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Primary & secondary
metabolites of fungi
The metabolism is defined as the sum of all
the biochemical reactions carried out by
an organism.
Primary metabolites
Are those that are essential for growth to
occur.
proteins
carbohydrates
nucleic acids
lipids
Associated with rapid initial growth phase
of the organism.
Examples on primary metabolites produced
in abundance include Enzymes, fats,
alcohol and organic acid.
Secondary metabolites
Compounds produced by an organism that
are not required for primary metabolic
processes and not used for growth.
It is used for survival or nutrition storage.
Produced when organism enter the
stationary phase.
Secondary metabolites are produced when the
cell is not operating under optimum
conditions e.g. when primary nutrient source
is depleted.
Metabolites produced in this phase are often
associated with differentiation and
sporulation and have biological activities.
Some examples of fungal secondary
metabolites and its usage
Antibiotics
Penicillin (Penicillium
chrysogenum)
Immunosuppressant
Cyclosporin (Trichoderma)
Agriculture
Growth promoter Zearalonone
(Gibberella zeae)
Enzymes
Amylase (Aspergillus niger)
Pigments
Ancaflavin (Monascus purpureus)
similarities between the pathways that
produce primary and secondary metabolites
1- The product of one reaction is the
substrate for the next and the first reaction
in each case is the rate-limiting step.
2- Also the regulation of secondary
metabolic pathways is interrelated in
complex ways to primary metabolic
regulation.
Clear reasons exist for studying
secondary metabolites
Many have been found to have use in
industry and medicine.
Six of the twenty most commonly prescribed
medications for humans are of fungal origin.
Some metabolites are toxic to humans and
other animals.
biosynthetic pathways
1-polyketides pathway
2- mevalonate pathway
3- amino acids pathway
* In addition, 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.
* Some of the 'global regulators' are also
involved with sporulation and hyphal
elongation.
Polyketide Metabolites
Acetate
‫ح‬
polymerization
‫ح‬
polyketide
processing
Staggering no. Of possible structure built
from the simple primer unit
Cyclisation,
lactonisation,
formation of amides
tetrahydroxy
naphthalene
orsellinic
acid
Sterigmatocyctin
polyketide
secondary
metabolites
fumonisin
statins
aflatoxins
Aflotoxins
are produced in members of the Aspergillus
parasiticus group via the polyketide
Aflatoxin B1 is one of the most toxic
compounds known.
The structure of Alphatoxin B1
- At lower levels and following prolonged
exposure, the toxins cause liver cancer in
humans.
- Animals tend to avoid contaminated feed, but
as B1 is so highly toxic, even large animals can
be killed by small, almost undetectable
quantities.
Patulin
antibiotic synthesized on
an acetate/malonate
pathway.
Its biosynthetic pathway is
still unclear, but it appears
that several alternate
pathways may result in the
same end product.
Structure of Patulin
It is now also known to be a compound
produced by Penicillium expansum in
contaminated apples.
Aromatic Compounds
Cyclic compounds can be synthesized via
the polyketide pathway.
Zearalenone is one interesting example
from this group. The compound regulates
perithecium formation in the fungus.
Structure of Zearalenone
Amino Acid Pathway
Antibiotics
penicillin &
cephalosporin
B lactams
antibiotics
plectasin
defensin
Peptides act
against bacteria
Toxins derived from amino acid synthesis
psilocybin (Psilocybe)
Bufotenine (Amanita).
act on nerve impulses
hallucinations
The fungus
Aminata
Plant Growth Regulators
Many pathogenic and benign fungi produce
auxin, cytokinins, gibberellins and
abscisic acid.
In fact the gibberlellins were first found in
the fungus Gibberella fujikuroi, a
pathogen that causes tall, straggly growth
of rice.
The gibberellins are diterpenes produced by
the mevalonate/isoprenoid pathway.
The function of these compounds in fungi
that colonise plants seem clear.
Modification of host tissue enhances
colonisation, releases nutrients for fungal
metabolism and regulates host
reproduction.
The function of plant growth regulation in
fungi found outside plants is unclear.
Some plant fungi appear to modify host
production of growth regulators, resulting
in alterations of host metabolism.
For instance, initiation of AM in roots
results in a slowing of root tip elongation
and increase in lateral formation.
The cause may be associated with a change
in concentrations of auxins and/or
cytokinins from the fungus or induced in
the host, an increase in local concentration
of phosphate due to the fungi, or a factor
influenced by either.
Increases in expression of plant hormones
may be direct or indirect.
Toxins
In general, toxins associated with fruiting
bodies are important because consumption
of the fruiting body can result in
poisoning.
Toxins associated with microfungi are
important because they become evident
after consumption of contaminated food.
In addition, toxins are produced from a
myriad of pathways, and have enormously
diverse effects.
That they may be produced at a different
point in the life cycle of a fungus is
simply another aspect of the complex
subject.
The function of toxins to fungi has been
subject of much speculation.
Colonization (contamination) of organic
materials is a prelude to the digestion of
the material by the fungus. The production
and expression of toxins is one
mechanism the microbe has to protect the
food, provided competitors detect the
presence of the microbe and toxin.
The conditioned response to the fungus thus
reduces the consumption of the fungal
substrate. The “detection” molecule may
be other than the toxin.
Objectionable flavors and smells may thus
be warnings to competitors.
Overall, the resultant reduction of feeding
increases the chances of the fungi
surviving
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