Overview
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
Category | Definition | Examples |
Essential Amino Acids | Cannot be synthesized by the body or plants and must be obtained from external sources (e.g., diet). | Phenylalanine, Tryptophan, Lysine, Valine, Leucine, Isoleucine, Threonine, Methionine, Histidine |
Non-Essential Amino Acids | Can be synthesized internally from other metabolic precursors. | Glutamate, Aspartate, Alanine, Glycine, Proline, Serine, Tyrosine, Asparagine, Glutamine, Cysteine, Arginine |
Key Steps and Main Routes Amino Acid Pathways
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.
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.
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
Alkaloids: Morphine, nicotine, vinblastine.
Sulfur Compounds: Glucosinolates.
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.