The occupation of a G-protein coupled receptor with a ligand promotes the:

Question

BlackTom AI · Accepted Answer

Ligand binding causes the GPCR to act as a guanine-nucleotide exchange factor (GEF) for the associated G protein, promoting release of GDP from the Gα subunit and allowing GTP to bind.

1) Relevant concepts
- Heterotrimeric G proteins consist of Gα (binds GDP/GTP) plus Gβγ. In the inactive state Gα·GDP is associated with Gβγ.
- Activated GPCRs catalyze GDP release from Gα (they function like GEFs). GTP is abundant in the cytosol and replaces GDP.
- Gα·GTP undergoes a conformational change, dissociates from Gβγ, and both can regulate downstream effectors.
- Gα has intrinsic GTPase activity that hydrolyzes GTP to GDP to terminate signaling.

2) Step-by-step reasoning
- Step 1: Ligand binds the extracellular domain of the GPCR, inducing a conformational change in the receptor.
- Step 2: The activated receptor interacts with the heterotrimeric G protein and lowers the affinity of Gα for GDP (i.e., acts as a GEF).
- Step 3: GDP dissociates and GTP binds to Gα (exchange of GDP for GTP).
- Step 4: Gα·GTP separates from Gβγ and both parts modulate effectors (adenylate cyclase, ion channels, etc.).
- Step 5: Gα hydrolyzes GTP to GDP (GTP hydrolysis) to reset the system.

3) Calculations
- No numeric calculations are required; the logic is sequential biochemical events.

4) Why the correct answer is correct
- The immediate molecular event directly promoted by receptor occupation is nucleotide exchange on Gα (GDP out, GTP in). That is precisely answer A.

5) Why the other options are incorrect
- B (dissociation of the hetero-trimer): This occurs after GDP→GTP exchange as a consequence, not the primary promoted step.
- C (hydrolysis of GTP): GTP hydrolysis is the termination step carried out by Gα’s intrinsic GTPase activity and happens later, not upon initial receptor occupation.
- D (phosphorylation of adenylate cyclase): Adenylate cyclase is regulated by Gα and Gβγ interactions (activation/inhibition), not by phosphorylation by the GPCR; phosphorylation of AC is not the immediate result of ligand binding.

6) Conclusion
- Therefore the correct choice is A. This matches the provided answer: A. exchange of GDP for GTP.

The occupation of a G-protein coupled receptor with a ligand promotes the:单项选择题

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