Terpenoids and Biosynthetic Pathways

Terpenoids

• Terpenoids, also called isoprenoids, are natural products constructed from isoprene units (2-methyl-1,3-butadiene).
• There are over 55,000 known terpenoids.
• Terpenoids play vital roles in all living organisms, ranging from structural components to metabolic and regulatory functions.

Biosynthetic Pathways of Terpenoids

• Mevalonic Acid Pathway (MVA): Found in animals, plants (cytosol), fungi, and archaea.
  • Starts with Acetyl-CoA.
  • Key intermediate is Mevalonic acid (MVA).
  • Produces Isopentenyl pyrophosphate (IPP) and Dimethylallyl pyrophosphate (DMAPP), the building blocks of terpenoids.
• Methylerythritol Phosphate Pathway (MEP): Found in eubacteria, green algae, and plastids of higher plants.
  • Uses Pyruvate and Glyceraldehyde-3-Phosphate.
  • Involves the conversion of methylerythritol 4-phosphate to 4-hydroxy-3-methylbutenyl diphosphate.

Terpenes vs. Terpenoids

• Terpenes: Simple hydrocarbons, built from isoprene units.
• Terpenoids: Modified terpenes with functional groups. They are more diverse than terpenes due to the presence of functional groups and varying attachments of their isoprene subunits.

Isoprene Rule

• Otto Wallach (1887) proposed the isoprene rule, stating that all naturally occurring terpenoids can be constructed from isoprene units.
• Support for the rule:
  • Many terpenoids have an empirical formula of C5H8.
  • Thermal decomposition of terpenoids yields isoprene as a product.
• Special Isoprene Rule: Proposed by Ingold (1925). It states that isoprene units are linked "head-to-tail" in terpenoids.

Violations of the Isoprene Rule

• Some terpenoids have carbon contents that are not a multiple of five.
• Examples: Cryptone, Lavandulol.

Classification of Terpenoids

• Terpenoids are classified based on the number of isoprene units they contain:
  • Monoterpenoids: 2 isoprene units (C10H16).
  • Sesquiterpenoids: 3 isoprene units (C15H24).
  • Diterpenoids: 4 isoprene units (C20H32).
  • Triterpenoids: 6 isoprene units (C30H48).
  • Tetraterpenoids: 8 isoprene units (C40H64).
• They are also classified based on their ring structure:
  • Acyclic: Open chain.
  • Monocyclic: One ring.
  • Bicyclic: Two rings.
  • Tricyclic: Three rings.
  • Tetracyclic: Four rings.

Monoterpenoids

• Found in essential oils of plants.
• Commonly used in fragrances and pharmaceuticals due to their scent and biological activities.
• Examples: Limonene, Citral, beta-Citronellol.
  • Iridoids: A specific type of monoterpenoid with a cyclopentanopyran structure. They are often glycosides.

Sesquiterpenoids

• Three isoprene units (C15H24).
• Found in linear, cyclic, bicyclic, and tricyclic forms.
• Commonly found in latex-producing plants.
• Exert antimicrobial and insecticidal actions.
• Examples: Farnecene, Chrysanthemulide A, Drimenin.

Triterpenoids

• Six isoprene units (C30H48).
• Often exhibit biological activity.
• Examples: Squalene, Lanosterol, Lupeol.

Meroterpenoids

• Secondary metabolites with a partial terpenoid skeleton.
• Derived from the mevalonic acid pathway.
• Classified based on their biosynthetic origin (polyketide-terpenoids and nonpolyketide-terpenoids).
• Example: Amestolkolide B

Isolation of Terpenoids

• Methods:
  • Steam distillation: Used for volatile terpenoids in essential oils.
  • Solvent extraction: Using suitable solvents to isolate terpenoids.
  • Maceration: Soaking plant material in a solvent to extract terpenoids.

Structure Elucidation

• UV spectroscopy: Identifies conjugation.
• IR spectroscopy: Detects functional groups (e.g., hydroxyl, carbonyl). Distinguishes between cis and trans isomers.
• NMR spectroscopy: Identifies the type of hydrogen and carbon atoms.
• Mass spectroscopy: Determines the molecular weight.

Uses of Terpenoids

• Medicine:
  • Some terpenoids are used as anticancer drugs (e.g., Taxol).
  • Artemisinin and its derivatives are used as antimalarials.
• Fragrances:
  • Terpenes are used in perfumes, cosmetics, and flavorings due to their pleasant aromas.

Physical Properties

• Generally colorless liquids.
• Soluble in organic solvents, insoluble in water.
• Optically active.
• Volatile in nature.
• Boiling points range from 150 - 180 °C.

Chemical Properties

• Unsaturated compounds, susceptible to addition reactions.
• Undergo polymerization and dehydrogenation.
• Upon thermal decomposition they yield isoprene.

Explore the world of terpenoids, their structures, and their roles in living organisms through this quiz. Learn about the two main biosynthetic pathways: the Mevalonic Acid Pathway and the Methylerythritol Phosphate Pathway. Test your knowledge on the differences between terpenes and terpenoids.