G-protein-linked receptors.
<Li> The  G-protein is inactive when it is attached to a GDP (guanosine
diphosphate) molecule.
<Li> 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.
<Li> The  activated  G-protein binds to another enzyme and activates it
to initiate a cellular response.
<Li> The G-protein hydrolyzes the GTP and returns to an inactive state.