27.5 Terpenoids

Terpenoid Biosynthesis and Reactions

Terpenoids (also called isoprenoids) are a huge class of natural products built from repeating five-carbon isoprene units. Understanding how they're assembled from simple precursors and then cyclized into complex structures is central to lipid biochemistry. This section covers the biosynthetic pathway from mevalonate to the major terpene classes, along with the key cyclization mechanisms.

Terpenoid Biosynthesis Pathway

All terpenoids trace back to two C5 building blocks: isopentenyl diphosphate (IPP) and dimethylallyl diphosphate (DMAPP). These are structural isomers, and an isomerase enzyme interconverts them. DMAPP is the "starter unit," while IPP is the "extender."

Prenyltransferase enzymes then catalyze head-to-tail condensation of IPP units onto DMAPP, building up the carbon chain in five-carbon increments:

• Geranyl diphosphate (GPP, C10) = DMAPP + 1 IPP → monoterpene precursor
• Farnesyl diphosphate (FPP, C15) = DMAPP + 2 IPP → sesquiterpene precursor
• Geranylgeranyl diphosphate (GGPP, C20) = DMAPP + 3 IPP → diterpene precursor

Notice the pattern: each addition of IPP adds five carbons. This is why terpene carbon counts always come in multiples of five.

Terpene synthase enzymes then convert GPP, FPP, and GGPP into the enormous variety of actual terpene structures through cyclizations, rearrangements, and other modifications. Some well-known products:

• Limonene (monoterpene, C10) — citrus scent
• Farnesene (sesquiterpene, C15) — found in apple skin
• Taxadiene (diterpene, C20) — precursor to the anticancer drug taxol

Mevalonate to Isopentenyl Diphosphate Conversion

IPP is synthesized from mevalonate through three sequential reactions. Each step requires ATP:

1. Mevalonate kinase phosphorylates mevalonate at the C5 hydroxyl, using ATP as the phosphate donor, to yield mevalonate 5-phosphate.
2. Phosphomevalonate kinase adds a second phosphate group (again from ATP), producing mevalonate 5-diphosphate.
3. Diphosphomevalonate decarboxylase catalyzes an ATP-dependent decarboxylation of mevalonate 5-diphosphate to give IPP.

That third step is the most mechanistically interesting. It proceeds through a carbocation intermediate: the loss of $CO_2$ generates a transient carbocation, which then collapses to form the double bond in IPP. The decarboxylation is thermodynamically favorable because $CO_2$ is released as a gas, pulling the equilibrium forward.

Mechanisms of Monoterpenoid Cyclization

Monoterpene cyclization from GPP follows a carbocation-driven mechanism. The steps are worth knowing in detail because they illustrate classic organic chemistry principles (carbocation stability, rearrangements, elimination).

1. Carbocation formation: The diphosphate leaving group departs from GPP, generating a resonance-stabilized allylic carbocation.
2. 1,6-Cyclization: The allylic carbocation attacks an internal double bond, forming a six-membered ring and producing a new tertiary carbocation. This cyclization is favorable because it creates a more stable (tertiary) cation.
3. Carbocation fate: The tertiary carbocation can then follow different paths depending on which terpene synthase is involved:
   • Proton elimination generates a double bond (yielding the final terpene)
   • Hydride shifts or alkyl migrations (Wagner-Meerwein rearrangements) move the carbocation to a new position before elimination or trapping

Different fates of that carbocation produce different monoterpenes:

• Limonene — 1,6-cyclization followed by proton elimination. Found in citrus peels.
• Menthol — 1,6-cyclization, then a hydride shift, then stereospecific reduction of the carbocation. The key flavor compound in peppermint oil.
• $\alpha$-Pinene and $\beta$-pinene — 1,6-cyclization followed by a Wagner-Meerwein rearrangement (an alkyl migration that forms the strained four-membered ring in the bicyclic skeleton), then proton elimination. Major components of pine resin and turpentine.

The takeaway: a single precursor (GPP) gives rise to structurally diverse products because different enzymes direct the carbocation through different rearrangement and elimination pathways.

Terpenoids as Natural Products and Secondary Metabolites

Terpenoids are classified as secondary metabolites, meaning they aren't required for basic cellular survival (like primary metabolites such as amino acids or sugars) but serve specialized ecological functions:

• Defense — many terpenoids are toxic or repellent to herbivores and pathogens
• Communication — volatile terpenoids act as chemical signals between plants or between plants and insects
• Pollinator attraction — floral scent terpenoids draw in bees and other pollinators

Their remarkable structural diversity comes from the enzymatic modifications described above: cyclizations that build complex ring systems, skeletal rearrangements that shuffle the carbon framework, and oxidation/reduction reactions that install or modify functional groups. With over 80,000 known terpenoid structures, this is one of the largest classes of natural products.