Cholesterol Biosynthesis
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Direct answer
All 27 carbons of cholesterol originate from acetyl-CoA through the mevalonate pathway, whose committed, rate-limiting step — reduction of hydroxymethylglutaryl-CoA to mevalonate by HMG-CoA reductase on the endoplasmic reticulum — is the target of statin drugs. Two acetyl units make acetoacetyl-CoA, a third joins to form HMG-CoA, and after mevalonate the pathway runs through isopentenyl pyrophosphate, six isoprene units to squalene, and cyclisation to lanosterol en route to cholesterol. Every nucleated cell can synthesise cholesterol, with liver, intestine, adrenal cortex and gonads leading; the sterol then becomes bile acids, steroid hormones, membrane structure or vitamin D precursor. The LDL receptor-mediated endocytosis described by Brown and Goldstein clears most plasma cholesterol, and its failure produces familial hypercholesterolaemia.
What you must remember
- Enzyme order: thiolase (two acetyl-CoA to acetoacetyl-CoA), HMG-CoA synthase (cytosolic), HMG-CoA reductase (rate-limiting, NADPH-consuming, two molecules per mevalonate), mevalonate kinase onwards to isoprenoids.
- Statins: competitive (transition-state analogue) inhibitors of HMG-CoA reductase taken at night because enzyme activity peaks nocturnally; the hepatic cholesterol depletion then upregulates LDL receptors, lowering plasma LDL twice over.
- Layered regulation of the reductase: sterols accelerate enzyme degradation and suppress SREBP-2-mediated gene transcription; glucagon-driven phosphorylation inactivates it while insulin dephosphorylates it — the classic short-term and long-term control viva answer.
- Locations and quantification: cytosol with the final steps membrane-bound in the ER; in humans the liver contributes roughly half, the intestine most of the rest; about one gram is made daily, balancing a similar intake.
- Fates: bile acids are the major output (cholic and chenodeoxycholic, conjugated with glycine or taurine); 7-dehydrocholesterol in skin converts to cholecalciferol under ultraviolet light; adrenal and gonadal steroidogenesis draws on the same pool.
- Isoprenoid branch points: farnesyl pyrophosphate anchors Ras proteins, ubiquinone, dolichol and haem oxygenase-related products — why complete blockade distal to mevalonate would be toxic.
- Familial hypercholesterolaemia: LDL receptor gene on chromosome 19; heterozygotes (about 1 in 500) with LDL around 300 milligrams per decilitre and tendon xanthomas, homozygotes with values above 600 and childhood coronary disease; PCSK9 gain-of-function is the phenocopy, treated now with inclisiran or PCSK9 antibodies.
How to work through a hypercholesterolaemia case
A 32-year-old presents with tendon xanthomas over the knuckles and corneal arcus; his father died of myocardial infarction at 40. Reason along the receptor pathway: LDL (the main cholesterol carrier, one apolipoprotein B100 per particle) normally binds the LDL receptor, is internalised in clathrin-coated pits, and releases cholesterol that suppresses HMG-CoA reductase and receptor synthesis through SREBP. A defective receptor (or its apolipoprotein B100 ligand, familial defective apoB100) removes this clearance, plasma LDL climbs, and the artery wall macrophages take up oxidised LDL instead — foam cells, fatty streaks, premature atheroma.
Treatment logic follows the biochemistry directly. A statin lowers hepatocyte cholesterol; the liver, sensing depletion via SREBP-2, manufactures more LDL receptors, so plasma clearance rises even in heterozygotes with one healthy allele. Ezetimibe blocks the NPC1L1 sterol transporter to cut reabsorption of recycled bile cholesterol; bile-acid sequestrants force the liver to convert more cholesterol to bile acids. Homozygous receptor-null disease needs LDL apheresis or the newer agents, and India's rising premature coronary burden makes recognising the xanthoma-plus-family-history pattern a genuine clinical skill.
Where students slip
Students confuse the cytosolic HMG-CoA synthase of cholesterogenesis with its mitochondrial isoenzyme of ketogenesis — different fates from an identical intermediate, and a favourite MCQ pair. Second, statins are "competitive inhibitors" in name but are actually transition-state analogues with enormous affinity; quoting them as reversible competitors of the substrate is acceptable, calling them allosteric is not. Third, remember that HMG-CoA reductase is controlled by regulated proteolysis as well as phosphorylation — many candidates recite only the kinase arm. Finally, LDL is cholesterol-rich but not made of cholesterol alone; apolipoprotein B100 quantitation (each particle carries exactly one molecule) is the modern measure students should be able to name.
Frequently asked questions
What is the rate-limiting enzyme of cholesterol synthesis and its inhibitors?
HMG-CoA reductase on the endoplasmic reticulum, inhibited by statins and by sterol-accelerated proteolysis, activated by insulin and SREBP-2 when cell cholesterol falls.
How do statins lower plasma LDL besides reducing synthesis?
Depleting hepatic cholesterol upregulates LDL receptor expression, so circulating LDL is cleared faster — the dominant clinical effect.
Which intermediate links cholesterol synthesis to protein anchoring?
Farnesyl pyrophosphate and related isoprenoids prenylate Ras and other proteins, also feeding ubiquinone and dolichol synthesis.
Where does vitamin D synthesis connect to the pathway?
Skin 7-dehydrocholesterol, the immediate precursor of cholesterol, photolyses to cholecalciferol on ultraviolet exposure.
What causes familial hypercholesterolaemia biochemically?
Loss-of-function of the LDL receptor (chromosome 19) blocks endocytotic clearance of LDL; PCSK9 gain-of-function accelerates receptor degradation and mimics it.