Cholesterol can be eliminated from the body only after conversion into bile acids or direct biliary secretion. The rate-controlling enzyme of the classical bile acid synthetic pathway is:
- A Cholesterol 7-alpha-hydroxylase (CYP7A1) ✓
- B HMG-CoA reductase
- C 27-hydroxylase (CYP27A1)
- D Sterol regulatory element binding protein-2
Correct answer: A. Cholesterol 7-alpha-hydroxylase (CYP7A1)
Explanation
Cholesterol 7-alpha-hydroxylase, a microsomal cytochrome P450 enzyme, catalyzes the committed first step converting cholesterol to 7-alpha-hydroxycholesterol on the way to cholic and chenodeoxycholic acids. It is feedback-inhibited by bile acids returning via enterohepatic circulation and is transcriptionally induced by cholesterol via the LXR-FXR axis. CYP27B1 acts later in the pathway, and SREBP-2 regulates HMG-CoA reductase and LDL receptor genes rather than catalyzing any step.
Reference: Harper's Illustrated Biochemistry, 32nd ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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