2nd Messengers

2nd Messengers

1. cyclic AMP

   • formed from cytosolic ATP by enzyme adenylyl cyclase
     • adenylyl cyclase is anchored in the plasma membrane
       • enzyme is inactive until it binds to an activated Gs alpha sub-unit
         • Gs alpha sub-unit activated by receptor-ligand stimulated acquisition of GTP and release from Gs beta gamma
       • Gi alpha sub-unit inhibits adenylyl cyclase
       • G proteins respond quickly to changes in ligand [] bc they remain active for only a short time before the G alpha subunit hydrolyzes its bound GTP and converts to the inactive state
         • once G protein becomes inactive, adenylyl cyclase ceases to make cAMP
           • enzyme phosphodiesterase degrades the cAMP
   • main intracellular target: enzyme protein kinase A
     • CAMP regulates activity of PKA by separating the regulatory and catalytic subunits
     • PKA phosphorylates proteins by transferring a phosphate from ATP to a ser or thr

2. calcium ions