Fig. 1.

 A schematic representation of MEV/MVA and MEP biosynthetic pathways of triterpenoids and their types. (A) The MVA pathway originating from the cytosol of the cell from Acetyle-CoA. It is induced into peroxisome via a series of steps at the point of MVP to MVPP. The enzymes proposed to have the highest degree of control over the metabolic flux via the MVA pathway (HMGR) are underlined. The negative regulators of the MVA pathway at the post-transcriptional level are colored in pink. (B) The MEP pathway occurs in the plastids. It starts from pyruvate and G3P molecules and ends when GPP is form via a series of steps. The enzymes proposed to have the highest degree of control over the metabolic flux via the MEP pathway (DXS) are underlined. The positive and negative regulators of the MEP pathway at the posttranscriptional level are colored in pink and red, respectively. (C) In the endoplasmic reticulum, squalene is synthesized via the fusion of both pathways. GPP leads to the synthesis of 2,3-oxidosqualene. 2,3-Oxidosqualene is the most common compound for pentacyclic as well as tetracyclic products. The enzymes that catalyze the steps are shown along with the arrows. (D) The biosynthetic pathway of pentacyclic triterpenoid scaffolds. Abbreviations: Oxidosqualene cyclase (OSC), chair-chair-chair (C-C-C), multi-functional oxidosqualene cyclase (MOSC), lupeol synthase (LUP) and β-amyrin synthase (BAS). (E) The possible biosynthetic routes for tetracyclic triterpenoids. Abbreviations: (C-C-C) chair-chair-chair manner, (C-B-C) chair-boat-chair manner, (OSC) oxidosqualene synthase cyclases, (LAS) lanosterol synthase, (LAS1) lanosterol synthase 1, (LAS2) lanosterol synthase 2, (FMO) flavin-containing monooxygenase and (GST) glutathione S-transferase