Biosynthesis of Terpenoids

Introduction
  • Class format: Discussing the biosynthesis of terpenoids.
  • Reminder for students to fill out the National Student Survey (NSS) to give feedback for course improvement.
Acetyl Coenzyme A (Acetyl CoA)
  • Acetyl CoA: Key compound in the biosynthesis of terpenoids; reactive like an acid chloride.
  • Commonly generated in the mitochondria, not in highly aqueous conditions.
  • Note on leaving groups: CoA is a good leaving group due to the reactive carbonyl group.
  • pKa of acetyl CoA: ~12, indicating weak acidity but notable enolate formation capability.
Mevalonate Pathway Overview
  • Important pathway for terpenoid synthesis.
  • Focus: Step-by-step mechanism starting from acetyl CoA, leading to terpenoid precursors.
  • Not all details (like curly arrow mechanisms) may be required for the exam but will be discussed for understanding.
Step 1: Formation of Acetoacetyl CoA
  • Reaction involves:
    • Two molecules of acetyl CoA.
    • One acetyl CoA acts as a nucleophile (enolate), attacking the carbonyl of the other.
  • Result: Formation of a tetrahedral intermediate and subsequent loss of CoA leads to acetoacetyl CoA.
Commitment to Terpene Synthesis
  • Acetoacetyl CoA can lead to several compounds (polyketides, fatty acids); further steps lead to specialized pathways.
  • Formation of HMG CoA (hydroxymethyl-glutaryl CoA) is the first committed step toward terpenoids:
    • Catalyzed by HMG CoA synthase.
    • Involves another acetyl CoA and results in the formation of a carboxylic acid.
Step 2: HMG CoA to Mevalonic Acid
  • HMG CoA is reduced by HMG CoA reductase using NADPH:
    • Important for producing mevalonic acid, the precursor of the mevalonate pathway.
    • Note: Drug relevance (e.g., Statins inhibit this enzyme to lower cholesterol levels).
Steps 3-5: Phosphorylation of Mevalonic Acid
  • Phosphorylation with ATP occurs in multiple steps to produce:
    • Mevalonate 5-Phosphate.
    • Involves two kinases adding phosphate groups.
  • The importance of phosphorylating mevalonate:
    • Sets the stage for decarboxylation and formation of acyclic isoprenoid precursor.
Final Steps: Formation of Isopentenyl Pyrophosphate (IPP)
  • Further phosphorylation and decarboxylation of mevalonate leads to isopentenyl pyrophosphate (IPP).
  • IPP is the foundational building block for terpenoids, comprising 5 carbon units.
Isopentenyl-Diphosphate Isomerization
  • Enzyme isopentenyl diphosphate isomerase converts IPP into dimethylallyl pyrophosphate (DMAPP).
Incorporating Carbon Labeling in Terpenoid Synthesis
  • Experimentation with isotopically labeled acetate to trace carbon incorporation in terpenoids.
    • Importance of using minimal media to ensure the labeled acetate is the only carbon source.
  • Pathway exploration through labeling helps in deducing compound structures and biosynthetic pathways.
Final Thoughts on Terpene Biosynthesis
  • Asteroidal and cardiac-related drugs exemplify importance of biochemistry in medicine.
  • Understanding the synthesis processes helps in the engineering and production of valuable natural products.