In cAMP signaling, which enzyme is directly activated by cAMP to modify cellular functions?

• A. Adenylyl cyclase
• B. Protein kinase A (PKA)
• C. Phospholipase C
• D. Protein kinase C

Answer: (B)

cAMP acts as a second messenger by directly activating protein kinase A. When cAMP levels rise, it binds to the regulatory subunits of PKA, causing the holoenzyme to release the active catalytic subunits. These catalytic subunits then phosphorylate serine/threonine residues on many target proteins, altering their activity to bring about cellular responses such as changes in metabolism, gene expression, and transport.

Adenylyl cyclase is the enzyme that makes cAMP from ATP, not the enzyme directly activated by cAMP. Phospholipase C and protein kinase C participate in separate signaling pathways—PLC is activated by Gq-coupled receptors to generate IP3 and DAG, and PKC is activated by DAG (and Ca2+)—not directly by cAMP.