Cholesterol biosynthesis HMG-CoA reductase Malonyl-CoA and regulation of fatty acid oxidation

Cholesterol

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• Cellular Location: Cytosol and the Smooth Endoplasmic Reticulum (SER).

• Primary Organ: Liver (accounts for the vast majority of de novo synthesis).

• Starting Substrate: Acetyl-CoA (requires 18 molecules of Acetyl-CoA to build one molecule of cholesterol).

• Energy Requirement: Highly endergonic, requiring substantial ATP and NADPH (from the Pentose Phosphate Pathway).

Condensation

2 X Acetyl-CoA → Acetoacetyl-CoA.

Formation of HMG-CoA

Acetoacetyl-CoA + Acetyl-CoA → 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA). (Enzyme: HMG-CoA synthase).

The Rate-Limiting Step

HMG-CoA is reduced to Mevalonate. (Enzyme: HMG-CoA reductase).

Isoprenoid Formation

Mevalonate is phosphorylated and decarboxylated to form active isoprenoids (5-carbon units).

Squalene Formation

Isoprenoids condense into Squalene (a 30-carbon linear molecule)

Cyclization

Squalene cyclizes to form Lanosterol, which undergoes multiple reactions to finally yield Cholesterol (27 carbons).

Hormonal (Covalent Modification)

Activator (Increases Synthesis): Insulin: Activates a phosphatase that dephosphorylates (activates) the enzyme.

Inhibitor (Decreases Synthesis): Glucagon / Epinephrine: Activate AMPK, which phosphorylates (inactivates) the enzyme.

Allosteric / Feedback

Activator (Increases Synthesis): Depleted intracellular cholesterol levels.

Inhibitor (Decreases Synthesis): Intracellular Cholesterol: High levels trigger the degradation of the enzyme and inhibit its gene transcription.

Statins

(e.g., Atorvastatin, Simvastatin) are structural analogs of HMG-CoA. They act as competitive inhibitors of HMG-CoA reductase, halting de novo cholesterol synthesis. This lowers intracellular cholesterol, forcing the liver to upregulate LDL receptors to pull LDL cholesterol out of the bloodstream.

Malonyl-CoA

is the first committed intermediate in fatty acid synthesis (lipogenesis). It is formed from Acetyl-CoA in the cytosol by the rate-limiting enzyme Acetyl-CoA Carboxylase (ACC)

Carnitine Shuttle

Fatty acid degradation (beta-oxidation) happens inside the mitochondria. However, long-chain fatty acids in the cytosol cannot pass through the inner mitochondrial membrane on their own. They require a specialized transporter known as the

Carnitine Palmitoyltransferase I (CPT-I)

The gatekeeper of this shuttle is the enzyme ‘BLANK’, also known as Carnitine Acyltransferase I (CAT-1), located on the outer mitochondrial membrane

In the Fed State (High Insulin)

Insulin activates ACC. Acetyl-CoA is converted to Malonyl-CoA. High Malonyl-CoA inhibits CPT-I. Fatty acids cannot enter the mitochondria. Fatty acid synthesis proceeds; beta-oxidation is blocked

In the Fasting State (High Glucagon/Epinephrine)

Glucagon activates AMPK, which phosphorylates and inhibits ACC. Malonyl-CoA levels drop sharply. The inhibition on CPT-I is lifted. Fatty acids enter the mitochondria. $\beta$-oxidation proceeds; fatty acid synthesis is blocked.