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Cholesterol synthesis- Steps and regulation- A quick revision
Cholesterol is derived from
- De novo synthesis and
- Hydrolysis of cholesteryl esters
- A little more than half the cholesterol of the body arises by synthesis (about 700 mg/d), and the remainder is provided by the average diet.
Site of synthesis
- The liver and intestine account for approximately 10% each of total synthesis in humans.
- Virtually all tissues containing nucleated cells are capable of cholesterol synthesis, which occurs in the endoplasmic reticulum and the cytosol.
Steps of synthesis of cholesterol
Remember the formula for cholesterol synthesis
4+2 = 6
5+5 = 10
15 +15 = 30
30-3 = 27
The representation of the numbers (Compounds with given carbon numbers) is as follows-
2 – Acetyl co A
4- Acetoacetyl co A
5- Isopentenyl pyrophosphate (IPP)
5+5 – Isopentenyl pyrophosphate +Dimethyl allyl pyrophosphate
10- Geranyl pyrophosphate
15- Farnesyl pyrophosphate
30 –Squalene and lanosterol
Details of steps
- Acetyl co A acts as a precursor of cholesterol.
- All 27 carbon atoms of cholesterol are derived from acetyl CoA in a three-stage synthetic process
Stages of cholesterol synthesis
Stage-1- Synthesis of Isopentenyl pyrophosphate, an activated isoprene unit that is the key building block of cholesterol
Stage-2- Condensation of six molecules of Isopentenyl pyrophosphate to form squalene
Stage-3- Cyclization of squalene to form lanosterol and subsequent conversion to cholesterol
a) Formation of Mevalonate
- Initially, two molecules of acetyl-CoA (2+2) condense to form Acetoacetyl-CoA (4) catalyzed by cytosolic thiolase.
- Acetoacetyl-CoA condenses with a further molecule of acetyl-CoA (4+2) catalyzed by HMG-CoA synthase to form HMG-CoA that is reduced to mevalonate (6) by NADPH catalyzed by HMG-CoA reductase (figure-1).
Figure-1- These reactions take place in the cytosol, formation of 3-hydroxy-3-methylglutaryl CoA (HMG CoA) takes place from acetyl CoA.
- The synthesis of mevalonate is the committed step in cholesterol formation.
- The enzyme catalyzing this irreversible step, 3-hydroxy-3-methylglutaryl CoA reductase (HMG-CoA reductase), is an important control site in cholesterol biosynthesis,
- It is the site of action of the most effective class of cholesterol-lowering drugs, the HMG-CoA reductase inhibitors (statins).
b) Formation of IPP (Isopentenyl pyrophosphate)
- Mevalonate is converted into 3-isopentenyl pyrophosphate in three consecutive reactions requiring ATP (figure-2)
- Decarboxylation (6-1 =5) yields Isopentenyl pyrophosphate (5), an activated isoprene unit that is a key building block for many important biomolecules (figure-2).
Figure-2- Mevalonate is phosphorylated first to form Mevalonate- 5-P by Mevalonate kinase, subsequently it is phosphorylated at 5th and 3rd position in two different steps to form 5-Pyrophospho mevalonate and 3-phosho-5-pyrophospho mevalonate respectively. 3-phosho-5-pyrophospho mevalonate is highly unstable and undergoes decarboxylation and loses phosphate also to form Isopentenyl pyrophosphate (IPP).
Stage-2- Formation of Squalene
- Squalene (C30) is synthesized from six molecules of Isopentenyl Pyrophosphate (C5).
- This stage starts with the isomerization of Isopentenyl pyrophosphate (C5) to dimethylallyl pyrophosphate (C5).
- Isopentenyl pyrophosphate is isomerized by a shift of the double bond to form dimethylallyl pyrophosphate that condenses with another molecule of Isopentenyl pyrophosphate (5+5 =10) to form the ten-carbon intermediate geranyl pyrophosphate (10).
- A further condensation with Isopentenyl pyrophosphate forms Farnesyl pyrophosphate (15). Two molecules of Farnesyl pyrophosphate (15+15) condense at the pyrophosphate end to form squalene (30) –figure-3
Figure-3- Initially, inorganic pyrophosphate is eliminated, forming presqualene pyrophosphate, which is then reduced by NADPH with elimination of a further inorganic pyrophosphate molecule.
Stage-3 –Formation of cholesterol
a) Formation of Lanosterol
- Squalene can fold into a structure that closely resembles the steroid nucleus
- Before ring closure occurs, squalene is converted to squalene 2,3-epoxide by a mixed-function oxidase in the endoplasmic reticulum, squalene epoxidase.
- The methyl group on C14 is transferred to C13 and that on C8 to C14 as cyclization occurs, catalyzed by oxidosqualene: lanosterol cyclase (figure-4).
- The newly formed cyclized structure is Lanosterol
Figure-4- Formation of Cholesterol from Lanosterol involves three processes-i) loss of three carbons, ii) Shifting of double bond and iii) Saturation of the double bond in the side chain.
b) Formation of cholesterol from Lanosterol (30 – 3 = 27)
- The formation of cholesterol from lanosterol takes place in the membranes of the endoplasmic reticulum and involves changes in the steroid nucleus and side chain.
- The methyl groups on C14 and C4 are removed to form 14-desmethyl lanosterol and then zymosterol.
- The double bond at C8–C9 is subsequently moved to C5–C6 in two steps, forming desmosterol.
- Finally, the double bond of the side chain is reduced, producing cholesterol (figure-4).
Regulation of cholesterol biosynthesis
Regulation of cholesterol synthesis is exerted near the beginning of the pathway, at the HMG-CoA reductase step. Following mechanisms are involved at the regulatory step-
1) Competitive inhibition
- Statins (Lovastatin, Mevastatin, Atorvastatin etc.) are the reversible competitive inhibitors of HMG Co A reductase (figure-1)
- They are used to decrease plasma cholesterol levels in patients of hypercholesterolemia.
2) Feed back inhibition
- HMG Co A reductase is inhibited by Mevalonate, bile acids and Cholesterol (Figure-1)
- Mevalonate is the immediate product of HMG Co A reductase catalyzed reaction whereas Cholesterol is the ultimate product of the reaction pathway.
3) Covalent modification (Role of hormones)- Figure-5
- Phosphorylation decreases the activity of the reductase.
- Glucagon favors formation of the inactive (phosphorylated form) form, hence decreases the rate of cholesterol synthesis
- In contrast , insulin favors formation of the active (dephosphorylated)form of HMG Co A reductase and results in an increase in the rate of cholesterol synthesis
- Cholesterol synthesis ceases when the ATP level is low.
Figure-5- HMG Co A reductase exists in two forms- Dephosphorylated form is the active while phosphorylated form is the inactive form. Phosphorylation is brought about by reductase kinase enzyme, that itself is phosphorylated by reductase kinase kinase enzyme to be active to bring about subsequent phosphorylation. The ultimate regulator of phosphorylation cascade is c-AMP which is released by Glucagon. Insulin brings about dephosphorylation by activating phosphatases.
4) Sterol mediated regulation of transcription
- The synthesis of cholesterol is also regulated by the amount of cholesterol taken up by the cells during lipoprotein metabolism.
- Chylomicron remnants internalized by liver cells, and low density lipoproteins internalized by liver cells and peripheral tissues provide cholesterol which causes a decrease in the transcription of HMG CoA reductase gene, leading to a decrease in cholesterol synthesis.
- Excess dietary intake thus suppresses endogenous cholesterol synthesis, whereas low cholesterol diet increases transcription of HMG Co A reductase gene causing more availability of enzyme to increase the synthesis of cholesterol.