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

The primary techniques used to elucidate biosynthetic pathways include the use of radioactive isotopes (e.g., 14C, 3H), stable isotopes (e.g., 13C, 15N, 2H) for NMR and mass spectrometry, mutant strain analysis, enzyme inhibition studies, and cell-free system analysis.

Tracer Technique (Isotopic Labeling): This is the most common method. A potential precursor molecule, labeled with a radioactive or stable isotope at a specific position, is administered to the plant. After a period, the plant is harvested, and the secondary metabolite is isolated. The location of the label in the final product is determined using techniques like NMR or degradation studies. If the label appears in the expected position, it confirms the precursor's role in the pathway. For example, feeding 14C-labeled mevalonic acid to plants helps trace its incorporation into terpenoids.

The shikimic acid pathway is the primary route for the biosynthesis of aromatic amino acids (phenylalanine, tyrosine, and tryptophan) and various phenolic compounds in plants.

1. Formation of Shikimic Acid: The pathway begins with the condensation of phosphoenolpyruvate (PEP) and erythrose-4-phosphate to form 3-deoxy-D-arabino-heptulosonate-7-phosphate (DAHP), which eventually cyclizes to form shikimic acid.
2. Conversion to Chorismate: Shikimic acid is phosphorylated and reacts with another molecule of PEP to form 5-enolpyruvylshikimate-3-phosphate (EPSP), which is then converted into chorismate.
3. Branching Point: Chorismate serves as the critical branch point. It can be converted into prephenate, which leads to the synthesis of phenylalanine and tyrosine via transamination and decarboxylation. Alternatively, it can react with glutamine to form anthranilate, leading to the synthesis of tryptophan.
4. Secondary Metabolites: The phenylalanine and tyrosine produced are precursors for the phenylpropanoid pathway, leading to the formation of lignins, flavonoids, coumarins, and tannins.