Cholesterol Biosynthesis: Pathway, Steps, Regulation, HMG-CoA Reductase & Clinical Significance

 

Table of Contents

• Introduction to Cholesterol Biosynthesis
• Site and Significance of Cholesterol Biosynthesis
• The Pathway Breakdown: From Acetyl-CoA to Cholesterol
• Regulation of Biosynthesis: Hormonal Control and the SREBP Pathway
• Clinical Pharmacology: How Statins Inhibit Cholesterol Synthesis
• The Fate of Cholesterol: Bile Acid Synthesis and Esterification
• Conclusion 
• Reference 

Ready Made Presentation

Introduction to Cholesterol Biosynthesis

• Cholesterol is the most important sterol present in the human body and plays essential structural and metabolic roles.
• Approximately half of the body’s cholesterol is produced internally through de novo biosynthesis, rather than being obtained from dietary sources.
• The liver and intestines are the major sites responsible for endogenous cholesterol production.
• Among these, the liver is the primary site, contributing nearly 80% of the total cholesterol synthesized in mammals.
• The key regulatory and rate-limiting enzyme in cholesterol biosynthesis is HMG-CoA reductase (3-hydroxy-3-methylglutaryl-coenzyme A reductase), which controls the overall pathway.
• Cholesterol synthesis begins with acetyl-CoA, a central metabolic intermediate derived from the metabolism of proteins, carbohydrates, and fats.
• The biosynthetic pathway is highly energy-intensive, requiring significant amounts of metabolic energy in the form of ATP and reducing power in the form of NADPH.
• For the synthesis of one mole of cholesterol, the pathway requires 18 moles of acetyl-CoA, 36 moles of ATP, and 16 moles of NADPH, highlighting the high energetic cost of this process.

Site and Significance of Cholesterol Biosynthesis

• Cholesterol biosynthesis primarily takes place in the cytoplasm and endoplasmic reticulum of liver cells, where the necessary enzymes and substrates are localized.
• This synthesis supplies cholesterol that is essential for cellular homeostasis, particularly by maintaining membrane structure, contributing to both rigidity and fluidity of the lipid bilayer.
• Cholesterol serves as a vital precursor for steroid hormones, including glucocorticoids, mineralocorticoids, and sex hormones, which are produced in endocrine tissues and regulate numerous physiological processes.
• It is also converted into bile acids, a process that not only aids in the excretion of excess cholesterol but also plays a crucial role in the digestion and absorption of dietary lipids.
• Additionally, cholesterol acts as the backbone for vitamin D synthesis, which is essential for calcium homeostasis and proper bone mineralization.

The Pathway Breakdown: From Acetyl-CoA to Cholesterol

Cholesterol biosynthesis can be divided into five key stages:

1. Synthesis of HMG-CoA
2. Formation of mevalonate (6C)
3. Production of isoprenoid units (5C)
4. Synthesis of squalene (30C)
5. Conversion of squalene to cholesterol (27C)

1. Formation of HMG-CoA: The Initial Condensation Steps

• The process begins with two molecules of acetyl-CoA, which condense to form acetoacetyl-CoA.
• This reaction is catalyzed by thiolase (acetyl-CoA acetyl transferase 2, ACAT2) in the cytosol.
• Acetoacetyl-CoA then reacts with a third molecule of acetyl-CoA to produce HMG-CoA.
• The enzyme cytosolic HMG-CoA synthase (encoded by HMGCS1) catalyzes this step.
• HMG-CoA serves as the precursor for mevalonate synthesis, the next stage of cholesterol biosynthesis.

2. The Rate-Limiting Step: Synthesis of Mevalonate by HMG-CoA Reductase

• HMG-CoA is converted into mevalonate by HMG-CoA reductase (HMGR), an endoplasmic reticulum-bound enzyme.
• This reaction requires two molecules of NADPH, making it NADPH-dependent.
• It is the rate-limiting and most important regulatory step in cholesterol biosynthesis.

