TERPENES: CHARACTERISTICS, STRUCTURE, FUNCTION, CLASSIFICATION - BIOLOGY - 2024

The terpenes or isoprenoid compounds formed are organic repeating units of isoprene molecules, which are only hydrocarbons structured 5 carbon atoms. That is, they are isoprene polymers of different lengths.

Initially the word "terpene" was used specifically to refer to natural organic compounds derived from a single isoprene unit, but today the terms "terpene" and "isoprenoid" are used interchangeably with much frequency to refer to substances of one or more isoprene units.

Structure of an isoprene unit (Source: Edgar181, via Wikimedia Commons)

The term "terpene" originates from the English word "turpentine", which derives from the Latin words used to describe turpentine, a compound derived from the resin of certain species of conifers.

Turpentine contains some hydrocarbon compounds that were first coined as "terpenes," so traditionally all natural compounds made from isoprene units and derived from plants are denoted as terpenes.

These molecules are synthesized from acetyl coenzyme A as a precursor molecule. Initially, the condensation of two of these molecules occurs, forming acetoacetyl-CoA, which then reacts with another acetyl-CoA to form β-hydroxy-β-methylglutaryl-CoA.

The enzymatic reduction of the previous compound in the presence of water and dependent on NADPH produces mevalonic acid, whose phosphorylation yields the diphosphate form of mevalonic acid. The latter compound is decarboxylated and dehydrated to yield isopentenyl pyrophosphate (IPP), an activated isoprene unit.

The isomerization of the IPP, some successive chemical modifications of the same and the condensation with other IPP units produce the different terpenes that are known. The side chains of vitamin A, E and K, of beta-carotene and of squalene are examples of known terpenes.

Characteristics of terpenes

- Terpenes are generally volatile aromatic compounds.

- They are unsaturated hydrocarbon molecules (they have double bonds between some of their carbon atoms) that are found in all living organisms, but they are particularly abundant in the essential oils of many vegetables.

- Those terpenes that have oxygen atoms are known as "terpenoids". These compounds perform different functions in living organisms and are surprisingly diverse.

- They are not only produced by plant species, but they are also abundant in many animals, where they perform equally important functions.

Structure

More than 30,000 species of terpenes are known, or at least that is the approximate number of those that have been cited in the literature.

Its basic structure follows a general principle consisting of 2-methylbutane residues, often known as isoprene units, which are molecules made up of 5 carbon atoms; which is why they are also known as "isoprenoids".

Terpenes usually occur in nature as hydrocarbons, alcohols and their glycosides, ethers, aldehydes, ketones, carboxylic acids, and esters.

Depending on the number of 2-methylbutane residues, the following are distinguished:

- Hemiterpenes (C5)

- Monoterpenes (C10)

- Sesquiterpenes (C15)

- Diterpenes (C20)

- Sesterpenes (C25)

- Triterpenes (C30)

- Tetraterpenes (C40)

- Polyterpenes (C5) n, with more than 8 units

The isopropyl portion of each isoprene unit is known as the "head" and the ethyl portion is known as the "tail." In the mono-, sesqui-, di- and sesterpenes, the isoprene units are linked together in a head-tail sequence, but the tri- and tetraterpenes have a tail-tail connection in the center.

It is also important to note that terpenes logically have a number of carbon atoms that is always a multiple of 5, since their structural "blocks" are made up of isoprenes of 5 carbon atoms.

Function

As active compounds

- Terpenes are the substances responsible for the smell, taste and some of the specific pharmacological activities of certain conifers and citrus fruits, coriander and lavender, eucalyptus, lemongrass, lilies, pepper, and some flowers, leaves and roots of a wide variety of plants.

- These substances are used by plants to drive away insects or predators and also to attract pollinators and dispersers of their seeds, since they are often volatile compounds that emit specific aromas (the less volatile ones tend to provide bitter or toxic flavors).

- They are also useful compounds from the point of view of signaling and regulating plant growth.

In industry and biomedicine

- Terpenes are used for the preparation and production of perfumes, for which natural compounds such as menthol, citrus fruits and many spices are used.

- They are also useful for the manufacture of aromatherapy systems of natural origin, which have traditionally been used to improve the mood of individuals and, in addition, have a repellent effect against some insects.

- Medicinally, terpenes derived from eucalyptus oil, for example, have stimulating properties for the secretion of mucus, which is why they are commonly used in the formulation of expectorant substances.

Structure of the different forms of metol (Source: H Padleckas, via Wikimedia Commons)

- Many terpenes also have diuretic properties and help in relieving gastrointestinal spasms.

- They are also incorporated into lotions and creams to reduce pain and urticaria, as well as to reduce microbial infections, especially antibiotic-resistant microorganisms, such as yeast and fungi.

- The terpenes consumed in infusions of medicinal plants can help eliminate flatulence and indigestion. Furthermore, these substances are also included in many pesticide products.

Classification

Terpenes are classified, according to the number of isoprene units that compose them, as previously mentioned: hemiterpenes (C5); monoterpenes (C10); sesquiterpenes (C15); diterpenes (C20); sesterpenes (C25); triterpenes (C30); tetraterpenes (C40); polyterpenths (C5) n, with more than 8 units.

