Plant Secondary Metabolites

• The sum of all chemical reactions in an organism is metabolism
• Lipids, proteins, nucleic acids, and carbohydrates are primary metabolites
• Most plants divert carbon and energy to synthesize organic molecules without obvious roles in normal cell function

Secondary Metabolites

• Molecules not required for general organism functioning but have specialized functions
• They are highly diverse
• Compounds involved in plant defenses are often toxic secondary metabolites
• Most medicinal and drug compounds are secondary metabolites

Biosynthetic Relationships

• Principal primary and secondary metabolites are circled in a schematic illustration
• Photosynthesis produces sugars from CO2 and H2O
• Sugars undergo respiration, producing acetyl-CoA and amino acids
• Acetyl-CoA is converted into terpenoids/sterols and fatty acids/lipids
• Amino acids are converted into proteins and phenols
• Phenols can also come from Malonyl-CoA
• Sugars are converted into Saponins and Cardiac glycosides
• Amino acids can be turned into alkaloids
• Phenols become Flavonoids, Tannins, and Lignin
• Sugars can be converted into Cyanogenic glycosides or Glucosinolates

Plant Metabolic Pathways

• Products and intermediates from primary metabolism (photosynthesis, respiration) feed into secondary pathways
• The Calvin cycle intermediate product is Erythrose-4-phosphate
• The Glycolysis intermediate product is phosphoenolpyruvate
• The Citric acid cycle intermediate product is Acetyl CoA
• Erythrose-4-phosphate and phosphoenolpyruvate can create a shikimic acid pathway
• Acetyl CoA can create a Malonic Acid Pathway or Mevalonic Acid Pathway
• Aromatic amino acids produce alkaloids
• Phenolic compounds and Terpenes

Amino Acid Biosynthesis

• 20 amino acids are found in proteins.
• 9 essential amino acids exist in humans
• Amino acids are precursors to several secondary compounds

Shikimate Pathway

• Tryptophan is a precursor in the synthesis of auxins (plant hormone)
• Phenylalanine is a precursor in the synthesis of flavonoid pigments
• Herbicide Round-Up blocks their synthesis of metabolites; without these amino acids, plants die

Other key facts on Primary vs Secondary Metabolites

• The distinction can be difficult to make
• Primary and secondary metabolites share several of the same intermediates
• They are derived from the same core metabolic pathways
• Secondary metabolites are not essential for cellular structure or function
• Secondary metabolites generally occur in relatively low quantities
• Production can be widespread or restricted to particular families, genera, or species
• These compounds are also known as natural products
• Natural products have a significant economic and medicinal value
• They are used as folk remedies, soaps, essences, drugs, medicinal products, etc.
• Natural products also have significant ecological functions, like protection against microbial or insect attacks

Major Plant Secondary Metabolic Pathways

• Mevalonic Acid Pathway and MEP (non-mevalonic acid) produces Terpenes
• Shikimic Acid Pathway produces aromatic amino acids

Secondary metabolites can be created by:

• Alkaloids
• Phenolics

Four Major Classes of Secondary Metabolites

• Terpenes: include hormones, pigments, essential oils, steroids, and rubber
• Phenolic compounds: include coumarins, flavonoids, lignin, and tannins
• Glycosides: Include saponins, cardiac glycosides, cyanogenic glycosides, and glucosinolates
• Alkaloids

Terpenes

• Made of Monoterpenes (C10)
• Sesquiterpenes (C15)
• Diterpenes (C20)
• Triterpenes (C30)
• Polyterpenes (C5)n

Phenolic Compounds are made of:

• Coumarin
• Furano-coumarins
• Ligin
• Flavonoids
• Isoflavonoids
• Tanins

Sulphur-containing secondary metabolites are made up of

• GSH
• GSL
• Phytoalexins
• Defensins
• Thionins
• Lectins

Nitrogen-containing secondary metabolites are made up of

• Alkaloids
• Cyanogenic glycosides
• Non-protein amino acids

Terpenes/Terpenoids

• The largest class of secondary metabolites.
• 25,000 have been described
• Some act as primary metabolites for the plant
• They are necessary for cell function for:
  • Cell growth modulation
  • Light harvesting
  • Photoprotection
  • Control of membrane fluidity
• They are components of essential oils i.e. the "essences" of plants
• They have numerous applications in medicine

