This causes the G protein to expel its GDP molecule and replace it with GTP. The GTP causes a small loop (shown here in red) to change shape, and the G protein falls into two pieces. The freed alpha subunit, with its bound GTP, then moves along the membrane until it finds the enzyme adenylyl cyclase. The little loop then binds to the enzyme and activates it. The activated adenylyl cyclase then produces lots of cyclic AMP, which spreads the signal through the cell. Eventually, the GTP in the active alpha subunit will break down into GDP, and the G protein will reform into its inactive, resting state.

One major advantage of this approach is that it allows the signal to be amplified. In the signaling chain shown here, a single molecule of adrenaline can stimulate the production of many molecules of cyclic AMP. By incorporating an enzyme (like adenylyl cyclase) into the chain, a weak signal from outside the cell can be translated into a strong signal throughout the inside of the cell.

When you go to look at these structures yourself, keep this in mind: structures are available at each stage of this interesting process, but you have to be willing to cobble together examples from a number of different signaling pathways. Four PDB structures were used in this picture: 1f88, 1got, 1cul, and 1tbg, from left to right. These aren't the exact proteins that respond to adrenaline, but they give an idea of what the signaling pathway is like.