Hemiterpenes

About 50 types of hemiterpenes are known, which are terpenes made up of a single isoprene unit. 3-methyl-2-buten-1-ol (prenol) is well known and is found in the oil extracted from the flowers of the Cananga odorata tree and hops (Humulus lupulus).

Monoterpenes

Monoterpenes are terpenes made up of 2 isoprene units. There are acyclic and cyclic ones and, in total, there are about 1,500 documented chemical species.

Chemical structure of limonene, a monoterpene (Source: Leyo, via Wikimedia Commons)

Most acyclic monoterpenes are derived from 2,6-dimethyloctane and have their isoprene units attached "head-to-tail". Some of these compounds enrich the floral tissues of geraniums (Pelargonium sp.) And are in the wax that covers the plumage of some birds.

Monocyclic monoterpenes are those that have a cyclic portion in their structure and the most common are those that have cyclopropane, cyclobutane and cyclohexane rings. There are also the bicyclic monoterpenes, which have two rings.

Sesquiterpenes

Sesquiterpenes are terpenes made up of 3 isoprene units, so they have 15 carbon atoms.

There are the Farneseans (which are responsible for the flavors of the covering of apples, pears and other fruits), the Germacranians and Elemans, the Humulans, the Eudesmans and Furanoeudesmans, the Eremofilanes, Furanoeremophilanes and Valerans, the Cardinans, the Drimans, the guainanos and cicloguainanos, among many others.

Diterpenes

Diterpenes have 4 isoprene units (C20) and are also a quite varied group of terpenes, which include phytanes, cyclophytans, labdans, pimarans and isopimarans, beyerans, atisans, giberelans, cembrans, etc.

Sesterpenes

They are composed of 5 isoprene units and can be cyclic or acyclic. These compounds rarely occur in "higher" plants and have been reported in some marine sponges and other sea animals.

Triterpenes

Composed of 6 isoprene units (C30), triterpenes are characteristic molecules of some plants, as they are part of brassinosteroids, phytoalexins, some toxins and certain protective waxes.

Many of these are linear compounds, most of which are derived from squalene, with two farnesene units linked tail to tail; There are also cyclic triterpenes.

Tetraterpenes

Tetraterpenes are made up of 7 isoprene units, so they have 35 carbon atoms. In this group are carotenoids, which are the most abundant in nature.

Carotenoids are all structural variants or derivatives of the degradation of β-carotene that have between 11 and 12 conjugated double bonds. Carotenoids are found in the leaves, stems, and roots of higher plants, accounting for up to 0.1% of the dry weight of a whole plant.

Structure of B-carotene, a tetraterpene (Source: NEUROtiker / Public domain, via Wikimedia Commons)

They work as "color filters" for photosynthesis in leaves, and are responsible for the yellowish and reddish colorations of the leaves of some trees during autumn, as they degrade more slowly than chlorophyll (green).

Although they do not have the same number of functions as anthocyanins and flavonoids in plants, carotenoids contribute to the coloring of the flowers and fruits of many vegetables.

Polyterpenes

These are the isoprenoids of more than 8 isoprene units. The "natural rubber", which is obtained from the coagulation of the latex of Hevea brasiliensis, is essentially made up of cis-polyisoprenoids.

Dolichols with 14-20 isoprene units are found as phosphoric acid esters in the lipid membranes of nerve cells and some endocrine glands. They function in the transfer of oligosaccharides for the synthesis of glycolipids and glycoproteins.

Examples of terpenes

- Triene

The oil extracted from the leaves of basil, bay leaf and bitter orange is rich in acyclic monoterpenoids known as "triene", among which the β-myrcene and the conformational isomers of β-ocimene stand out.

Chemical structure of myrcene, a monoterpene (Source: Jan Herold, Leyo / Public domain, via Wikimedia Commons

- Limonene

Limonene, a monoterpene present in the peel of citrus fruits and in the leaves of their trees, as well as in plants such as mint, juniper, marijuana, rosemary and pine, is industrially exploited in pharmaceuticals, cosmetology, biotechnology and in the food industry.

It has antidepressant, anxiolytic, antibacterial and immunostimulating properties, making it very attractive from many points of view.

- Carotenoids

Carotenoids are terpenes that function as natural pigments in many plant, fungal, and bacterial species. They are essential for the synthesis of vitamin A in animals and must be consumed with food, since the human body and that of other mammals is unable to synthesize them.

References

1. Bouvier, F., Rahier, A., & Camara, B. (2005). Biogenesis, molecular regulation and function of plant isoprenoids. Progress in lipid research, 44 (6), 357-429.
2. Breitmaier, E. (2006). Terpenes: flavors, fragrances, pharmaca, pheromones. John Wiley & Sons.
3. Holstein, SA, & Hohl, RJ (2004). Isoprenoids: remarkable diversity of form and function. Lipids, 39 (4), 293-309.
4. Kandi, S., Godishala, V., Rao, P., & Ramana, KV (2015). Biomedical significance of terpenes: an insight. Biomedicine, 3 (1), 8-10.
5. Tetali, SD (2019). Terpenes and isoprenoids: a wealth of compounds for global use. Plant, 249 (1), 1-8.