3. From Mevalonate to Squalene: Formation of Isoprenoid Units

• Mevalonate is phosphorylated by mevalonate kinase and phosphomevalonate kinase to form mevalonate-5-pyrophosphate, requiring ATP.
• Mevalonate-5-pyrophosphate is decarboxylated by mevalonate-5-pyrophosphate decarboxylase to produce isopentenyl pyrophosphate (IPP) and CO₂.
• IPP is isomerized to dimethylallyl pyrophosphate (DMAPP) by isopentenyl pyrophosphate isomerase.
• IPP and DMAPP condense to form geranyl pyrophosphate (GPP, 10C), catalyzed by geranyl pyrophosphate synthase.
• GPP combines with another IPP to produce farnesyl pyrophosphate (FPP, 15C) via farnesyl pyrophosphate synthase (FDPS).
• Finally, two FPP molecules condense through squalene synthase (FDFT1) to generate squalene (30C).

4. Cyclization of Squalene: Creating the Steroid Nucleus (Lanosterol)

• Squalene is cyclized in two steps to produce lanosterol.
• First, squalene epoxidase (SQLE) introduces an epoxide at the 2,3 position, forming 2,3-oxidosqualene; this reaction requires NADPH and O₂.
• Second, lanosterol synthase (LSS) catalyzes the cyclization of 2,3-oxidosqualene to form lanosterol, establishing the four-ring steroid nucleus, essential for all downstream cholesterol and steroid derivatives.

5. Final Processing: Conversion of Lanosterol to Cholesterol

• Conversion of lanosterol to cholesterol involves 19 enzymatic reactions, including demethylation, desaturation, isomerization, and reduction.
• The Kandutsch-Russell pathway is the primary human pathway, producing 7-dehydrocholesterol as the immediate precursor.
• Key reactions include:
• Reduction of carbons from 30 to 27
• Removal of two methyl groups at C4 and one at C14
• Shift of a double bond from C8 to C5
• Reduction of the double bond between C24 and C25
• Important intermediates include desmethyl lanosterol, zymosterol, cholestadienol, and desmosterol.
• The final step converts 7-dehydrocholesterol into cholesterol via DHCR7.

Regulation of Biosynthesis: Hormonal Control and the SREBP Pathway

Hormonal Control

• HMG-CoA reductase is the rate-limiting enzyme in cholesterol biosynthesis and plays a central role in regulating cholesterol production.
• The enzyme exists in two forms: dephosphorylated (active) and phosphorylated (inactive).
• Insulin and thyroxine stimulate cholesterol synthesis by promoting the formation of the active dephosphorylated enzyme.
• Glucagon and glucocorticoids inhibit cholesterol synthesis by stabilizing the phosphorylated (inactive) form of HMG-CoA reductase.

SREBP Pathway (Transcriptional Regulation)

• SREBPs (Sterol Regulatory Element-Binding Proteins) are transcription factors synthesized as inactive precursors in the endoplasmic reticulum (ER) membrane.
• Under low cellular cholesterol conditions, SREBPs bind to SCAP (SREBP cleavage-activating protein) and are transported to the Golgi apparatus.
• In the Golgi, Site-1 and Site-2 proteases cleave SREBP, releasing its active domain, which translocates to the nucleus.
• Nuclear SREBP activates transcription of genes encoding HMG-CoA reductase, HMG-CoA synthase, and other enzymes in the mevalonate pathway, thereby promoting cholesterol synthesis.
• When ER cholesterol levels rise, cleavage of SREBP is inhibited by INSIG proteins, reducing transcription of cholesterol biosynthetic genes.
• Additionally, oxysterols (e.g., 25-hydroxycholesterol) act as feedback inhibitors of SREBP, further controlling cholesterol biosynthesis.

Clinical Pharmacology: How Statins Inhibit Cholesterol Synthesis

• Statins are FDA-approved medications used to treat hyperlipidemia and hypercholesterolemia.
• They act as reversible competitive inhibitors of HMG-CoA reductase, the rate-limiting enzyme in cholesterol synthesis.
• Structurally, statins are analogs of HMG-CoA, allowing them to compete with the natural substrate for binding to the enzyme.
• Inhibition of HMG-CoA reductase prevents the conversion of HMG-CoA to mevalonate, thereby blocking de novo cholesterol synthesis in the liver.
• The resulting low intracellular cholesterol triggers SREBP-mediated transcriptional regulation, which increases the expression of LDL receptors on cell surfaces.
• Elevated LDL receptor levels enhance cellular uptake and clearance of blood LDL cholesterol.
• The overall pharmacologic effect is a reduction in plasma LDL cholesterol and a lowered risk of cardiovascular disease.
• Common statins include atorvastatin, simvastatin, and lovastatin, all of which are fungal-derived HMG-CoA reductase inhibitors.

