Cholesterol synthesis + reverse cholesterol pathway + cholesterol information

structures and steps

synthesis of cholesterol

• All of its carbon atoms are provided by acetate (activated form is acetyl-CoA)

• Isoprene units are essential intermediates in the pathway

four stages of cholesterol synthesis from acetyl-CoA quick summary

1. Condensation of three acetate units to form mevalonate

2. Conversion of mevalonate to activated isoprene units

3. Polymerization of six isoprene units to form the 30-carbon linear squalene

4. Cyclization of squalene to form the four rings of the steroid nucleus

Stage 1 of cholesterol synthesis—> 3 steps in detail (including 3 enzymes and structures)

synthesis of mevalonate from acetate

1. acetyl-CoA acetyl transferase = catalyzes the condensation of two acetyl-CoA molecules

2. HMG-CoA synthase = catalyzes the condensation of acetyl-CoA with acetoacetyl-CoA to form 𝛽-hydroxy-𝛽-methylglutaryl-CoA (HMG-CoA)

3. HMG-CoA reductase = an integral membrane protein of the smooth ER that catalyzes the reduction of HMG-CoA to mevalonate

   1. The major point of regulation on the pathway to cholesterol

   2. Catalyzes the committed step 

   3. Requires 2 molecules of NADPH

   4. Enzyme is inhibited by statin

stage 2 of cholesterol of synthesis

Conversion of mevalonate to two activated isoprenes

• 1 phosphate is not sufficient 

• Final product has a good leaving group, CO₂ and one phosphate on its beta carbon and the other two on the 1st carbon

• Three phosphate groups are transferred from three ATP molecules to mevalonate to form 3-phospho-5-pyrophosphomevalonate

• CO₂ and Pi leave to produce a double bond in ∆³- isopentenyl pyrophosphate

  • The first activated isoprene

• Isomerization of ∆³-isopentenyl pyrophosphate yields dimethylallyl pyrophosphate

  • The second activated isoprene 

stage 3 of cholesterol of synthesis

condensation of six activated ispoprene units to form squalene

• The two activated isoprenes undergo head-to-tail condensation to form geranyl pyrophosphate

• Geranyl pyrophosphate undergoes head-to-tail condensation with isopentenyl pyrophosphate to form farnesyl pyrophosphate

• Two molecules of farnesyl pyrophosphate join head-to-head to form squalene

stage 4 of cholesterol of synthesis

Conversion of squalene to the four-ring steroid nucleus

• Squalene monoocygenase = adds one oxygen atom from O₂ to the end of the squalene chain to form squalene 2,3-epoxide

  • Mixed-function oxidase

  • Requires NADPH

• Cyclization of squalene 2,3-epoxide forms lanosterol

• A series of approx. 20 reactions converts lanosterol to cholesterol

• In plants, the epoxide cyclizes to other sterols, such as ergosterol

cholesterol has several fates

• Cholesterol is synthesized primarily in the liver

• Most of it is exported as:

  • Bile acids

  • Biliary cholesterol

  • Cholesteryl esters

    • Formed in the liver by acyl-CoA-cholesterol acyltransferase (ACAT)

export of cholesterol as bile acids

• Bile acids= the principal components of bile, a fluid stored in the gallbladder

  • Excreted into the small intestine to aid in the digestion

  • Relatively hydrophilic cholesterol derivatives that serve as emulsifiers

  • Made from cholesterol

  • Ex. taurocholic acid

export of cholesterol as choelsteryl esters

• Cholestereyl esters = cholesterol molecule with a fatty acid from coenzyme A attached to its hydroxyl group

  • Formed in the liver by acyl-CoA-cholesterol acyltransferase (ACAT)

  • Cannot function appropriately in membranes due to high hydrophobicity

  • Transported to other tissues or stored in the liver in lipid droplets

the reverse cholesterol pathway

• HDL acts as the main carrier in this pathway, collecting excess cholesterol from cholesterol-rich extrahepatic tissues and transporting it back to the liver.

• In the liver, cholesterol can be:

  • Converted to bile salts and stored in the gallbladder, or

  • Excreted into bile for elimination.

• Macrophages in arterial walls can engulf LDL particles; the cholesterol esters are hydrolyzed to free cholesterol. If excess cholesterol accumulates, it can crystallize. When macrophages rupture, this contributes to plaque formation (atherogenesis).

