G-proteins

Question

G-proteins

BlackTom AI · Accepted Answer

G-proteins are molecular switches that transfer signals from ligand-bound G protein–coupled receptors (GPCRs) to downstream effector proteins.

1. Relevant knowledge points and concepts
- G-proteins are heterotrimeric proteins (α, β, γ subunits) that cycle between inactive (GDP-bound) and active (GTP-bound) states.
- GPCRs are the recognition sites (receptors) for extracellular ligands; effectors include enzymes (adenylyl cyclase, phospholipase C) or ion channels that produce second messengers (cAMP, IP3, DAG).
- The primary role of G-proteins is signal transduction: conveying information from receptor to effector.

2. Step-by-step reasoning process
- A ligand binds the GPCR (recognition site).
- The activated GPCR functions as a guanine nucleotide exchange factor (GEF) for the G-protein, causing GDP on the α-subunit to be replaced by GTP.
- GTP binding induces dissociation (or rearrangement) of the G-protein subunits (α and βγ), which then interact with effector proteins.
- The effector’s activity (e.g., adenylyl cyclase) changes, producing or modulating second messengers that drive cellular responses.
- Intrinsic GTPase activity of the α-subunit hydrolyzes GTP to GDP, returning the G-protein to its inactive state.

3. Intermediate steps and examples
- Example: Ligand → GPCR → Gs activation (αs·GTP) → stimulation of adenylyl cyclase → ↑ cAMP → PKA activation.
- Example: Gq activation → phospholipase C → IP3 + DAG → Ca2+ release and PKC activation.

4. Why the correct answer is correct
- This sequence shows G-proteins physically and functionally sit between the recognition site (GPCR) and the effector (adenylyl cyclase, PLC, ion channels), so they “act as transducers between recognition site and effector site.”

5. Why the other options are incorrect
- (a) “represent the effector site”: incorrect — effectors are separate enzymes or channels, not the G-protein itself.
- (b) “represent the recognition site”: incorrect — recognition is performed by GPCRs, not G-proteins.
- (d) “act as transducer between effector site and second messenger”: incorrect — G-proteins activate effectors that produce second messengers; they do not primarily act between effector and second messenger (the effector produces the messenger).

6. Conclusion
- Therefore the correct choice is c. act as transducer between recognition site and effector site, which matches the provided answer.

G-proteins单项选择题

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