Cholesterol Synthesis
| Category | Mechanisms |
|---|---|
| Also known as | sterol biosynthesis, mevalonate pathway |
| Last updated | 2026-04-14 |
| Reading time | 4 min read |
| Tags | mechanismmetabolismlipid |
Overview
Cholesterol synthesis is a complex, multi-step pathway that builds the 27-carbon sterol from acetyl-CoA units, proceeding through the key intermediate mevalonate. Cholesterol is an essential component of eukaryotic membranes, a precursor to steroid hormones, bile acids, and vitamin D, and a regulator of signaling (notably in Hedgehog and some GPCR systems). Humans synthesize roughly 700-900 mg of cholesterol per day, primarily in the liver, with dietary intake contributing a smaller amount.
The pathway is best known for its regulation by the sterol regulatory element binding protein (SREBP) family and for being the target of the statin class of drugs. HMG-CoA reductase, the rate-limiting enzyme, is inhibited by statins, lowering hepatic cholesterol synthesis and upregulating LDL receptor expression, which in turn reduces circulating LDL cholesterol. Statins are among the most widely prescribed medications in the world.
The mevalonate pathway also supplies isoprenoid intermediates (farnesyl pyrophosphate, geranylgeranyl pyrophosphate) used to prenylate small GTPases (Ras, Rho, Rab family), modify proteins, and produce quinones, dolichols, and heme A. Thus statin effects extend beyond cholesterol to other aspects of cell biology.
Mechanism / Process
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Acetyl-CoA source. Cytoplasmic acetyl-CoA from citrate cleavage or from acetate activation feeds the pathway.
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Mevalonate formation. Two acetyl-CoA molecules are condensed by thiolase to acetoacetyl-CoA, and a third acetyl-CoA is added by HMG-CoA synthase (cytosolic isoform, HMGCS1) to form HMG-CoA. HMG-CoA reductase (HMGCR) then reduces HMG-CoA to mevalonate in a two-step, NADPH-dependent reaction — the rate-limiting and committed step.
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Isoprenoid formation. Mevalonate is phosphorylated twice and decarboxylated to isopentenyl pyrophosphate (IPP). IPP is the universal five-carbon building block. Isomerization produces dimethylallyl pyrophosphate (DMAPP).
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Chain elongation. IPP and DMAPP condense to form geranyl pyrophosphate (C10), then farnesyl pyrophosphate (FPP, C15). Two FPP molecules condense tail-to-tail to form squalene (C30), catalyzed by squalene synthase.
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Cyclization. Squalene is epoxidized by squalene monooxygenase and then cyclized by lanosterol synthase to lanosterol, the first sterol in the pathway.
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Conversion to cholesterol. Lanosterol is converted to cholesterol through approximately 20 additional enzymatic steps, including demethylations, double bond reductions, and saturation, via two possible routes (Bloch and Kandutsch-Russell).
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Regulation. HMGCR is tightly regulated by sterol-dependent transcription (SREBP-2), protein stability (ubiquitination triggered by sterols and lanosterol via Insig/HRD1), and phosphorylation by AMPK (inhibitory under energy stress).
Key Players / Molecular Components
- HMG-CoA reductase (HMGCR). Rate-limiting; target of statins.
- HMG-CoA synthase (HMGCS1). Cytosolic isoform for cholesterol synthesis.
- Squalene synthase (SQS/FDFT1). Branch point committing to sterols.
- Squalene monooxygenase (SQLE). Emerging second regulatory node.
- Lanosterol synthase, CYP51, DHCR7, DHCR24. Sterol modification enzymes.
- SREBP-2, Insig, SCAP. Sterol-sensing transcriptional machinery.
- HRD1 and MARCHF6. E3 ligases mediating sterol-responsive degradation of HMGCR.
Clinical Relevance / Therapeutic Targeting
Statins (atorvastatin, rosuvastatin, simvastatin, and others) are cornerstone therapy for hyperlipidemia and cardiovascular disease prevention. They reduce cardiovascular events in primary and secondary prevention. Additional cholesterol-lowering drugs include ezetimibe (inhibits intestinal absorption), bile acid sequestrants, PCSK9 inhibitors (monoclonal antibodies or siRNA that preserve LDL receptor), bempedoic acid (inhibits ATP-citrate lyase, upstream of HMGCR), and inclisiran (siRNA). Inherited disorders include familial hypercholesterolemia, Smith-Lemli-Opitz syndrome (DHCR7 deficiency), and desmosterolosis (DHCR24 deficiency). Statins also affect protein prenylation via the isoprenoid branch.
Peptides That Target This Pathway
- PCSK9-targeting peptides — preserve LDL receptor, enhancing cholesterol clearance.
- ApoA1 mimetic peptides — promote reverse cholesterol transport.
- Thyroid hormone-related peptides — thyroid hormone modulates cholesterol metabolism.
- Bile acid-mimetic peptides — influence cholesterol through FXR signaling.
- Leptin — integrates lipid metabolism with energy balance.
Related Topics
Related entries
- Fatty Acid Synthesis— Fatty acid synthesis is the metabolic pathway that converts excess dietary carbohydrates and amino acids into fatty acids for energy storage, membrane synthesis, and signaling — a process tightly regulated by insulin and relevant to metabolic peptide therapeutics.
- Bile Acid Synthesis— The hepatic conversion of cholesterol into bile acids, a major route of cholesterol disposal that produces signaling molecules regulating metabolism.
- Fatty Acid Synthesis— The cytoplasmic pathway that builds long-chain fatty acids from acetyl-CoA, coordinated with dietary and hormonal signals.
- Steroidogenesis— The enzymatic conversion of cholesterol into steroid hormones in adrenal, gonadal, and other steroidogenic tissues.
- Vitamin D Signaling— Vitamin D signaling governs calcium and phosphate homeostasis, bone mineralization, and a broad range of immune and cellular differentiation programs via the nuclear vitamin D receptor.