Cytoplasmic response to a signal: the stimulation of glycogen breakdown by epinephrine.

 In this signaling system, the hormone epinephrine acts through a G–protein–linked receptor to activate a succession of relay molecules, including cAMP and two protein kinases (see also Figure 11.10). The final protein to be activated is the enzyme glycogen phosphorylase, which releases glucose–1–phosphate units from glycogen. This pathway amplifies the hormonal signal, because one receptor protein can activate about 100 molecules of G protein, and each enzyme in the pathway can act on many molecules of its substrate, the next molecule in the cascade. The number of activated molecules given for each step is approximate.