Amino Acid Pathways

Overview of Amino Acid Pathways:

• Amino acid-derived pathways leverage the plant’s pool of protein-building amino acids (e.g., lysine, tyrosine, tryptophan, ornithine) to construct a wide array of nitrogen-containing secondary metabolites, especially alkaloids.

Essential and Non-Essential Amino Acids:

Key Steps and Main Routes:

1. Transamination and Amino Acid Pool Formation

   • Amino acids are synthesized from:
     • Shikimic acid pathway (e.g., phenylalanine, tyrosine, tryptophan).
     • Glutamate and aspartate families.
   • These amino acids serve as precursors for nitrogen-containing secondary metabolites.

2. Alkaloid Biosynthesis

   • Alkaloids are nitrogen-containing ring structures derived from specific amino acids:
     • Tryptophan → Indole alkaloids (e.g., vinblastine).
     • Tyrosine → Isoquinoline alkaloids (e.g., morphine).
     • Ornithine/Arginine → Tropane alkaloids (e.g., atropine) and pyrrolidine alkaloids.
   • Biosynthesis involves decarboxylation and cyclization of amino acids into complex structures.

3. Cyanogenic Glycosides and Glucosinolates

   • Cyanogenic glycosides: Derived from amino acids like tryptophan or phenylalanine and release cyanide upon tissue damage, aiding in plant defense (e.g., amygdalin in stone fruits).
   • Glucosinolates: Sulfur-containing compounds derived from amino acids like methionine and play a role in pest and pathogen resistance (e.g., in Brassicaceae plants).

Major Secondary Metabolites from Amino Acid Pathways:

1. Alkaloids: Morphine, nicotine, vinblastine.
2. Sulfur Compounds: Glucosinolates.
3. Cyanogenic Glycosides: Amygdalin.

Biological Significance:

• Defense: Alkaloids deter herbivory (e.g., nicotine).
• Pharmacology: Medicinal alkaloids (e.g., morphine, vincristine).
• Signaling: Nitrogen-containing compounds function as signals or hormonal precursors.

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