Shikimic Acid Pathway

Shikimic Acid Pathway:

Overview:

• The Shikimic acid pathway is a central biosynthetic route responsible for the production of aromatic amino acids (phenylalanine, tyrosine, tryptophan) and various aromatic secondary metabolites.
• It is not present in animals, making it a major target for herbicides and antimicrobial agents.

Key Steps:

1. Formation of Shikimic Acid

   • Starting materials:
     • Phosphoenolpyruvate (PEP): Derived from glycolysis.
     • Erythrose-4-phosphate: Derived from the pentose phosphate pathway.
   • These precursors undergo condensation and multiple enzymatic reactions to form shikimic acid.

2. Formation of Chorismate

   • Shikimic acid is phosphorylated to form shikimate-3-phosphate, which is then converted to chorismate.
   • Chorismate is a crucial branching point leading to the biosynthesis of aromatic amino acids and other secondary metabolites.

3. Aromatic Amino Acid Biosynthesis

   • Phenylalanine and Tyrosine: Chorismate is converted into prephenate, which serves as a precursor for these amino acids.
   • Tryptophan: Chorismate is converted to anthranilate, a precursor to tryptophan.
   • Phenylalanine and Tyrosine are important for secondary metabolic pathways like the phenylpropanoid pathway.

Major Secondary Metabolites from the Shikimic Pathway:

1. Phenylpropanoids (via phenylalanine): Include flavonoids, lignins, coumarins, and phenolic acids.
2. Alkaloids (some classes are derived from tryptophan or tyrosine).
3. Indole derivatives (e.g., indole-3-acetic acid, the plant hormone auxin, derived from tryptophan).
4. Benzoic acids (e.g., salicylic acid).

Biological Significance:

1. Plant Growth and Development: Aromatic amino acids (phenylalanine, tyrosine, tryptophan) are precursors for proteins, hormones, and pigments.
2. Defense Mechanisms: Secondary metabolites (e.g., phytoalexins, lignin, salicylic acid) protect plants from pathogens and herbivores.
3. Pollinator Attraction: Flavonoids, from phenylpropanoids, contribute to flower pigmentation and attract pollinators.
4. Structural Integrity: Lignins, derived from phenylalanine, provide strength to cell walls.

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