Substrate Preparation and Cellular Localization
The starting building block for the cholesterol molecule is acetyl-CoA. It is formed inside mitochondria (e.g., via oxidative decarboxylation of pyruvate or $\beta$-oxidation of fatty acids). Because cholesterol synthesis itself occurs in the cytoplasm, acetyl-CoA must be transported across the mitochondrial membrane. This transport is carried out as citrate—in the exact same manner as during fatty acid synthesis.
The initial reactions of cholesterol synthesis are chemically identical to the early steps 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: Formation of Mevalonate
This stage involves the conversion of two-carbon acetyl-CoA fragments into the 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, releasing free coenzyme A (HS-CoA) as a byproduct.
- Synthesis of HMG-CoA. The enzyme HMG-CoA synthase adds another molecule of acetyl-CoA and a water molecule to acetoacetyl-CoA. This produces the intermediate $\beta$-hydroxy-$\beta$-methylglutaryl-CoA (HMG-CoA), releasing another molecule of HS-CoA.
- Formation of mevalonate. This is the primary regulatory step of the entire pathway. The enzyme HMG-CoA reductase reduces HMG-CoA to mevalonate (containing 6 carbon atoms). The reaction requires energy: 2 molecules of NADPH + 2H⁺ are oxidized to 2 NADP⁺ as coenzymes.
Stage II: Formation of Squalene
The objective of the second stage is the formation of a 30-carbon long-chain hydrocarbon. It begins with the activation of mevalonate and ends with the assembly of a linear polymer.
- Activation of mevalonate. The six-carbon mevalonate undergoes simultaneous phosphorylation (consuming 3 ATP molecules, which are converted to 3 ADP) and decarboxylation (cleavage of a CO₂ molecule). This produces the active isoprenoid derivative isopentenyl pyrophosphate, containing 5 carbon atoms.
- Condensation. Six molecules of mevalonate, previously converted into isopentenyl pyrophosphate, sequentially condense with one another. The end 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 water-insoluble. Consequently, further synthesis continues not in the cytosol, but in the membrane layer of the endoplasmic reticulum (ER) via microsomal oxidation enzymes.
- Cyclization. The linear squalene molecule undergoes complex spatial cyclization. This forms a rigid polycyclic core, with lanosterol being the first cyclic product.
- Modification. Lanosterol undergoes a series of chemical modifications during which three carbon atoms are removed from its skeleton.
The result is the formation of the final product—the cholesterol molecule, consisting of 27 carbon atoms and featuring a characteristic hydroxyl group at the C3 position.
Regulation of Cholesterol Biosynthesis
The rate of cholesterol production within the cell is tightly controlled. The primary regulatory target is the enzyme HMG-CoA reductase.
The enzyme shifts to an active state upon:
- Increased insulin secretion. This hormone stimulates intracellular phosphatases.
- The action of phosphatases, 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).
- Excess dietary cholesterol intake. Exogenous cholesterol acts as an allosteric inhibitor and represses the genes responsible for enzyme synthesis.
Additionally, an important step in cholesterol metabolism is its conversion into bile acids. The key enzyme for this process is $7\alpha$-hydroxylase (catalyzing the formation of $7\alpha$-hydroxycholesterol). It is inhibited by its end product, bile acids, via classical negative feedback.