Substrate preparation and localization of the process
The initial building block for the cholesterol molecule is acetyl-CoA. It is formed inside mitochondria (for example, as a result of pyruvate oxidative decarboxylation or $\beta$-oxidation of fatty acids). Since cholesterol synthesis itself takes place in the cytoplasm, acetyl-CoA must be transported across the mitochondrial membrane. This transport is carried out as part of citrate, exactly as it occurs during fatty acid synthesis.
The first reactions of cholesterol synthesis are chemically identical to the initial reactions of ketone body formation. However, there is a fundamental difference in localization: ketogenesis occurs exclusively in mitochondria, whereas the assembly of the cholesterol carbon skeleton begins in the cell cytosol.
Stage I: Mevalonate formation
At this stage, two-carbon acetyl-CoA fragments are converted into six-carbon mevalonic acid (mevalonate). The process includes three sequential reactions:
- Condensation of acetyl groups. The enzyme thiolase catalyzes the condensation of two molecules of acetyl-CoA. This yields acetoacetyl-CoA, and free coenzyme A (HS-CoA) is cleaved off as a byproduct.
- HMG-CoA synthesis. The enzyme HMG-CoA synthase adds another molecule of acetyl-CoA and a water molecule to acetoacetyl-CoA. This forms the intermediate $\beta$-hydroxy-$\beta$-methylglutaryl-CoA (HMG-CoA). HS-CoA is released again in the process.
- Mevalonate formation. This is the main regulatory reaction of the entire pathway. The enzyme HMG-CoA reductase reduces HMG-CoA to mevalonate (containing 6 carbon atoms). The reaction requires energy consumption: 2 molecules of NADPH + 2H⁺ are used as a coenzyme and oxidized to 2 NADP⁺.
Stage II: Squalene formation
The objective of the second stage is the formation of a long carbon chain of 30 carbon atoms. It begins with the activation of mevalonate and ends with the assembly of a linear polymer.
- Mevalonate activation. Six-carbon mevalonate undergoes simultaneous phosphorylation (consuming the energy of 3 ATP molecules, which are converted to 3 ADP) and decarboxylation (cleavage of a CO₂ molecule). This yields an active isoprenoid derivative, isopentenyl pyrophosphate, containing 5 carbon atoms.
- Condensation. Six molecules of mevalonate, previously converted into the form of isopentenyl pyrophosphate, undergo sequential condensation with each other. The final product of this stage is squalene, a long hydrophobic 30-carbon compound with a linear structure.
Stage III: Conversion of squalene to cholesterol
Starting from squalene, all intermediate metabolites become completely insoluble in water. Because of this, further synthesis continues not in the cytosol, but in the membrane layer of the endoplasmic reticulum (ER) with the participation of microsomal oxidation enzymes.
- Cyclization. The linear squalene molecule undergoes complex spatial cyclization. As a result, a rigid polycyclic core is formed, and the first cyclic product is lanosterol.
- Modification. Lanosterol goes through a series of chemical modifications, during which three carbon atoms are cleaved from its skeleton.
The result is the formation of the final product—a cholesterol molecule consisting of 27 carbon atoms and having a characteristic hydroxyl group at position 3.
Regulation of cholesterol biosynthesis
The rate of cholesterol formation in the cell is tightly controlled. The main regulatory target is the enzyme HMG-CoA reductase.
The enzyme transitions to an active state upon:
- Increased secretion of insulin. This hormone stimulates the activity of intracellular phosphatases.
- The action of phosphatases themselves, which cleave the inhibitory phosphate group from the enzyme (dephosphorylation converts the reductase into its active form).
The enzyme is inhibited (becomes inactive) upon:
- The action of kinases, which phosphorylate the enzyme. This process is actively stimulated by the counter-regulatory hormone glucagon.
- A decrease in blood glucose concentration (e.g., during fasting).
- Dietary intake of excess cholesterol. Exogenous cholesterol acts as an allosteric inhibitor and causes repression of the genes responsible for enzyme synthesis.
Additionally, an important step in metabolism is the conversion of cholesterol itself into bile acids. The key enzyme of this process is $7\alpha$-hydroxylase (catalyzing the formation of $7\alpha$-hydroxycholesterol). It is inhibited by the end product, i.e., bile acids, via a classical negative feedback mechanism.
Briefly: answers to topical questions
In which organs and tissues of the body does cholesterol biosynthesis proceed most actively?
Cholesterol biosynthesis proceeds most actively in the liver and small intestine, although it can occur in most cells of the body.
Main organs and tissues:
- Liver — synthesizes about 75–80% of all cholesterol, being its main supplier for other tissues.
- Small intestine — provides the synthesis of approximately 15% of cholesterol.
- Skin — participates in biosynthesis along with other tissues.
- Glands — the adrenal cortex and gonads, which produce steroid hormones.
Intracellularly, the process is localized in the cytosol and the endoplasmic reticulum.
Which vitamins and coenzymes are required for all stages of cholesterol biosynthesis?
The main required cofactor for cholesterol biosynthesis is the coenzyme NADPH, while other compounds are also used in the process.
Coenzymes and participants of synthesis:
- NADPH — acts as a coenzyme in the regulatory reaction of reducing HMG-CoA to mevalonate. Its main source is the pentose phosphate pathway of glucose catabolism.
- Coenzyme A (CoA) — carries acetyl groups, being part of the initial substrate (acetyl-CoA). Coenzyme A includes vitamin B₅ (pantothenic acid) in its structure.
- ATP — is necessary for phosphorylation reactions (e.g., mevalonate activation).
Class and exam questions
What is the main difference between the initial stages of cholesterol synthesis and ketogenesis?
Although the chemistry of the initial reactions (formation of acetoacetyl-CoA and HMG-CoA) is the same, they occur in different cell compartments. Ketone body synthesis occurs in mitochondria, whereas cholesterol synthesis occurs in the cytoplasm.
Why do the final stages of cholesterol synthesis take place in the ER?
Starting from the squalene stage, intermediate metabolites become insoluble in water (hydrophobic). Therefore, cyclization and modification reactions can take place only in the lipid bilayer of endoplasmic reticulum membranes.
How does fasting affect cholesterol biosynthesis?
During fasting, glucose levels drop and glucagon is produced. It activates kinases that phosphorylate the key enzyme, HMG-CoA reductase, thereby inactivating it and halting synthesis.
What else to study on this topic
- Digestion and transport of exogenous cholesterol
- Biological functions of cholesterol in cell membranes
- Role and mechanisms of lipid microsomal oxidation
- Transport of acetyl-CoA from mitochondria to the cytoplasm
- Synthesis of bile acids from cholesterol
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