Secondary Products

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Secondary Products
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Secondary Products

• Carbohydrates, lipids, proteins, and nucleic
  acids
• primary metabolites in all organisms
• occur in primary metabolic pathways
• Plants and fungi produce a variety of other
  chemical compounds that are produced on secondary
  pathways and are called secondary products
  (secondary compounds or secondary metabolites)

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Glycolysis
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Pyruvate ? Acetyl-CoA
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Pentose Phosphate Pathway

• Pathway for the breakdown of glucose that occurs
  in the cytoplasm
• It is an alternative to Glycolysis and Krebs
• Used to produce intermediates although it can be
  used as an alternative to Glycolysis
• Most important products
• Ribose-5-PO4 for nucleotides and nucleic acids
• NADPH for lipid synthesis

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Secondary Products

• No direct function in growth and development of
  the plant
• Often limited distribution among plants
• A particular metabolite may be restricted to one
  species or one genus
• Usually synthesized from primary products
• Variety of functions recognized today but
  originally thought to be waste products

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Secondary Products

• About 100,000 have been identified
• Often accumulated and stored in high
  concentrations -- 1 to 3 of dry weight
• Hydrophilic cmpds stored in vacoules
• Hydrophobic cmpds stored in resin ducts,
  laticifers, trichomes, oil cells, or in cuticle
• Sites of synthesis and sites of storage often
  different - transported in the plant

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Functions of Secondary Products

• Protect the plant against herbivory
• deter grazing animals
• deter insects
• Inhibit bacterial or fungal pathogens
• Inhibit growth of competing plants
• Attract pollinators
• Attract seed dispersing animals

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Defense Compounds

• Plants cant run and hide, therefore they must
  use chemical defenses
• Vulnerable tissues defended more than old
  senescing tissues
• Seeds, seedlings, buds, and young tissues often
  have large amounts of SP
• Organs important for survival (flowers, fruits,
  and seeds) often rich source as well

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Defense Compounds

• SP will only work as a defense cmpd if they are
  able to influence herbivore or microbe in a
  negative way
• they often mimic the structure of a compound in
  the animal such as a hormone or neurotransmitter
• others inhibit protein synthesis, membrane
  structure, DNA or RNA synthesis

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Major Secondary Products

• Four Classes of Compounds
• Terpenes
• Phenolics
• Glycosides
• Alkaloids
• In some respects, these are artificial categories
• Some compounds in these categories are primary
  metabolites

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Examples of Secondary Products
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Terpenes (Terpenoids)

• Diverse group of lipids about 15,000 to 30,000
• Insoluble in water
• Synthesized from acetyl-CoA or Glycolysis
  intermediates
• Derived from the union of 5-carbon isoprene units
  (C5H8)
• terpenes also called isoprenes or isoprenoids
• Called terpenoids if other elements included
  (especially oxygen)

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Isoprene Molecule
H3C CH-CHCH2 H2C
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Not all terpenes are SP

• Largest class of secondary compounds
• Many terpenes are primary metabolites
• Including steroids
• Sterols cholesterol, sitosterol, ergosterol
• Carotenoid pigments
• Plant hormones gibberellic acid and abscissic
  acid

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Two Pathways of Terpene Synthesis

• Mevalonic Acid Pathway 3 molecules of
  acetyl-CoA are joined together to form mevalonic
  acid
• Mevalonic acid is a 6-carbon intermediate
• Mevalonic acid is
• phosphorylated with two phosphate groups -
  pyrophosphate (P-P)
• decarboxylate and dehydrated to produce isopentyl
  pyrophosphate (isopentyl diphosphate) - IPP
• IPP the activated building block
• Mevalonate-Independent Pathway
• Uses glycolysis intermediates to synthsize IPP

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Isoprene Synthesis
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Types of terpenes

• Classified by number of isoprene units
• sometimes compounds highly modified so its
  difficult to pick out isoprenes
• Hemiterpenes - contain 1 isoprene unit (5-C)
• Monoterpenes - contain 2 isoprene units (10-C)
• Sesquiterpenes - 3 isoprenes (15-C)
• Diterpenes - 4 isoprenes (20-C)
• Triterpenes - 6 isoprenes (30-C)
• Tetraterpenes - 8 isoprenes (40-C)
• Polyterpenes - over 10 isoprenes

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Function of Terpenes as SP

• Hydrophobic substances stored in resin ducts, oil
  cells or glandular trichomes
• Readily interact with membranes often increasing
  membrane fluidity
• efflux of ions and metabolites
• disrupt membrane proteins and receptors
• cell leakage
• Activity non-specific and acts on a wide range of
  organism microbes to vertebrates
• Some specific action because mimic hormones

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Glandular trichomes in geranium
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Trichomes
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Monoterpenes C-10

• Many monoterpenes function as insect toxins
• Pyrethrins neurotoxins
• Conifer resins mixture of monoterpenes
• Essential oils - many are insecticidal
• Recent study showed that some essential oils
  released after the first insect attack
  essential oil attracted predators to attacking
  insect
• Many essential oils are also antibacterial and
  antifungal
• Some essential oils attract pollinators

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Sesquiterpenes C-15

• Strong feeding repellants to insects and mammals
• Some antimicrobial as well
• Often have a lactone ring
• Gossypol in cotton responsible for insect
  resistance in some varieties
• Artemisinin produced by Artemesia annua
• Lactone with potent anti-malarial activity

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Diterpenes C-20

• Toxins and feeding deterrants
• High boiling point so not volatile essential oil
  - referred to as resins
• Some diterpenes skin irritants
• Cytotoxic lactone from Podocarpus have
  antileukemia activity
• Taxol from Taxus is antimitotic - used to treat
  ovarian and breast cancer

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Triterpenes C-30

• Includes structurally diverse cmpds including
  steroids
• Some important glycosides have a triterpene as
  the aglycone digitalis, saponins
• Some mimic insect hormones and interfere with
  development
• Other triterpenes are antifeedants
• Azadiractin from oil in neem tree best known

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Terpenes and air pollution

• Recent interest in volatile terpenes produced by
  plants
• Hemiterpenes, monoterpenese, sesquiterpenes given
  off from vegetation especially in warm weather
• Contribute to the VOCs in atmosphere that are
  involved in photochemical smog and production of
  ozone