G Proteins as Molecular Switches in Signaling

Steps:

• Recall that G proteins are heterotrimeric proteins associated with G protein-coupled receptors (GPCRs) in the cell membrane.
• When a ligand binds to the GPCR, it activates the G protein by exchanging GDP for GTP on the alpha subunit.
• The activated G alpha subunit dissociates and interacts with effectors like adenylyl cyclase to release second messengers such as cAMP.
• This on-off GTPase activity allows G proteins to toggle signaling pathways precisely.

Why B is correct:

• G proteins act as switches by binding GTP to activate and hydrolyzing it to GDP to inactivate, directly triggering second messenger release as defined in GPCR signal transduction pathways.

Why the others are wrong:

• A: G proteins are not enzymes; they are GTP-binding regulators, while enzymes like adenylyl cyclase are downstream effectors.
• C: Amplification occurs via second messengers and cascades, not directly by G proteins themselves.
• D: Ligand binding changes receptor shape to activate G proteins; G proteins do not alter the receptor.

Final answer: B