Secondary metabolite

Source: Wikipedia, the free encyclopedia.
Structural formula for the amino acid pipecolic acid, which contrary to other amino acids is not used as a building block in proteins. In some plants, pipecolic acid act as a defense compound against microorganisms.[1] Because of its limited presence, pipecolic acid is considered a secondary metabolite.
Structural formula for the amino acid proline, that in all living beings is a building block in proteins. Because of its universal presence, proline is considered a primary metabolite.

Secondary metabolites, also called specialised

phylogenetic group. Secondary metabolites often play an important role in plant defense against herbivory and other interspecies defenses. Humans use secondary metabolites as medicines, flavourings, pigments, and recreational drugs.[2]

The term secondary metabolite was first coined by

nitrogen metabolism.[4]

Secondary metabolites commonly mediate antagonistic interactions, such as

milkweed (Asclepias) despite the presence of toxic cardiac glycosides.[8] The butterflies are not only resistant to the toxins, but are actually able to benefit by actively sequestering them, which can lead to the deterrence of predators.[8]

Plant secondary metabolites

Further information: Plant secondary metabolism

Plants are capable of producing and synthesizing diverse groups of organic compounds and are divided into two major groups: primary and secondary metabolites.

phenylpropanoids (i.e. phenolics), polyketides, and alkaloids.[11]

Chemical classes

Terpenoids

Structural formula for isopentenyl pyrophosphate (IPP), the carbon atoms of which constitute building blocks in terpenes. Because of the presence of five carbon atoms, the derived building block it is termed a C5 unit.
Structural formula for humulene, a monocyclic sesquiterpene (called so because it contains 15 carbon atoms) that is built from three C5 units derived from isopentenyl pyrophosphate (IPP).
taxol
, an anticancer drug.

sterols) are primary metabolites. Some terpenoids that may have originated as secondary metabolites have subsequently been recruited as plant hormones, such as gibberellins, brassinosteroids, and strigolactones
.

Number of isoprene units Name Carbon atoms
1 Hemiterpene C5
2 Monoterpene C10
3 Sesquiterpenes C15
4 Diterpene C20
5 Sesterterpene C25
6 Triterpene C30
7 Sesquarterterpene C35
8 Tetraterpene C40
More than 8 Polyterpene

Examples of

oligomerization
are:

Phenolic compounds

Phenolics are a chemical compound characterized by the presence of aromatic ring structure bearing one or more hydroxyl groups. Phenolics are the most abundant secondary metabolites of plants ranging from simple molecules such as phenolic acid to highly polymerized substances such as tannins. Classes of phenolics have been characterized on the basis of their basic skeleton.

No. of carbon atoms Basic skeleton Class
6 C6 Simple phenols
7 C6 - C1 Phenolic acids
8 C6 - C2 Acetophenone, Phenyle acetic acid
9 C6 - C3 Phenylepropanoids, hydroxycinnamic acid, coumarins
10 C6 - C4 Naphthoquinone
13 C6 - C1- C6 Xanthone
14 C6 - C2 - C6 Stilbene, anthraquinone
15 C6 - C3 - C6 Flavonoids, isoflavanoids
18 (C6 - C3 ) 2 lignans, neolignans
30 ( C6 - C3 - C6)2 Biflavonoids

An example of a plant phenol is:

Alkaloids

aspartate, whereas that in the pyrrolidine ring (to the right) derives from arginine (or ornithine)-[12]
Skeletal formula of solanine, a toxic alkaloid which builds up in potatoes.

Alkaloids are a diverse group of nitrogen-containing basic compounds. They are typically derived from plant sources and contain one or more nitrogen atoms. Chemically they are very heterogeneous. Based on chemical structures, they may be classified into two broad categories:

  • Non heterocyclic or atypical alkaloids, for example
    taxol
  • Heterocyclic or typical alkaloids, for example quinine, caffeine, and nicotine

Examples of alkaloids produced by plants are:

  • Hyoscyamine, present in Datura stramonium
  • deadly nightshade
  • Cocaine, present in Erythroxylum coca the Coca plant
  • Scopolamine
    , present in the Solanaceae (nightshade) plant family
  • opium poppy
  • rosy periwinkle

Many alkaloids affect the central nervous system of animals by binding to neurotransmitter receptors.

Glucosinolates

Structural formula for glucosinolates. The side group R can vary. The structure includes a glucose molecules (to the left), a nitrogen atom derived from an amino acid, and two sulfur atoms, among which one derives from glutathione and the other from sulfate (seen to the left).[13]

Glucosinolates are secondary metabolites that include both sulfur and nitrogen atoms, and are derived from glucose, an amino acid and sulfate.

An example of a glucosinolate in plants is Glucoraphanin, from broccoli (Brassica oleracea var. italica).

Plant secondary metabolites in medicine

Many drugs used in modern medicine are derived from plant secondary metabolites.

Extraction of taxol from barks of Pacific Yew.