Terpenoid Synthesis

• Accomplished via acetyl CoA via the mevalonic acid pathway
• Acetyl CoA is converted into mevalonic acid
• Terpenoid synthesis is created with different intermediates via the MEP pathway
• This usually occurs in chloroplasts
• Compounds comply with the isoprene rule
• Terpenes build by combining 5 carbon units
• Resulting in structures with 10,15,20,30, or 40 carbons
• The basic unit is 5-Carbon Isoprene

Types of Terpenoids

• Act as toxic and feeding deterrents e.g. Acetyl-coA or glycolytic intermediates
• Composed of a mixture of essential oils includes volatile compounds, limonene, and menthol
• Lends a characteristic odor to their foliage
• Found in glandular hairs that project from the epidermis
• Stored in the modified extracellular space in the cell wall

Linear vs Cyclic Terpenes

• Linear and Cyclic terpenes exist structurally
• Cyclization requires the action of specific enzymes
• non-cyclic terpene, Geraniol exists in Geraniums
• cyclic terpene, Limonene exists in Orange peels
• Both instances of monoterpenes, contain 10 carbons

Hemiterpenes

• iso-Amyl Alcohol exists in Mentha and Eucalyptus
• iso-Valeraldehyde exists in Eucalyptus

Monoterpenes

• They exhibit strong insecticidal responses to insects
• Popular ingredient in commercial insecticides
• Pyrethroids: in the leaves and flowers of chrysanthemum
• Gymnosperms (conifers): pine and fir
• Resin ducts: in the needles, twings, and trunk
• Cucurbitacin: exists in cucurbits

Limonene and Menthol

• Structures of limonene and menthol
• These two well-known monoterpenes protect against insects and other organisms

Pinene, myrcene, and menthol

• They are monoterpenes
• Pinene and myrcene: found in some conifer resin
• Menthol: the principal constituent of the essential oil of peppermint (Mentha piperta)
• Pinene: has demonstrated insecticidal properties

Diterpenes

• Abietic acid is in pines and leguminous plants.
• Diterpenes present in or along with resins in resin canals.
• When these canals are pierced by feeding insects, the outflow of resin may block feeding:
  • Providing a chemical deterrent that continues predation.
• Cannabis plants use diterpenes against predators such as insects and animals

Sesquiterpenes

• Sesquiterpenes act as antiherbivore agents, primarily by the compositae family.
• Have a strong feeding repellence that affects many herbivorous insects

Triterpenes

• Milkweeds produce glucosides (sterols)
• Limnoid: in family rutaceae from citrus fruits; act as antiherbivore compounds
• Azadirachtin: complex limnoid from Azadirachta indica; acts as a feeding deterrent to some insects and toxic effects

Terpene Antiherbivore Compounds

• Azadirachtin acts as the active ingredient
• It's a complex limonoid comes from the neem tree (Azadirachta indica)
• Potent feeding deterrent to some insects at doses as low as 50 PPB
• It is an insect control commercial agent w/ low levels of toxicity

Terpene Antiherbivore Compounds: Phytoecdysones

• Phytoecdysones are from common fern
• A group of steroids that have the same basic structure as insect molting hormones

Polyterpenes

• Larger terpenes include the tetraterpenes and the polyterpenes
• Principal tetraterpenes are carotenoids = pigment family
• Rubber: protection mechanism for wound healing and against herbivores
• Rubber is found in long vessels = laticifers

Carotenoids

• Xanthophylls: contains oxygen (Lutein)
• Carotenes: unoxygenated (carbon and hydrogen only) e.g. (α-carotene, β- carotene, lycopene)

Most common type of carotene:

• β-carotene: orange/yellow pigment (carrots & citrus fruits)
• Lycopene: red pigment (peppers & tomatoes0

Monoterpenes and sesquiterpenes:

• Commonly found in glandular hairs on the plant surface