Activated receptors (R*) are the primary activators of G proteins.
Here's a more detailed explanation:
G proteins, specifically heterotrimeric G proteins (comprising α, β, and γ subunits), reside on the inner surface of the cell membrane. They act as molecular switches, relaying signals from activated receptors to downstream effector proteins. The activation process involves a conformational change in the receptor, typically triggered by the binding of a ligand (e.g., a hormone or neurotransmitter).
How Activated Receptors Activate G Proteins:
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Receptor Activation: A signaling molecule (ligand) binds to a G protein-coupled receptor (GPCR) on the cell surface. This binding induces a conformational change in the receptor.
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G Protein Binding: The activated receptor (R*) interacts with the inactive G protein (α subunit bound to GDP).
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GDP Release and GTP Binding: The activated receptor promotes the release of GDP from the α subunit of the G protein. Because GTP (guanosine-5'-triphosphate) is much more abundant in the cytoplasm than GDP, GTP rapidly binds to the α subunit.
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G Protein Dissociation: GTP binding causes a conformational change in the α subunit, leading to its dissociation from both the receptor and the βγ complex.
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Activation of Downstream Effectors: Both the GTP-bound α subunit and the βγ complex can now interact with and regulate the activity of various downstream effector proteins, such as enzymes or ion channels, initiating a cellular response.
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GTP Hydrolysis and Inactivation: The α subunit possesses intrinsic GTPase activity. It hydrolyzes GTP to GDP, leading to the re-association of the α subunit with the βγ complex, returning the G protein to its inactive state. The cycle can then repeat upon further receptor activation.
In summary, the activation of G proteins is a direct consequence of the conformational change in a receptor induced by ligand binding and its subsequent interaction with the G protein, facilitating GDP release and GTP binding.
