Terpenoid

The terpenoids, also known as isoprenoids, are a class of naturally occurring organic chemicals derived from the 5-carbon compound isoprene and its derivatives called terpenes, diterpenes, etc. While sometimes used interchangeably with "terpenes", terpenoids contain additional functional groups, usually containing oxygen.^[1] When combined with the hydrocarbon terpenes, terpenoids comprise about 80,000 compounds.^[2] They are the largest class of plant secondary metabolites, representing about 60% of known natural products.^[3] Many terpenoids have substantial pharmacological bioactivity and are therefore of interest to medicinal chemists.^[4]

Plant terpenoids are used for their aromatic qualities and play a role in traditional herbal remedies. Terpenoids contribute to the scent of eucalyptus, the flavors of cinnamon, cloves, and ginger, the yellow color in sunflowers, and the red color in tomatoes.^[5] Well-known terpenoids include citral, menthol, camphor, salvinorin A in the plant Salvia divinorum, ginkgolide and bilobalide found in Ginkgo biloba and the cannabinoids found in cannabis. The provitamin beta carotene is a terpene derivative called a carotenoid.

The steroids and sterols in animals are biologically produced from terpenoid precursors. Sometimes terpenoids are added to proteins, e.g., to enhance their attachment to the cell membrane; this is known as isoprenylation. Terpenoids play a role in plant defense as prophylaxis against pathogens and attractants for the predators of herbivores.^[6]

Structure and classification

Terpenoids are modified terpenes,^[7] wherein methyl groups have been moved or removed, or oxygen atoms added. Some authors use the term "terpene" more broadly, to include the terpenoids. Just like terpenes, the terpenoids can be classified according to the number of isoprene units that comprise the parent terpene:

Terpenoids	Analogue terpenes	Number of isoprene units	Number of carbon atoms	General formula	Examples^[8]
Hemiterpenoids	Isoprene	1	5	C5H8	DMAPP, isopentenyl pyrophosphate, isoprenol, isovaleramide, isovaleric acid, HMBPP, prenol
Monoterpenoids	Monoterpenes	2	10	C10H16	Bornyl acetate, camphor, carvone, citral, citronellal, citronellol, geraniol, eucalyptol, hinokitiol, iridoids, linalool, menthol, thymol
Sesquiterpenoids	Sesquiterpenes	3	15	C15H24	Farnesol, geosmin, humulone
Diterpenoids	Diterpenes	4	20	C20H32	Abietic acid, ginkgolides, paclitaxel, retinol, salvinorin A, sclareol, steviol
Sesterterpenoids	Sesterterpenes	5	25	C25H40	Andrastin A, manoalide
Triterpenoids	Triterpenes	6	30	C30H48	Amyrin, betulinic acid, limonoids, oleanolic acid, sterols, squalene, ursolic acid
Tetraterpenoids	Tetraterpenes	8	40	C40H64	Carotenoids
Polyterpenoid	Polyterpenes	>8	>40	(C5H8)n	Gutta-percha, natural rubber

Terpenoids can also be classified according to the type and number of cyclic structures they contain: linear, acyclic, monocyclic, bicyclic, tricyclic, tetracyclic, pentacyclic, or macrocyclic.^[8] The Salkowski test can be used to identify the presence of terpenoids.^[9]

• Selected terpenoids
•  
  Paclitaxel is a diterpenoid anticancer drug.
•  
  Terpineols are monoterpenoids.
•  
  Humulones are classified as sesquiterpenoids.
•  
  Retinol is a diterpenoid.
•  
  Hinokitiol is a monoterpenoid, a tropolone derivative.
•  
  Limonin, a common limonoid, is a triterpenoid.
•  
  Geosmin is a sesquiterpenoid.

Biosynthesis

Further information: Terpene § Biosynthesis

Terpenoids, at least those containing an alcohol functional group, often arise by hydrolysis of carbocationic intermediates produced from geranyl pyrophosphate. Analogously hydrolysis of intermediates from farnesyl pyrophosphate gives sesquiterpenoids, and hydrolysis of intermediates from geranylgeranyl pyrophosphate gives diterpenoids, etc.^[10]

See also

• List of antioxidants in food
• List of phytochemicals in food
• Nutrition
• Phytochemistry
• Secondary metabolites

References

1. Chemistry, International Union of Pure and Applied. IUPAC Compendium of Chemical Terminology. IUPAC. doi:10.1351/goldbook.T06279.
2. Christianson, David W. (2017). "Structural and Chemical Biology of Terpenoid Cyclases". Chemical Reviews. 117 (17): 11570–11648. doi:10.1021/acs.chemrev.7b00287. PMC 5599884. PMID 28841019.
3. Firn R (2010). Nature's Chemicals. Oxford: Biology.
4. Ashour, Mohamed; Wink, Michael; Gershenzon, Jonathan (2010). "Biochemistry of Terpenoids: Monoterpenes, Sesquiterpenes and Diterpenes". Biochemistry of Plant Secondary Metabolism. pp. 258–303. doi:10.1002/9781444320503.ch5. ISBN 9781444320503.
5. Specter M (September 28, 2009). "A Life of Its Own". The New Yorker.
6. Singh, Bharat; Sharma, Ram A. (April 2015). "Plant terpenes: defense responses, phylogenetic analysis, regulation and clinical applications". 3 Biotech. 5 (2): 129–151. doi:10.1007/s13205-014-0220-2. ISSN 2190-572X. PMC 4362742. PMID 28324581.
7. Houghton, Isaac. "The Physiology of Cannabis Terpenes and Terpenoids – A Brief Overview". Elliot Barker. Retrieved 3 May 2016.
8. Ludwiczuk, A.; Skalicka-Woźniak, K.; Georgiev, M.I. (2017). "Terpenoids". Pharmacognosy: 233–266. doi:10.1016/B978-0-12-802104-0.00011-1. ISBN 9780128021040.
9. Ayoola GA (2008). "Phytochemical Screening and Antioxidant Activities of Some Selected Medicinal Plants Used for Malaria Therapy in Southwestern Nigeria". Tropical Journal of Pharmaceutical Research. 7 (3): 1019–1024. doi:10.4314/tjpr.v7i3.14686. hdl:1807/60332.
10. Davis, Edward M.; Croteau, Rodney (2000). "Cyclization Enzymes in the Biosynthesis of Monoterpenes, Sesquiterpenes, and Diterpenes". Topics in Current Chemistry. 209: 53–95. doi:10.1007/3-540-48146-X_2. ISBN 978-3-540-66573-1.

External links

• IUPAC definition of terpenoids