Overview
The Acetate pathway, often referred to as the polyketide pathway, involves the polymerization of acetyl-CoA or malonyl-CoA units.
It is crucial not only for primary metabolites like fatty acids but also for numerous plant secondary metabolites (e.g., flavonoids, certain antibiotics in microorganisms, and various phenolics in plants).
Key Steps of Acetate Pathway
Acetyl-CoA Generation
Acetyl-CoA is produced from:
Glycolysis: Through the pyruvate dehydrogenase complex.
Fatty acid β-oxidation.
Malonyl-CoA is formed from acetyl-CoA by the enzyme acetyl-CoA carboxylase.
Polyketide Chain Elongation
The acetate (acetyl-CoA) or malonate (malonyl-CoA) units undergo condensation reactions to form polyketide chains.
Malonyl-CoA donates two-carbon units to elongate the chain.
These steps are catalyzed by polyketide synthases (PKSs).
Cyclization and Modification
The linear polyketide chains undergo various modifications, including:
Cyclization: Formation of phenolic or aromatic ring structures.
Reduction, Oxidation, Methylation: Generate diverse molecular scaffolds.
These modifications create the structural diversity seen in polyketide-derived compounds.
Major Secondary Metabolites from the Acetate/Polyketide Pathway
Fatty Acids: Membrane components (primary metabolism).
Flavonoids: Pigments, antioxidants (linked to Shikimic pathway).
Quinones: E.g., anthraquinones.
Tannins: Derived from gallic acid.
Phenolics: Polyketide-based compounds.
Biological Significance
Structural Protection: Waxes and cutin from fatty acids form protective barriers on leaves and fruits.
Defense: Polyketide-derived phytoalexins, tannins, and flavonoids deter herbivores and pathogens.
Pigmentation: Flavonoids and anthraquinones enhance flower and fruit coloration, attracting pollinators.
Applications
Agriculture: Waxes and cutin for protective coatings.
Pharmaceuticals: Polyketides as precursors for antibiotics (e.g., erythromycin) and anticancer drugs.
Biotechnology: Engineering polyketide pathways for industrial use.