Cholesterol Biosynthesis Stages

• Acetyl-CoA serves as the only carbon source in cholesterol biosynthesis.

Location of Cholesterol Synthesis

• Cholesterol synthesis occurs in the endoplasmic reticulum and cytosolic compartments
• Cholesterol synthesis occurs in virtually all tissues containing nucleated cells.
• The liver and intestine account for ~10% each of the total cholesterol synthesis in humans.
• In vertebrates, the majority of cholesterol synthesis takes place in the liver.

Stages of Cholesterol Biosynthesis

• Cholesterol biosynthesis occurs in four stages.

Stage 1: Synthesis of Mevalonate from Acetyl-CoA

• This stage involves a six-carbon intermediate.
• Two molecules of acetyl-CoA condense to form acetoacetyl-CoA, a reaction catalyzed by acetyl-CoA acetyl transferase.
• Acetoacetyl-CoA condenses with a third molecule of acetyl-CoA to produce β-hydroxy-β-methylglutaryl-CoA (HMG-CoA), catalyzed by HMG-CoA synthase.
• HMG-CoA reductase catalyzes the reduction of HMG-CoA to mevalonate, which is a six-carbon compound.
• This reaction requires two molecules of NADPH as electron donors.
• The reduction of HMG-CoA to mevalonate is the committed and rate-determining step in cholesterol biosynthesis.
• HMG-CoA reductase is an integral membrane protein of the smooth endoplasmic reticulum.

Stage 2: Conversion of Mevalonate to Activated Isoprene Units

• This stage involves five-carbon units.
• Mevalonate-5-phosphotransferase phosphorylates mevalonate, using one molecule of ATP.
• Phosphomevalonate kinase converts mevalonate-5-phosphate to 5-pyrophosphomevalonate, using another molecule of ATP.
• Pyrophosphomevalonate decarboxylase catalyzes the ATP-dependent decarboxylation of 5-pyrophosphomevalonate, yielding isopentenyl pyrophosphate, involving the loss of a phosphate group.
• Isopentenyl pyrophosphate isomerase isomerizes isopentenyl pyrophosphate to dimethylallyl pyrophosphate.
• Isopentenyl pyrophosphate and dimethylallyl pyrophosphate are two five-carbon units which are activated isoprenes and are central to cholesterol formation.

Stage 3: Polymerisation of Isoprene Units to Form Squalene

• This stage involves six 5-carbon isoprene units forming the 30-carbon linear squalene.
• One molecule of dimethylallyl pyrophosphate condenses with one molecule of isopentenyl pyrophosphate to form geranyl pyrophosphate, which contains 10 carbons.
• Geranyl pyrophosphate condenses with another molecule of isopentenyl pyrophosphate to form farnesyl pyrophosphate, which contains 15 carbons.
• Squalene synthase catalyzes the reductive head-to-tail condensation of two molecules of farnesyl pyrophosphate to form squalene, a 30-carbon molecule, a reaction requiring NADPH.

Stage 4: Cyclisation of Squalene to Form Lanosterol

• This stage involves forming the four rings of the steroid nucleus, with further modifications to produce cholesterol.
• Squalene cyclase catalyzes the cyclization of squalene to form lanosterol, which is a 30-carbon molecule, the first cyclic intermediate in the pathway and requires molecular oxygen and NADPH.
• Lanosterol converts into cholesterol, a 27-carbon molecule, through a series of over 20 enzymatic reactions that involve:
  • Removal of three methyl groups.
  • Isomerization of double bonds.
  • Reduction of a double bond, requiring NADPH.
  • Desmosterol (24-dehydrocholesterol) is one intermediate in this process.

Regulation of Cholesterol Synthesis

• HMG-CoA reductase is the major point of regulation.
• HMG-CoA reductase activity and amount of the enzyme are controlled.
• High cholesterol levels in the cell reduce the transcription of the HMG-CoA reductase gene.
• Cholesterol promotes the degradation of HMG-CoA reductase.
• Statins inhibit HMG-CoA reductase, thereby reducing cholesterol synthesis.
• Cholesterol balance in cells is tightly regulated, involving factors that maintain the correct balance.

Fates of Cholesterol

• Cholesterol is incorporated into cellular membranes as an essential structural component.
• Cholesterol serves as a precursor for steroid hormones, such as gonadal and adrenal steroid hormones and for vitamin D.
• Cholesterol is a precursor for bile acids like cholic acid and chenodeoxycholic acid in the liver, which aid in fat digestion.
• Bile acids provide a major excretion route for cholesterol.
• Cholesterol 7α-hydroxylase (CYP7A1) catalyses the 7α-hydroxylation of cholesterol.
• 7α-hydroxylation is the first and principal regulatory step in bile acid biosynthesis.
• Acyl-CoA-cholesterol acyl transferase (ACAT) mediates the formation of cholesteryl esters in the liver, for transport in lipoproteins or storage.

Transport of Cholesterol

• Cholesterol and cholesteryl esters are transported in the blood as plasma lipoproteins.
• Chylomicrons transport dietary triacylglycerols and cholesterol from the intestine.
• Very-low-density lipoprotein (VLDL) carries triacylglycerols and cholesterol esters from the liver.
  • VLDL is converted to low-density lipoprotein (LDL) in the circulation.
• Low-density lipoprotein (LDL) is rich in cholesterol and its esters and delivers cholesterol to extrahepatic tissues via LDL receptors.
• High-density lipoprotein (HDL) is involved in reverse cholesterol transport and removes cholesterol from peripheral tissues and returns it to the liver.

Intermediates with Alternative Fates

• Isopentenyl pyrophosphate is a precursor for a wide array of other biomolecules, including vitamins A, E, and K, and plant pigments.
• Farnesyl pyrophosphate and geranylgeranyl pyrophosphate are used for protein prenylation, which anchors proteins to membranes.