Final answer:
The false statement about G proteins is that they are inactive when bound to GTP; in reality, GTP binding activates them. G proteins play a critical role in translating extracellular signals through G-protein-coupled receptors to cellular responses within various physiological systems.
Step-by-step explanation:
The statement regarding G proteins associated with G-protein-coupled receptors (GPCRs) that is false is "G proteins are inactive when bound to GTP." In fact, G proteins are activated when GTP replaces GDP on the alpha subunit. The activation of G proteins leads to downstream signaling, predominantly mediated by the alpha subunit but sometimes by the beta and gamma subunits as well. These proteins play a pivotal role in transmitting signals from external stimuli via the receptors to intracellular effectors, which results in cellular responses. Hydrolysis of GTP to GDP by the alpha subunit eventually terminates the signal, bringing the G protein back to its inactive state, ready to start the cycle once more with a new signal.
The process of cell signaling using GPCRs is a cycle where the receptor activation leads to conformational changes and the exchange of GDP for GTP on the G protein alpha subunit, causing the dissociation of the beta and gamma subunits. Once the GTP is hydrolyzed to GDP, the subunits reassociate, ending the signal and resetting the G protein.
G-protein-coupled receptor mechanisms are involved in various physiological processes, including vision, taste, immune system regulation, and inflammation, demonstrating their ubiquity and importance in cellular communication.