JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B. Pharmacy III Year PHARMACOGNOSY AND PHYTOCHEMISTRY - II Previous Question Paper (August - 2022 - R17)

The shikimic acid pathway is a primary metabolic route in plants, bacteria, and fungi for the biosynthesis of aromatic amino acids (phenylalanine, tyrosine, and tryptophan) and various secondary metabolites like lignins, tannins, and flavonoids. The pathway begins with the condensation of phosphoenolpyruvate (PEP) and erythrose-4-phosphate.

1. Condensation of PEP and erythrose-4-phosphate forms 3-deoxy-D-arabino-heptulosonate-7-phosphate (DAHP).
2. DAHP undergoes cyclization to form 5-dehydroquinic acid.
3. Dehydration of 5-dehydroquinic acid yields 5-dehydroshikimic acid, which is then reduced to shikimic acid.
4. Shikimic acid is phosphorylated and reacts with another molecule of PEP to form 5-enolpyruvylshikimate-3-phosphate (EPSP).
5. EPSP is converted into chorismic acid, which serves as the critical branch point for the synthesis of aromatic amino acids and other phenolic compounds.

Steroidal glycosides are synthesized primarily via the mevalonic acid pathway. The process involves the following key stages:

1. Acetyl-CoA units condense to form mevalonic acid, which is then converted into isopentenyl pyrophosphate (IPP) and dimethylallyl pyrophosphate (DMAPP).
2. These five-carbon units polymerize to form farnesyl pyrophosphate (C15) and subsequently squalene (C30).
3. Squalene undergoes cyclization to form the basic steroidal nucleus, typically starting with cycloartenol or lanosterol.
4. The steroidal nucleus undergoes structural modifications, including oxidation and hydroxylation, to form specific aglycones (genins) like pregnane or spirostane derivatives.
5. Finally, the aglycone is linked to a sugar moiety (glucose, rhamnose, or digitoxose) via a glycosidic bond at the C-3 position to form the active steroidal glycoside.