In adipose tissue during stress, catecholamines activate hormone-sensitive lipase through a Gs-coupled receptor cascade. What is the immediate molecular event that increases the enzyme's activity?
- A Dephosphorylation of the enzyme by protein phosphatase 2A
- B Direct allosteric activation by cyclic AMP
- C Phosphorylation of the enzyme by protein kinase A ✓
- D Proteolytic cleavage of the enzyme by caspases
Correct answer: C. Phosphorylation of the enzyme by protein kinase A
Explanation
Epinephrine binds beta-adrenergic receptors, raises cAMP, and activates protein kinase A, which phosphorylates hormone-sensitive lipase (and perilipin), converting it to its active form and permitting access of the lipase to stored triacylglycerol droplets. Insulin opposes this by activating phosphodiesterase and protein phosphatases, promoting dephosphorylation. Cyclic AMP acts through PKA rather than binding the lipase directly, which kills option B.
Reference: Lippincott's Illustrated Reviews: Biochemistry, 8th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.