## 2 Source and Structure of Triterpenoids  
Triterpenoids are metabolites of isopentenyl pyrophosphate oligomers that are chemically related to squalene, which is a large group of compounds having 30 carbon atoms arranged in five rings with several oxygen atoms attached. Triterpenoids are part of the largest group of plant products, Saponins can be chemically biosynthesized when one or more sugar moieties attach to aglycone. There are two types of saponins, steroidal aglycone and triterpenoid aglycone. Both steroid and triterpenoid systems  
\begin{table}
\begin{tabular}{l l} \hline
**Disease** & **Triterpenoid** \\ \hline Diabetes & Astragaloside, Cucurbitacin, Diosgenin, Ginsenoside, Amyrin, Asiatic acid, Avicin, \\  & Betulinic acid, Escin, Glycyrrhizin, Oleanolic acid, Platycodon D, Ursolic acid, \\  & Withanolide \\ Cardiovascular & Astragaloside, Cucurbitacin, Diosgenin, Ginsenoside, Gypenoside, Oleandrin, \\  & Betulinic acid, Escin, Glycyrrhizin, Lupeol, Oleanolic acid, Platycodon D, \\  & Saikosaponins, Ursolic acid, Withanolide \\ Arthritis & Cucurbitacin, Diosgenin, Ginsenoside, Amyrin, Boswellic acid, Celastrol, \\  & Glycyrrhizin, Lupeol, Oleanolic acid, CDDO-Me, Ursolic acid, Withanolide, \\  & Diosgenin, Gypenoside, Betulinic acid, Glycyrrhizin, Oleanolic acid, Ursolic acid \\ Obesity & Diosgenin, Ginsenoside, Betulinic acid, Escin, Glycyrrhizin, Platycodon D, \\  & Momordin, Oleanolic acid, Ursolic acid \\ Alzheimer & CDDO-MA, Alpha-onocerin \\ Parkinson & CDDO-MA \\ Multiple & Oleanolic acid \\ sclerosis & \\ Depression & Asiatic acid \\ Osteoporosis & Ursolic acid \\ Cerebral & Escin, Asiatic acid \\ ischemia & \\ Memory loss & CDDO-MA \\ \hline \end{tabular}
\end{table}
Table 3: Other uses of triterpenoid in treatment of chronic diseases.  
are found to be biosynthesized from a common precursor such as squalene [139]. Triterpenoids are synthesized from isopentenyl pyrophosphate (IPP) and its isomer dimethylallyl pyrophosphate (DMAPP). For this cyclization, three prenyltransferases synthesize the linear prenyl pyrophosphates geranyl pyrophosphate (GPP), farnesyl pyrophosphate (FPP), and geranylgeranyl pyrophosphate (GGPP). Squalene is in turn derived biosynthetically by the cyclization of a number of units of isoprene, (C\({}_{5}\)H\({}_{8}\))n, which undergo folding through 20 different patterns in the presence of prenyl pyrophosphates to form monocyclic, dicyclic, tricyclic, tetracyclic, or pentacyclic derivatives [140]. A family of oxidosqualene cyclases may produce only a single product, such as lupeol cyclases, but there are also multifunctional oxidosqualene cyclases that use dammanenyl cation intermediates to produce many products. Once squalene undergoes cyclization, it goes through the cytosolic mevalonate pathway to make a proximate tetracyclic C30 compound, lanosterol (Figure 4), which further undergoes oxidation and catabolic metabolism to form cholesterol.  
The variety of triterpenoids in nature is a result of the evolution of a large terpene synthase superfamily. One study analyzed the amino acid sequences of terpene synthase genes and found that all originated from an ancestral diterpene synthase. It was also found that the diversity of these triterpenoids is due to the structural features of their catalyst enzymes. Terpenes and their metabolites are widely distributed in various plant systems that depend on various biotic and abiotic environmental factors. Terpenes and their metabolites are used in several developmental and physiological functions on the basis of the differential expression profiles of terpene synthase genes. Terpenes and their metabolites play a very important role in a plant's defense mechanism. They protect the plants from both constitutive and induced defensive responses against insects and environmental stress [141, 142]. Hence, triterpenoids provide a very good protection shield for plants, indicating their potential for use in the prevention of various cancers and inflammatory diseases in humans.