01G protein-coupled receptors
A ligand binds a GPCR, which activates a G protein (swaps GDP for GTP). The G protein activates an enzyme like adenylyl cyclase, which converts ATP into cAMP, a second messenger.
cAMP activates protein kinase A, starting a phosphorylation cascade: each kinase phosphorylates (activates) the next. Phosphatases remove phosphates to switch the pathway off.
02Amplification
One ligand → many cAMP → many kinases → millions of product molecules. This is why tiny hormone amounts have huge effects (epinephrine → glycogen breakdown in liver).
Receptor tyrosine kinases (growth factors) and ligand-gated ion channels (neurotransmitters) are other receptor types.
03Notebook box
Epinephrine pathway: epinephrine → GPCR → G protein → adenylyl cyclase → cAMP → PKA → phosphorylase kinase → glycogen phosphorylase → glucose released.
Worked example: A drug blocks phosphodiesterase (the enzyme that breaks down cAMP). Effect? → cAMP stays high and the response continues longer.