Triterpenoids have 30 carbons and six isoprene units. They are derived from (S)-2,3-epoxysqualene. They may contain rings or be acyclic, depending on the bonds formed by the loss of the diphosphate group. First, the terpenoid backbone is synthesized, producing farnesyl pyrophosphate. Two molecules of farnesyl pyrophosphate then join together to form presqualene diphosphate, catalyzed by squalene synthase 1. Then, the same enzyme removes the pyrophosphate group and replaces it with a hydrogen ion, forming squalene. Squalene then undergoes oxidation of one of its bonds via squlene monooxygenase 1, to form (S)-2,3-epoxysqualene. This may then proceed to the steroid biosynthesis pathway or may react with an isomerase or lyase to form a chair-chair-chair-boat triterpenoid. Similarly, squalene may interact with an isomerase or lyase to form a chair-chair-chair-chair triterpenoid. After the backbone is complete, (S)-2,3-epoxysqualene can interact with many enzymes in order to form the triterpenoids. It can interact with camelliol C synthase to form camelliol C, thalianol synthase to form thalianol, baruol synthase to form baruol, tirucalladienol synthase to form tirucalla-7,24-dien-3-beta-ol, amyrun synthase LUP2 to form lupeol, alpha- and beta-amyrin synthases to form alpha- and beta-amyrin respectively. It can also interact with lupan-3beta,20-diol synthase to add a water molecule to form lupan-3beta,20-diol, alpha- and beta-seco-amyrin synthases to form alpha- and beta-seco-amyrin respectively, marneral synthase to form marneral, and finally arabidiol synthase to add a water molecule and form arabidiol.

Triterpenoid Biosynthesis References