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Terpenoids Biosynthesis

Primary metabolites are found in all plants and perform essential metabolic roles. Secondary metabolites are substances produced by plants that do not have the appearance of having a hand in growth or development. Isopentenyl diphosphate (IPP) is a five-carbon precursor that terpenoids are derived from. Terpenoids include both primary and secondary metabolites (around 25,000). The monomers in the isoprene units often yield alkene gas isoprene due to the decomposition of the terpenoid substances. Isoprene can then, under suitable chemical circumstances, can activate isoprene to polymerize in multiples of five carbons. This creates a diverse amount of terpenoid skeletons. The different terpenoids are classified by the number of carbons; hemiterpenoids (C5), monoterpenoids (C10), sesquiterpenoids (C15), diterpenoids (C20), sesterterpenoids (C25), triterpenoids (C30), tetraterpenoids (C40), and polyterpenoids (C>40). Hemiterpenoids are a volatile hydrocarbon isoprene found in some synthetic versions of rubber. Monoterpenoids are often found in essential oils extracted from various plants. Sesquiterpenoids are the most prevailing class of terpenoids and include a range of biological functions such as antitumor agents, anti-inflammatory and antimicrobial. Diterpenoids are non-volatile hydrocarbons known for having a microbial host. A common diterpenoid is Taxol, which is used in cancer treatment. Sesterterpenoids are fairly rare and found in waxes on fungi and insects. Triterpenoids can have a wide range of structures and known to be used in cosmetic products. Tetraterpenoids are known for their antioxidant activity and commercial use as food colorants. Overall, there is four steps to the biosynthesis of terpenoids.

First, the precursor IPP is synthesized. Compared to animals and microbes, plants cultivate a larger assortment of terpenoids. The complex organization of plants on the cellular, sub cellular and genetic level are the reason for this. The cytosolic IPP pathway involves a two-step condensation of three molecules. Acetyl-CoA is catalyzed by thiolase and hydroxymethylglutaryl- CoA synthase. Although regulation of the pathways is sometimes difficult to acquire, plastids can supply IPP to the cytosol. The product of this pathway is then reduced in several coupled reactions. 3-hydroxy-3 methylglutaryl- CoA is reduced by HMG-CoA reductase to form mevalonic acid. To yield IPP from mevalonic acid two consecutive ATP dependent phosporylations must occur and a subsequent elimination assisted decarboxylation. The repetition of addition of IPP must then occur to form the intermediate precursor of several classes of terpenoids. The intermediate precursor is a series of prenyl diphosphate homolog. To yield terpenoid skeletons the amplification of these allylic prenyl diphospahtes by certain terpenoid synthases then occur. The skeletons can then be modified. This mainly occurs by redox reactions, which create the chemical diversity and functional properties terpenoids are best known for.