzyme adenylyl cyclase, which converts ATP into cyclic AMP (cAMP), a second messenger, which is a small molecule that relays the signal inside the cell. The cAMP then activates protein kinase A (PKA) by binding to it and releasing its regulatory subunits, and PKA then adds phosphate groups to other proteins, switching them on or off. When a GPCR is activated, it makes the G protein swap GDP for GTP, and the GTP-bound alpha subunit separates from the beta-gamma complex and goes on to act on its effector enzyme. The three main alpha subunit types therefore have different effects: Gs raises cAMP, Gi lowers it, and Gq raises calcium through IP3. Memory aid: s for stimulate and i for inhibit adenylyl cyclase.
- Gs: the stimulatory alpha subunit activates adenylyl cyclase.
- Product: adenylyl cyclase converts ATP into cAMP, which activates protein kinase A.
- Other subunits: Gi lowers cAMP and Gq raises calcium through IP3.
Why the other options are incorrect
- B. G-alpha 12/13 subunit activating RhoGEF: The G-alpha 12/13 subunit activates RhoGEF and downstream Rho signalling, which regulates the cytoskeleton rather than adenylyl cyclase, so it does not raise cAMP. Your answer
- C. G-alpha t subunit activating cyclic GMP phosphodiesterase: Transducin, the G-alpha t subunit, activates cyclic GMP phosphodiesterase in photoreceptor cells, which lowers cGMP; it does not act on adenylyl cyclase or raise cAMP. Your answer
- D. G-alpha q subunit activating phospholipase C: Gq alpha activates phospholipase C, which leads to IP3, DAG and a rise in intracellular calcium. It does not raise cAMP. Your answer
- E. G-alpha i subunit inhibiting adenylyl cyclase: Gi alpha inhibits adenylyl cyclase, which lowers cAMP production. It has the opposite effect to the one the stem describes. Your answer
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More questions from this set. Start the quiz to answer them and see each explanation.
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1
Which statement best describes the concept of signal amplification in cell signalling?
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2
Which statement best describes the structural organisation of a typical G-protein-coupled receptor (GPCR) and its associated G-protein during signal transduction?
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