Final answer:
GPCRs facilitate cell signaling by undergoing a conformational change upon ligand binding, allowing the associated G protein to switch GDP for GTP in the alpha subunit, which activates the G protein and leads to various cellular responses.
Step-by-step explanation:
When a signaling molecule, such as a hormone or neurotransmitter, binds to a G-protein-coupled receptor (GPCR), it triggers a conformational change in the receptor. This change enables the associated G protein to bind to the activated receptor. Upon this interaction, the G protein undergoes its own conformational shift, causing the alpha subunit of the G protein to release guanosine diphosphate (GDP) and pick up guanosine triphosphate (GTP), effectively activating the G protein. Subsequently, the activated G protein dissociates into two fragments: the GTP-bound alpha subunit and the beta-gamma (By) dimer. These components can then go on to activate or inhibit other downstream effector proteins within the cell, leading to various cellular responses. The activation is typically short-lived because the intrinsic GTPase activity of the alpha subunit rapidly hydrolyzes the bound GTP back to GDP, which deactivates the alpha subunit, allowing it to reassociate with the beta-gamma dimer and rendering the G protein inactive, ready for a new cycle of activation.