Biochemistry · Enzyme
Cholesterol 7α-hydroxylase (CYP7A1) !
Liver CYP7A1 takes the first step in turning cholesterol into bile acids and sets the pace of that pathway.
Updated 2026-11-01
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Key facts
- Cholesterol + O₂ + NADPH → 7α-hydroxycholesterol: the rate-limiting step of the classic bile acid pathway.
- An endoplasmic reticulum cytochrome P450 found only in hepatocytes.
- Bile acids returning through the enterohepatic circulation repress it (FXR, SHP and ileal FGF19).
- Turning cholesterol into bile acids is the body's main route for getting rid of cholesterol.
- Also called
- CYP7A1
- EC number
- 1.14.14.23
- Genes
- CYP7A1
- Structure (PDB)
- 3SN5
Structure
- A 504-residue P450 haemoprotein anchored to the ER membrane.
- Structure 3SN5 shows the human enzyme with the steroid cholest-4-en-3-one in its pocket next to the haem.
Regulation
- Bile-acid-activated FXR induces SHP in the liver and FGF19 in the ileum: both shut CYP7A1 off.
- It rises when the enterohepatic circulation is interrupted (bile acid sequestrants).
- It follows a circadian rhythm.
Inhibitors and toxins
- Bile acids (physiological feedback)
Clinical notes
- Bile acid sequestrants cut the feedback: the liver uses more cholesterol and raises its LDL receptors.
- Variants that reduce CYP7A1 are linked to high LDL and gallstones.
- When bile holds more cholesterol than bile salts and phospholipids can carry, cholesterol stones form.
Pathways
Related conditions
Related
Test yourself
Think of the answer, then tap the question to see it.
What is the rate-limiting step of bile acid synthesis?
Cholesterol 7α-hydroxylase (CYP7A1), which makes 7α-hydroxycholesterol.
How do bile acids slow CYP7A1?
They activate FXR, which induces SHP in the liver and FGF19 in the ileum; both repress its gene.
Why does a bile acid sequestrant lower LDL?
With bile acids lost, CYP7A1 is switched on, the liver uses cholesterol and raises its LDL receptors.
Educational information, not medical advice. For any symptom, see a health professional.
Sources: CYP7A1: Cytochrome P450 7A1 (human) (UniProt, 2026); 3SN5: Crystal structure of human CYP7A1 in complex with cholest-4-en-3-one (RCSB Protein Data Bank, 2026); Synthesis of bile acids and bile salts (R-HSA-192105) (Reactome, 2026).
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Representative model, for educational purposes only.