Terpenes Terpenes are a diverse class of lipids. More than 20,000 terpenes are known. They are hydrocarbons that have isoprene units as the building block. Biochemical modifications such as oxidation or rearrangement produce the related terpenoids. Certain terpenes and terpenoids have been used as spices, perfumes, and medicines for many thousands of years. 1 Investigation showed that their structures are consistent with the assumption that they were made by joining together isoprene units, usually in a head-to-tail fashion. (branched end of isoprene is the head, the unbranched end is the tail.) Isoprene is the common name for 2-methyl-1,3-butadiene, a compound containing five carbon atoms. The linking of isoprene units in a head-to-tail fashion to form terpenes is known as the isoprene rule. 2 carbon skeleton of two isoprene units with a bond between the tail of one and the head of another 3 4 For cyclic compounds, the linkage of the head of one isoprene unit to the tail of another is followed by an additional linkage to form the ring. The second linkage is not necessarily head-to-tail, but is whatever is necessary to form a stable five- or six membered ring. 5 Classification of Terpenes They are classified according to the number of carbons they contain. Monoterpenes are composed of two isoprene units, so they have 10 carbons. Sesquiterpenes, with 15 carbons, are composed of three isoprene units. Many fragrances and flavorings found in plants are monoterpenes and sesquiterpenes. These compounds are known as essential oils. 6 Classification of Terpenes Carbon atoms Isoprene units Classification 10 2 monoterpenes 15 3 sesquiterpenes 20 4 diterpenes 25 5 sesterterpenes 30 6 triterpenes 40 7 tetraterpenes 7 Monoterpenes Monoterpenes, with sesquiterpenes, are the main constituents of essential oils. most occur as complex mixtures. They have wound healing properties. They may be acyclic or cyclic. Acyclic monoterpenes They can be considered as derivatives of 2,6-dimethyloctane. 8 Monocyclic monoterpenes They are derived from cyclohexane with an isopropyl substituent. The most common ones are : 9 Bicyclic monoterpenes 10 Many monoterpenes possess antitumor activity in animal and cell models. They have also antioxidant properties, γ-terpene and hydroxytyrosol being among the most effective. 11 Sesquiterpenes They are a class of terpenes that consist of three isoprene units and often have the molecular formula C15H24. Like monoterpenes, sesquiterpenes may be acyclic or contain rings, including many unique combinations. Sesquiterpenes are found naturally in plants and insects, as semiochemicals, e.g. defensive agents or pheromones. 12 The reaction of geranyl pyrophosphate with isopentenyl pyrophosphate results in the 15-carbon farnesyl pyrophosphate (FPP), which is an intermediate in the biosynthesis of sesquiterpenes such as farnesene. 13 14 Zingiberene a sesquiterpene abundant in ginger. Cadinene isomeric hydrocarbons that occur in a wide variety of essential oil- producing plants. Name (Cadinene) is derived from the Cade juniper (Juniperus oxycedrus L.). 15 Diterpenes Diterpenes belong to a class of chemical compounds that is composed of two terpene units, often with the molecular formula C20H32. They consist of four isoprene subunits and are biosynthesized by plants, animals and fungi via the HMG-CoA reductase pathway, with geranylgeranyl pyrophosphate being a primary intermediate. 16 Diterpenes form the basis for biologically important compounds such as retinol, retinal, and phytol. They are known to be antimicrobial and antiinflammatory. Geranylgeranyl pyrophosphate, the starting material for the biosynthesis of diterpenes 17 Though a wide range of terpene structures exist, few of them are biologically retinol significant; by contrast, diterpenoids possess a rich pharmacological activities and include phytol important compounds such as retinol and phytol. Taxadiene a tricyclic diterpene Cembrene A 18 Sesterterpenes Sesterterpenes are terpene molecules containing a C25 skeleton, which are rare among terpene compounds. Sesterterpenoids are ubiquitous secondary metabolites in fungi, marine organisms and plants. Their structural diversity encompasses carbotricyclic ophiobolanes, polycyclic anthracenones, polycyclic furan-2-ones, polycyclic hydroquinones, among many other carbon skeletons. 19 20 Triterpenoids Triterpenoids are widely distributed in both edible plants and ethnomedicinal plants; they can also be found in any animals and marine organisms. These compounds are structurally diverse and they are characterized by a backbone of 30 C modified in multiple ways that enables a formation of more than 20,000 members. 21 Triterpenoids are the most important group of terpenoids because they exhibit a great diversity of biological activities and they are the major constituents in medicinal plant decoction and extracts. 22 Examples 23 Tetraterpenes Triterpenes and tetraterpenes have important biological roles. For example, squalene, a triterpene, is a precursor of steroid molecules. 24 Tetraterpenes are terpenes consisting of eight isoprene units with a molecular formula of C40H64 Tetraterpenoids are tetraterpenes that have been chemically modified, with oxygen-containing functional groups. Carotenoids are tetraterpenes. Lycopene, the compound responsible for the red coloring of tomatoes and watermelon, and β-carotene, the compound that causes carrots and apricots to be orange, are examples of carotenoids. 25 26 Phytoene Accumulation of this carotenoids in the skin may protect the skin by several mechanisms: acting as UV absorbers, as antioxidants, and as anti- inflammatory agents. 27 The most studied group of tetraterpenes is the carotenoids. They have important biological functions, in sight, antioxidative activity and protection against free radicals, synthesis of plant hormones and as structural components of the membranes. Apart from these biological relevance, carotenoids are also highly valued compounds in the food and pharmaceutical industries. 28 Terpenes Biosynthesis Reaction of dimethylallyl pyrophosphate with isopentenyl pyrophosphate forms geranyl pyrophosphate, a 10-carbon compound. First step of the reaction, isopentenyl pyrophosphate acts as a nucleophile and displaces a pyrophosphate group from dimethylallyl pyrophosphate. 29 Pyrophosphate is an excellent leaving group: The four OH groups have pKa values of 0.9, 2.0, 6.6, and 9.4. Thus, three of the four groups will mainly be in their basic forms at physiological (pH = 7.3). A proton is removed in the next step, which results in the formation of geranyl pyrophosphate. 30 31 The schemes below show how some of the many monoterpenes could be synthesized from geranyl pyrophosphate: 32 33 Geranyl pyrophosphate can react with another molecule of isopentenyl pyrophosphate to form farnesyl pyrophosphate, a 15-carbon compound. 34 35 Two molecules of farnesyl pyrophosphate form squalene, a 30-carbon compound. The reaction is catalyzed by the enzyme squalene synthase, which joins the two molecules in a tail-to-tail linkage. Squalene is the precursor of cholesterol, and cholesterol is the precursor of all other steroids. 36 37 Farnesyl pyrophosphate can react with another molecule of isopentenyl pyrophosphate to form geranylgeranyl pyrophosphate, a 20-carbon compound. Two geranylgeranyl pyrophosphates can join to form phytoene, a 40-carbon compound. Phytoene is the precursor of the carotenoid (tetraterpene) pigments in plants. 38 References https://en.wikipedia.org/wiki/Tetraterpene https://en.wikipedia.org/wiki/Phytoene Organic Chemistry 4th Ed. by Bruice Organic Chemistry 7th ed. by Wade, L. G. 39.