Isoprenoid Compounds

Cholesterol is synthesized from acetyl‐CoA in the liver. Cholesterol and a number of natural products from plants (including rubber) are isoprenoid compounds. The isoprenoid unit is a 5‐carbon structure.

Isoprenoid compounds are synthesized from a common intermediate, mevalonic acid. Mevalonate is synthesized from acetyl‐CoA and then serves as the precursor to isoprenoid units.

acetyl‐CoA → mevalonate

The key enzyme in this pathway is HMG‐CoA reductase in connection with ketone body formation. The reactions leading to HMG‐CoA are shared with that pathway.

Acetyl‐CoA can be derived directly from metabolism, especially lipid degradation, or by the acetate thiokinase reaction:

Two acetyl‐CoA molecules condense to form acetoacetyl‐CoA.

Finally, the enzyme 3‐hydroxy‐3‐methylglutaryl‐CoA synthase (HMG‐CoA synthase) adds a third acetyl‐CoA.