Protein kinase A exists as an inactive tetrameric holoenzyme. What happens when cyclic AMP levels rise within the cell?

• A cAMP phosphorylates the regulatory subunits directly
• B cAMP binds the regulatory subunits, freeing active catalytic subunits ✓
• C cAMP dissociates the alpha and gamma subunits of the enzyme
• D cAMP transports the holoenzyme into the nucleus

Explanation

PKA is a tetramer of two regulatory and two catalytic subunits. Each regulatory subunit binds two molecules of cAMP, undergoes a conformational change, and releases the catalytic subunits, which then phosphorylate serine and threonine residues on target enzymes such as phosphorylase kinase. The regulatory subunit is not itself a phosphoprotein target, ruling out option A.

Reference: Lippincott's Illustrated Reviews: Biochemistry, 8th ed.

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