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Secondary metabolites are organic compounds produced by bacteria, fungi, or plants which are not directly involved in the normal growth, development, or reproduction of the organism. Unlike primary metabolites, absence of secondary metabolites does not result in immediate death, but rather in long-term impairment of the organism's survivability, fecundity, or aesthetics, or perhaps in no significant change at all. Specific secondary metabolites are often restricted to a narrow set of species within a phylogenetic group. Secondary metabolites often play an important role in plant defense against herbivory and other interspecies defenses. Humans use secondary metabolites as medicines, flavorings, pigments, and recreational drugs.^[1]

Secondary metabolites aid a host in important functions such as protection, competition, and species interactions, but are not necessary for survival. One important defining quality of secondary metabolites is their specificity. Usually, secondary metabolites are specific to an individual species,^[2] though there is considerable evidence that horizontal transfer across species or genera of entire pathways plays an important role in bacterial (and, likely, fungal) evolution.^[3] Research also shows that secondary metabolic can affect different species in varying ways. In the same forest, four separate species of arboreal marsupial folivores reacted differently to a secondary metabolite in eucalypts.^[4] This shows that differing types of secondary metabolites can be the split between two herbivore ecological niches.^[4] Additionally, certain species evolve to resist secondary metabolites and even use them for their own benefit. For example, monarch butterflies have evolved to be able to eat milkweed (Asclepias) despite the toxic secondary metabolite it contains.^[5] This ability additionally allows the butterfly and caterpillar to be toxic to other predators due to the high concentration of secondary metabolites consumed.^[5]

Human health implications[edit]

Most polyphenol nutraceuticals from plant origin must undergo intestinal transformations, by microbiota and enterocyte enzymes, in order to be absorbed at enterocyte and colonocyte levels. This gives rise to diverse beneficial effects in the consumer, including a vast array of protective effects against viruses, bacteria, and protozoan parasites.^[6]

Secondary metabolites also have a strong impact on the food humans eat. Some researchers believe that certain secondary metabolite volatiles are responsible for human food preferences that may be evolutionarily based in nutritional food.^[7] This area of interest has not been thoroughly researched, but has interesting implications for human preference. Many secondary metabolites aid the plant in gaining essential nutrients, such as nitrogen. For example, legumes use flavonoids to signal a symbiotic relationship with nitrogen fixing bacteria (rhizobium) to increase their nitrogen uptake.^[5] Therefore, many plants that utilize secondary metabolites are high in nutrients and advantageous for human consumption.

Categories[edit]

Most of the secondary metabolites of interest to humankind fit into categories which classify secondary metabolites based on their biosynthetic origin. Since secondary metabolites are often created by modified primary metabolite synthases, or "borrow" substrates of primary metabolite origin, these categories should not be interpreted as saying that all molecules in the category are secondary metabolites (for example the steroid category), but rather that there are secondary metabolites in these categories.

Small "small molecules"[edit]

• 
  Alkaloids (usually a small, heavily derivated^{[clarification needed]} amino acid):
  
  • 
    Hyoscyamine, present in Datura stramonium
    
  
  
  • 
    Atropine, present in Atropa belladonna, Deadly nightshade
    
  
  
  • 
    Cocaine, present in Erythroxylum coca the Coca plant
  
  
  • 
    Scopolamine, present in the Solanaceae (nightshade) plant family
  
  
  • 
    Codeine and Morphine, present in Papaver somniferum, the opium poppy
    
  
  
  • 
    Tetrodotoxin, a microbial product in Fugu and some salamanders
    
  
  
  • 
    Vincristine & Vinblastine, mitotic inhibitors found in the Rosy Periwinkle
    
  
  


• 
  Terpenoids (come from semiterpene oligomerization):
  
  • 
    Azadirachtin, (Neem tree)
  
  
  • 
    Artemisinin, present in Artemisia annua Chinese wormwood
  
  
  • 
    tetrahydrocannabinol, present in cannabis
    
  
  
  • 
    Steroids (Terpenes with a particular ring structure)
    
    • 
      Saponins (plant steroids, often glycosylated)
  
  


• 
  Flavonoids (or bioflavonoids) (from the Latin word flavus meaning yellow, their color in nature) are a class of plant and fungus secondary metabolites):
  
  • 
    isoflavonoids & neoflavonoids, flavone, flavanones
    


• 
  Glycosides (heavily modified sugar molecules):
  
  • Nojirimycin
  
  
  • Glucosinolates
  
  


• 
  Natural phenols:
  
  • Resveratrol


• 
  Phenazines:
  
  • Pyocyanin
  
  
  • Phenazine-1-carboxylic acid (and derivatives)
  
  


• 
  Biphenyls and dibenzofurans are phytoalexins of the Pyrinae^[8]

Big "small molecules", produced by large, modular, "molecular factories"[edit]

• 
  Polyketides:
  
  • Erythromycin
  
  
  • 
    Lovastatin and other statins
  
  
  • Discodermolide
  
  
  • Aflatoxin B1
  
  
  • Avermectins
  
  
  • Nystatin
  
  
  • Rifamycin
  
  


• 
  Fatty acid synthase products :
  
  • 
    Jawsamycin (FR-900848)
  
  
  • U-106305
  
  
  • phloroglucinols
  
  


• 
  Nonribosomal peptides:
  
  • Vancomycin
  
  
  • Ramoplanin
  
  
  • Teicoplanin
  
  
  • Gramicidin
  
  
  • Bacitracin
  
  
  • Ciclosporin
  
  
  • Malacidin
  
  


• 
  Ribosomally synthesized and post-translationally modified peptides:
  
  • Thiostrepton


• 
  Hybrids of the above three:
  
  • Epothilone


• Polyphenols

Non-"small molecules": DNA, RNA, ribosome, or polysaccharide "classical" biopolymers[edit]

• 
  Ribosomal peptides:
  
  • Microcin-J25

See also[edit]

• Secondary metabolism


• 
  Hairy root culture, a strategy used in plant tissue culture to produce commercially viable quantities of valuable secondary metabolites


• Metabolite


• Primary metabolite

References[edit]

External links[edit]

• 
  Media related to Secondary metabolites at Wikimedia Commons