JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B. Pharmacy III Year PHARMACOGNOSY AND PHYTOCHEMISTRY-II Previous Question Paper (March-2021 - R17)

The Acetate pathway, also known as the polyketide pathway, is a primary biosynthetic route in plants and microorganisms responsible for the formation of diverse secondary metabolites, including fatty acids, polyketides, and phenolic compounds. The pathway begins with the precursor Acetyl-CoA.

The process involves the following key stages:

1. Activation: Acetyl-CoA is carboxylated to form Malonyl-CoA by the enzyme Acetyl-CoA carboxylase, requiring ATP and biotin.
2. Condensation: Malonyl-CoA units are added sequentially to an acetyl-CoA starter unit via a Polyketide Synthase (PKS) enzyme complex. This involves decarboxylative condensation, releasing CO2 and extending the carbon chain.
3. Cyclization and Modification: The resulting poly-β-keto chain undergoes various modifications such as reduction, dehydration, or cyclization to form stable aromatic structures like anthraquinones, flavonoids, or simple phenols.

This pathway is essential for the biosynthesis of numerous pharmacologically active compounds, including tetracyclines and various plant pigments.

Radioactive isotopes serve as tracers in biogenetic studies to elucidate the biosynthetic pathways of secondary metabolites in plants. By introducing a labeled precursor, researchers can track the movement of atoms through metabolic intermediates to the final product.

Key uses include:

1. Precursor-Product Relationship: Isotopes like 14C, 3H, or 32P are incorporated into suspected precursor molecules. If the radioactivity appears in the isolated secondary metabolite, it confirms the precursor's role in the biosynthetic pathway.
2. Elucidation of Metabolic Pathways: By measuring the specific activity of various intermediates at different time intervals, the sequence of enzymatic reactions can be determined.
3. Determination of Biogenetic Origin: Isotopes help identify which primary metabolites (e.g., acetate, mevalonate, or amino acids) provide the carbon skeleton for complex plant constituents like alkaloids or terpenoids.
4. Site-Specific Labeling: Using position-specific labeled precursors allows researchers to determine exactly which carbon or hydrogen atom in the precursor ends up at a specific position in the final molecule, providing insight into the mechanism of rearrangement or cyclization reactions.
5. Quantification: Radioactive counting techniques allow for the precise measurement of the rate of biosynthesis and the turnover of metabolites within the plant tissue.