The committed and rate-limiting step in the conversion of cholesterol to bile acids is catalysed by:
- A 7-alpha-hydroxylase (CYP7A1) ✓
- B 27-hydroxylase (CYP27A1)
- C HMG-CoA reductase
- D Cholesterol acyltransferase (ACAT)
Correct answer: A. 7-alpha-hydroxylase (CYP7A1)
Explanation
Cholesterol 7-alpha-hydroxylase, a microsomal cytochrome P450 in hepatocytes, performs the first committed step of the classic bile acid synthetic pathway and is its principal control point. It is transcriptionally repressed when bile acids returning via enterohepatic circulation activate FXR, inducing FGF19 signalling that suppresses CYP7A1. CYP27A1 acts later in side chain oxidation, HMG-CoA reductase governs cholesterol synthesis itself, and ACAT esterifies cholesterol for storage.
Reference: Harper's Illustrated Biochemistry, 32nd ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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