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- 1. Secondary metabolites of Fungi Marlia Singgih Wibowo
- 2. Fungal secondary metabolites Secondary metabolites are compounds produced by an organism that are not required for primary metabolic processes. Fungi produce an enormous array of secondary metabolites, some of which are important in industry
- 3. Many fungi express secondary metabolites that influence competitive outcomes. The compounds are expressed along with enzymes necessary for extracellular digestion. The precise function of many of these compounds in the natural environment, however, is unclear.
- 4. Secondary metabolites are generally produced following active growth, and many have an unusual chemical structure. Some metabolites are found in a range of related fungi, while others are only found in one or a few species. The restricted distribution implies a lack of general function of secondary metabolites in fungi.
- 5. six of the twenty most commonly prescribed medications are of fungal origin. These metabolites have been subjected to combinatorial chemistry following growth in selective media. Some metabolites are toxic to humans and other animals. Yet others can modify the growth and metabolism of plants.
- 6. The pathway Interestingly, the most important secondary metabolites seem to be synthesised from one or a combination of three biosynthetic pathways: polyketides arising from Acetyl Coenzyme A, mevalonate pathway that also arises from Acetyl Coenzyme A, and from amino acids.
- 7. genes for the synthesis of some important secondary metabolites are found clustered together, and expression of the cluster appears to be induced by one or a few global regulators.
- 8. Polyketide Metabolites Polymerisation of acetate may result in the formation of a fatty acid or a polyketide. Polyketides result when a primer other than acetate is included, and processing during chain elongation results in the inclusion of various other compounds. The chain may be further processed by cyclisation, lactonisation, or formation of thioesters or amides. The result is a staggering number of possile structures built from the simple primer units.
- 9. Included among the polyketide secondary metabolites are orsellinic acid, tetrahydroxynaphthalene (precursor for melanin), sterigmatocyctin, aflatoxins, statins, and fumonisin.
- 10. Aflatoxin Aflatoxins are produced in members of the Aspergillus parasiticus group via the polyketide pathway. The pathway has around 20 steps, and the end products include a diversity of related compounds (bisfuranocoumarins) that can be readily converted one to another.
- 11. Aflatoxin B1 is one of the most toxic compounds known. The toxin is formed commonly in plant materials held at relatively high moisture and temperature for long periods (ie growth in tropics and subtropics). Peanuts, corn and cotton are readily contaminated in the field. The Aspergillus parasiticus group of fungi are common in soil. The fungi can colonise roots and spread through the plant. When harvested, contaminated seed will become toxic if not dried immediately and held in a dry form.
- 12. Structure of Aflatoxin B1
- 13. Aflatoxins are toxic and carcinogenic. The LSD 50 for ducks is 0.33 mg/kg. At lower levels and following prolonged exposure, the toxins cause liver cancer in humans. Aflatoxin B1 is converted to Aflatoxin M (in milk) on passage through cows. Though less toxic, it does illustrate the potential damage caused by consumption of contaminated product.
- 14. Aromatic Compounds Cyclic compounds can be synthesised via the polyketide or shikimic acid pathways. Zearalenone is one interesting example from this group. The compound regulates perithecium formation in the fungus. It also has an oestrogenic effect in mammals.
- 16. Amino Acid Pathway Penicillin and cephalosporin are 硫 lactam antibiotics. 硫 lactam antibiotics are produced by a few Ascomycota and several bacteria. The precursors of these antibiotics are amino acids. Synthesis of active antibiotics is directed by the inclusion in the growth medium of different organic and fatty acids resulting in different side chains on the compound.
- 17. A second group of antibiotics derived from the amino acid pathway are the defensins. Defensins are peptides that act against bacteria. They are found in animals where they function to protect organs such as the gingiva where bacterial densities are very high. The first defensin found in fungi has been called plectasin. The role of plectasin in its host Pseudoplectania nigrella is unknown.
- 18. Toxins derived from amino acid synthesis include psilocybin (Psilocybe) and Bufotenine (Amanita). These compounds act on nerve impulses, resulting in hallucinations. The result is thought to be due to the similarities between the compounds and serotonin
- 19. Combination of Pathways Ergot alkaloids are synthesised from several pathways. Trypotophan from the shikimic acid pathway is attached to an isoprenoid moiety from the mevalonate pathway, and several amino acids from primary metabolism are added depending on the final product. Ergot alkaloids are produced as a complex mixture of related compounds from a branched pathway. The activity of the alkaloids is as varied as the compounds. In essence, the compounds may function as vasodilators, hormone regulators, and feeding deterrents. They can be active in mammals and insects.