Polyketides
also from acetate-malonate, the enzyme system is "homologous" and from the same evolutionary ancestors as fatty acid synthetases -- polyketide synthetases
also uses ACP-derivatives/ but reduction doesn't occur as in fatty acid biosynthesis
the degree of oxidation and the number of carbons varies -- but the chains are even numbered initially
cyclization is usually involved. Alkylation with methyl and prenyl groups is common. These reactions appear to occur while the precursor is still bound to the synthetic enzymes.
polyketide compounds are common in algae, bacteria, fungi, lichens, but also in higher plants
Aldol and Claisen type condensations
many modifications occur after the original cyclization
two basic types of condensations that parallel the classic organic reactions: aldol and Claisen types
the type condensation by which a compound arises can usually be determined by the substitution pattern of the product
Tetraketides
6-methylsalicylic acid (Penicillium griseofulvin) and orsellinic acid (Roccella and Lecanora, both lichens, and many fungi) arise by aldol condensations
acetyl-CoA, malonyl-CoA, NADPH are required for these reactions to occur in Penicillium patulum
the intermediates are not released
depsides - complex esters of polyketides
dimerization also occurs, as with usnic acid
Pentaketides
citrinin, a causative agent of "yellow rice disease" is produced by Penicillium citrinum. Also produced by Aspergillus species.
Hexaketides
the naphthoquinones plumbagin (from Plumbago capensis) and 7-methyljuglone
Heptaketides
griseofulvin, Penicillium griseofulvum
one end of the molecule arises from an aldol condensation and the other end from a Claisen condensation
Octaketides
emodin/helminthosporin
anthraquinones. Emodin is found in fungi (Penicillium and Aspergillus) and in higher plants. Helminthosporin is from Helminthosporin gramineum
Tetracyclines
from a "malonamide" starter unit. These are extremely important antibiotics from Streptomyces spp. (bacteria)
Dekaketides
aflatoxins from Aspergillus and other fungi. Common on spoiled peanuts and corn. Aflatoxins are among the most powerful carcinogenic substances known.
Macrolide antibiotics
begin with a propionyl-CoA starter
C2, C3, and C4 units are used to assemble them
e.g., erythromycin from Streptomyces erythreus
Acetogenins
antitumor, insecticidal, highly cytotoxic ED50 10-7 micrograms/ml
mostly from the Annonaceae (Asimina and Annona)
Unusual starter units
compounds from poison ivy (Toxicodendron radicans, Anacardiaceae), Ginkgoaceae, Proteaceae,
and the fruits of some grasses
C16 fatty acid-CoA + 3 acetyl-CoA (malonyl-CoA) units
fern compounds (mostly Dryopteris); "methylenebisphloroglucinols"
C4-starter units, SAM involved, dimerization
but similar compounds are known from Hypericum, Eucalyptus, Mallotus, and other plants.
Hyperphorin is one of the bioactive compounds of Hypericum perforatum
Mycotoxins
Mycotoxins are substances produced by fungi that are toxic to other organisms, in particular, to humans and domestic animals.
zearalenone and fumonosins are from Fusarium spp. Aflatoxin is from Aspergillus species and patulin is from Penicillium species ... although many mycotoxins can be made by a variety of fungi.
Phytotoxins
Phytotoxins are involved in breakdown of host cells by pathogenic fungi and bacteria. They are of many chemical types. Of the polyketide ones:
T-toxins come from Helminthosporium maydis race T
cytochalasins, from a number of fungal genera. Cytochalasin B disrupts microfilament assembly in dividing plant
cells
Phytoalexins
6-Methoxymellein, a pentaketide, is produced by carrots in response to microbial attack by Ceratocystis fimbriata, which is not a normal carrot pathogen.
Methyljuglone and Plumbagin Biosynthesis
Polyketide Anthraquinone Biosynthesis
Plants and Fungi with Polyketides
© David S. Seigler, Integrative Biology 425, Plant Secondary Metabolism, Department
of Plant Biology, 265 Morrill Hall, 505 S. Goodwin Ave., University of Illinois, Urbana,
Illinois 61801, USA. 217-333-7577. seigler@life.uiuc.edu.