The two most commonly known

Pacific Yew.[15]

opium poppy. It is mostly known for its strong analgesic effects, however, morphine is also used to treat shortness of breath and treatment of addiction to stronger opiates such as heroin.[16][17] Despite its positive effects on humans, morphine has very strong adverse effects, such as addiction, hormone imbalance or constipation.[17][18] Due to its highly addictive nature morphine is a strictly controlled substance around the world, used only in very severe cases with some countries underusing it compared to the global average due to the social stigma around it.[19]

Opium field in Afghanistan, the largest grower of opium.[20]

Codeine, also an alkaloid derived from the opium poppy, is considered the most widely used drug in the world according to World Health Organization. It was first isolated in 1832 by a French chemist Pierre Jean Robiquet, also known for the discovery of caffeine and a widely used red dye alizarin.[21] Primarily codeine is used to treat mild pain and relief coughing[22] although in some cases it is used to treat diarrhea and some forms of irritable bowel syndrome.[22] Codeine has the strength of 0.1-0.15 compared to morphine ingested orally,[23] hence it is much safer to use. Although codeine can be extracted from the opium poppy, the process is not feasible economically due to the low abundance of pure codeine in the plant. A chemical process of methylation of the much more abundant morphine is the main method of production.[24]

nightshade family. While atropine was first isolated in the 19th century, its medical use dates back to at least the fourth century B.C. where it was used for wounds, gout, and sleeplessness. Currently atropine is administered intravenously to treat bradycardia and as an antidote to organophosphate poisoning. Overdosing of atropine may lead to atropine poisoning which results in side effects such as blurred vision, nausea, lack of sweating, dry mouth and tachycardia.[25]

raspberries and peanuts. It is commonly taken as a dietary supplement for extending life and reducing the risk of cancer and heart disease, however there is no strong evidence supporting its efficacy.[26][27] Nevertheless, flavonoids are in general thought to have beneficial effects for humans.[citation needed] Certain studies shown that flavonoids have direct antibiotic activity.[28] A number of in vitro and limited in vivo studies shown that flavonoids such as quercetin have synergistic activity with antibiotics and are able to suppress bacterial loads.[29]

Digoxin is a cardiac glycoside first derived by William Withering in 1785 from the foxglove (Digitalis) plant. It is typically used to treat heart conditions such as atrial fibrillation, atrial flutter or heart failure.[30] Digoxin can, however, have side effects such as nausea, bradycardia, diarrhea or even life-threatening arrhythmia.

Fungal secondary metabolites

The three main classes of fungal secondary metabolites are:

polyketides, nonribosomal peptides and terpenes. Although fungal secondary metabolites are not required for growth they play an essential role in survival of fungi in their ecological niche.[31] The most known fungal secondary metabolite is penicillin discovered by Alexander Fleming in 1928. Later in 1945, Fleming, alongside Ernst Chain and Howard Florey, received a Nobel Prize for its discovery which was pivotal in reducing the number of deaths in World War II by over 100,000.[32]

Examples of other fungal secondary metabolites are:

mevalonate
.

Fungal secondary metabolites can also be dangerous to humans.

Itching, psychosis or gangrene. Currently, removal of ergot bodies requires putting the rye in brine solution with healthy grains sinking and infected floating.[36]

Bacterial secondary metabolites

Bacterial production of secondary metabolites starts in the

endotoxins
.

An example of a bacterial secondary metabolite with a positive and negative effect on humans is botulinum toxin synthesised by Clostridium botulinum. This exotoxin often builds up in incorrectly canned foods and when ingested blocks cholinergic neurotransmission leading to muscle paralysis or death. However, botulinum toxin also has multiple medical uses such as treatment of muscle spasticity, migraine and cosmetics use.

Examples of other bacterial secondary metabolites are:

Phenazine

Polyketides

Nonribosomal peptides

Ribosomal peptides

Glucosides

Alkaloids

  • pufferfish
    .

Biotechnological approaches

Plant tissue culture Oncidium leucochilum.

Selective breeding was used as one of the first biotechnological techniques used to reduce the unwanted secondary metabolites in food, such as naringin causing bitterness in grapefruit.[38] In some cases increasing the content of secondary metabolites in a plant is the desired outcome. Traditionally this was done using in-vitro plant tissue culture techniques which allow for: control of growth conditions, mitigate seasonality of plants or protect them from parasites and harmful-microbes.[citation needed] Synthesis of secondary metabolites can be further enhanced by introducing elicitors into a tissue plant culture, such as jasmonic acid, UV-B or ozone. These compounds induce stress onto a plant leading to increased production of secondary metabolites.

To further increase the yield of SMs new approaches have been developed. A novel approach used by Evolva uses recombinant yeast S. cerevisiae strains to produce secondary metabolites normally found in plants. The first successful chemical compound synthesised with Evolva was vanillin, widely used in the food beverage industry as flavouring. The process involves inserting the desired secondary metabolite gene into an artificial chromosome in the recombinant yeast leading to synthesis of vanillin. Currently Evolva produces a wide array of chemicals such as stevia, resveratrol or nootkatone.

Nagoya protocol

With the development of recombinant technologies the Nagoya Protocol on Access to Genetic Resources and the Fair and Equitable Sharing of Benefits Arising from their Utilization to the Convention on Biological Diversity was signed in 2010. The protocol regulates the conservation and protection of genetic resources to prevent the exploitation of smaller and poorer countries. If genetic, protein or small molecule resources sourced from biodiverse countries become profitable a compensation scheme was put in place for the countries of origin.[39]

See also

References

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External links

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