The Fate of Cholesterol: Bile Acid Synthesis and Esterification

Bile Acid Synthesis

• Cholesterol conversion to primary bile acids (cholic acid [CA] and chenodeoxycholic acid [CDCA]) occurs exclusively in the liver.
• There are two pathways for bile acid synthesis: the classic (neutral) pathway and the alternative (acidic) pathway.
• In the classic pathway, cholesterol 7α-hydroxylase (CYP7A1) catalyzes the rate-limiting first step, converting cholesterol to 7α-hydroxycholesterol.
• Sterol 12α-hydroxylase (CYP8B1) oxidizes C-12 to produce cholic acid (CA); absence of this enzyme results in CDCA formation.
• CYP27A1 catalyzes side-chain oxidation, producing C24 bile acids.
• In the alternative pathway, CYP27A1 first converts cholesterol to 27-hydroxycholesterol, which is then hydroxylated by CYP7B1 to form CDCA.
• Primary bile acids are conjugated with glycine or taurine in the liver and secreted into bile.
• Intestinal bacterial enzymes convert primary bile acids into secondary bile acids, such as deoxycholic acid and lithocholic acid.
• Bile acids are essential for lipid digestion, cholesterol elimination, and reverse cholesterol transport from peripheral tissues to the liver.

Esterification

• Cholesterol is esterified to increase hydrophobicity, allowing it to be safely stored or transported.
• In the liver, the enzyme ACAT (acyl-CoA cholesterol acyltransferase) catalyzes the addition of a fatty acid to cholesterol, forming cholesteryl esters (CEs).
• In plasma, LCAT (lecithin cholesterol acyltransferase) performs esterification associated with HDL particles.
• Cholesteryl esters are more hydrophobic than free cholesterol, enabling storage in lipid droplets or incorporation into VLDL for transport.
• VLDL carries cholesteryl esters, triglycerides, and phospholipids to tissues for membrane synthesis, steroid hormone production, and vitamin D synthesis.
• Esterification prevents toxic accumulation of free cholesterol in cells and facilitates regulated cholesterol distribution, maintaining cholesterol homeostasis and lipoprotein-mediated transport.

Conclusion 

• Cholesterol is a vital sterol in the human body, with almost half synthesized de novo, primarily in the liver and intestine.
• Cholesterol biosynthesis occurs in the cytoplasm and endoplasmic reticulum, requiring acetyl-CoA, ATP, and NADPH.
• The biosynthetic pathway proceeds through five key steps:
• Formation of HMG-CoA
• Mevalonate synthesis
• Formation of isoprenoid units
• Squalene formation
• Cholesterol formation
• The rate-limiting and most important regulatory enzyme is HMG-CoA reductase.
• Cholesterol is essential for membrane integrity, steroid hormone synthesis, bile acid production, and vitamin D synthesis.
• Hormonal regulation influences cholesterol production: insulin stimulates while glucagon suppresses its synthesis.
• The SREBP pathway provides transcriptional control in response to intracellular cholesterol levels.
• Statins reduce cholesterol by competitively inhibiting HMG-CoA reductase and upregulating LDL receptor expression, promoting LDL clearance.
• The liver converts cholesterol into bile acids, aiding in lipid digestion and cholesterol excretion.
• Esterification of cholesterol, facilitated by ACAT and LCAT, allows safe storage and transport of cholesteryl esters, maintaining cholesterol homeostasis.

Reference 

• Chiang, J. Y. L., Ferrell, J. M., Wu, Y., & Boehme, S. (2020). Bile acid and cholesterol metabolism in atherosclerotic cardiovascular disease and therapy. Cardiology Plus, 5(4), 159–170.
• Craig, M., Yarrarapu, S. N. S., & Dimri, M. (2023, August 8). Biochemistry: Cholesterol. In StatPearls.
• Jakubowski, H., & Flatt, R. (n.d.). Lipid biosynthesis. In Fundamentals of Biochemistry. LibreTexts Biology.
• Satyanarayana, U., & Chakrapani, U. (2017). Biochemistry (5th ed.). Elsevier India.
• Scitable. (n.d.). Cholesterol metabolism. Biology Ease.
• Vasudevan, D. M. (n.d.). Cholesterol synthesis, metabolism, and regulation. The Medical Biochemistry Page. Retrieved from https://themedicalbiochemistrypage.org/cholesterol-synthesis-metabolism-and-regulation/