G protein coupled receptors
When GDP is bound to the G protein, G protein is inactive. When the appropiate signaling molecule binds to the extracellular side of the receptor, the receptor is activated and changes shape. Its cytoplasmic side then binds an inactive G protein, causing GTP to displace the GDP. This activates the G protein. The activated G protein dissociates from the receptor, diffuses along the membrane, and then binds to an enzyme, altering the enzyme's shape and activity. Once activated, the enzyme can trigger a cellular response along the signal transduction pathway. Then the G protein hydrolyzes its bound GTP to GDP. Now inactive again, G protein leaves the enzyme and returns to its original state.