G-protein-linked receptors.

1. The G-protein is inactive when it is attached to a GDP (guanosine diphosphate) molecule.
2. A signal molecule binds to the receptor, which changes shape and binds to the inactive G-protein.
   A GTP molecule displaces the GDP, and activates the G-protein.
3. The activated G-protein binds to another enzyme and activates it to initiate a cellular response.
4. The G-protein hydrolyzes the GTP and returns to an inactive state.