The Fourth Stage of Synthesis

The Four Ring Steroid Basis From Squalene.

All sterols consist of 4 fused rings, and are alcohols, with the hydroxyl group at C-3.

It can be seen quite easily that the linear squalene molecule

Squalene
can be converted into the 4 fused ring structure of sterols.
Squalene in position to cyclise
  1. Squalene monooxygenase adds 1 oxygen atom to the end of the squalene chain, forming an epoxide.
  2. NADPH is an electron donor and this reduces the other oxygen of the oxygen molecule to water.
  3. The double bonds of squalene 2,3-epoxide are positioned such that a concerted reaction can occur, which converts the linear squalene into a cyclic structure, called Lanosterol.
  4. Lanosterol is then converted to cholesterol.
Cholesterol

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Synthesis of Cholesterol

Third Stage of Synthesis