• The SR-BI (scavenger receptor class B type I) receptor on hepatocytes binds HDL and mediates selective uptake of cholesterol, allowing HDL to unload its cargo without being fully degraded.

steps of receptor-mediated endocytosis for cholesterol esters entering cells

• LDL receptors are made in the rough ER and trafficked to the plasma membrane through the Golgi.

• LDL receptors on the cell surface recognize and bind apoB-100 on LDL particles 

• The receptor-LDL complex is brought into the cell via endocytosis

• The receptor is recycled back to the plasma membrane so it can capture more LDL

• The endosome with LDL fuses with a lysosome

• Lysosomal enzymes degrade apoB-100 and cholesteryl esters → releasing free cholesterol, amino acids, and triacylglycerols

This is the mechanism of LDL uptake into cells, which is part of the cholesterol delivery side of the cycle. Reverse cholesterol transport (HDL pathway) is the protective counterbalance that prevents overload of cholesterol in tissues by bringing it back to the liver

PCSK 9

PCSK 9 protein degrades LDL receptors so if PCSK9 isn’t inhibited, fewer receptors return to the membrane → higher LDL levels in blood

Low-density lipoprotein and LDL receptors def

• Low-density lipoprotein (LDL) = formed by triacylglycerol loss in VLDL

  • Rich in cholesterol and choelsteryl esters

  • Carries cholesterol to extrahepatic tissues and macrophages

• LDL receptors = receptors in the hepatocyte plasma membrane that take up LDL not taken up by peripheral tissues and cells

HDL def and role in formation of cholesteryl esters

• High-density lipoprotein (HDL) = lipoproteins that originate in the liver and small intestine as small, protein-rich particles

  • Contain lecithin-cholesterol acyltransferase (LCAT) to catalyze the formation of cholesteryl esters

  • Mediates cholesterol scavenging and transport back to the liver; ie. carries out reverse cholesterol transport

dysregulation of cholesterol metabolism can lead to…

cardiovascular disease

• Atherosclerosis = the obstruction of blood vessels from the pathological accumulation of cholesterol (plaques)

• Foam cells = form from macrophages

  • Macrophages stuffed with cholesterol → foam cells

• Shatters immune cell leads to cholesterol plaque build up in arteries

• Upregulating HDL doesn’t work to regulate cholesterol/heart disease

  • It is better to prevent synthesis of cholesterol 

statins

• Drug class used to treat patients with elevated serum cholesterol

  • Resemble mevalonate

  • Are competitive inhibitors of HMG-CoA reductase 

  • By blocking this step, all downstream steps in cholesterol biosynthesis are halted (since mevalonate is the precursor for cholesterol and other isoprenoids)

why is HMG-CoA reductase a good drug target for restricting cholesterol synthesis?

It is the commitment step in cholesterol synthesis

Familial hypercholesterolemia

• genetic linked disorder

• Characterized by extremely high blood levels of cholesterol

• Due to defective LDL receptor (causing rapid accumulation of LDL particles)

• Cholesterol accumulates in foam cells and contributes to the formation of artherosclerotic plaques

why does reverse cholesterol counter plaque formation and atherosclerosis?

Since HDL removes cholesterol from peripheral tissues and carries it to liver

familial HDL deficiency and Tangier disease 

• Familial HDL deficiency = HDL levels are very low (very high cholesterol guaranteed)

• Tangier disease = HDL levels are almost undetectable

• Both are a result of mutations in the ABCA1 protein

  • ABCA1 protein transports cholesterol into the HDL

  • apoA-I in cholesterol-depleted HDL cannot take up cholesterol from cells that lack ABCA1 protein 

  • HDL cannot mature → HDL levels in plasma are very low or absent

  • Cholesterol builds up inside cells, especially macrophages and tissues

True or False —> A diet low in cholesterol would make a meaningful difference in cholesterol build up in individuals with Tangier disorder, as they have a mutation/variant in a gene that is involved in cholesterol trafficking

FALSE since most cholesterol is synthesized rather than consumed so would need pharmacological treatment

elevated cholesterol and TAGs increases ____

LDL

Decreasing LDLs or increasing LDL uptake (recycling) back into the liver is protective against heart disease