Let us make an in-depth study of the structure, distribution and role of isoprenoids (Terpenoids) as secondary metabolites in plants.
1. Monoterpenes (C10):
These terpenes contain 10-C atoms and are built up of two iso- preneitnits. There structure may be (i) acyclic, (ii) cyclohexanoid (mono, bi or tricyclic) and (iii) cyclopentanoid.
Monoterpenes are chiefly found in resin ducts in leaves, twigs and trunks of conifers such as pines.
Important monoterpene components of conifer resins are α -pinene, β -pinene, limonene and myrcene which are toxic to large number of insects.
Monoterpenes also occur as important components of essential oils in special secretory glands in many flowering plants, (e.g., mint) and give a characteristic odour to their foliage which has insects repelling properties.
Menthol is chief monoterpene component of peppermint oil. Essential oils are obtained from plants by steam distillation and find their use in flavouring foods and in perfumery. Structures of some common monoterpenes are given in Fig. 24.4.
Monoterpene esters pyrethroids occurring in leaves and flowers of Chrysanthemum possess strong insecticidal properties and are used on commercial scale in making insecticides.
2. Sesquiterpenes (C15):
These are largest group of isoprenoids which have great structural variations. Many sesquiterpenes co-exist with monoterpenes in essential oils in higher plants. Some sesquiterpene lactones such as costunolide found in the glandular hairs of sage brush and sunflower are feeding deterrents to herbivores. An aromatic sesquiterpene dimer gossypol found in cotton is known to provide resistance to insect, fungal and bacterial pathogens. The structures of above mentioned sesquiterpenes are given in Fig. 24.5.
3. Diterpenes (C20):
Plant resins produced by conifers such as pines and certain leguminous trees such as Hymenaea courbrail contain appreciable amount of diterpene abietic acid (Fig. 24.6). These diterpenes function as chemical deterrents to predators and also help in healing the wounds caused by insect bites.
4. Triterpenes (C30):
Triterpenes and their derivatives such as steroids represent another vast group of isoprenoids or terpenoid compounds. The steroids usually have a tetracyclic or pentacyclic molecular structure and many of them are modified to contain fewer than 30 – C atoms.
Some steroids such as plant sterols (e.g., sitosterol) have primary function in plant cells being part of their cell-membranes while others are defensive secondary products. Examples of the latter category are various phytoecdysones, limonoids, cardenolides, sapogenins, sterol alkaloids and steroid hormones.
A brief account of all these is as follows:
(i) Phytoecdysones (Ecdysteroids):
These have highly polar structure. Ponasterone A isolated from Podocarpus (a conifer) has the same basic structure as insect moulting hormones such as a-ecdysone and is therefore, a strong insect deterrent.
(ii) Limnoids (Bitter principles from citrus fruit):
These substances have highly complex structure. Azadirachtin from neem tree is a strong deterrent to insect feeding and other herbivores.
(iii) Cardenolides:
These are steroid glycosides highly toxic to higher animals and have important pharmacological effects on heart muscles. These substances are found in more than 10 families of higher plants. The glycoside parts of these steroids are complex and contain unique sugars such as digitoxose and acetyl digitoxose. Digitoxigenin is aglycone of dipitoxin that is obtained from Digitalis and is prescribed for heart ailments.
(iv) Sapogenins (Saponins):
These are also steroid glycosides found in many plants. They have detergent properties and ability to disrupt membranes and cause haemolysis of red blood cells. A saponin called yamogenin is obtained from Dioscorea and is used in making oral contraceptives.
(v) Sterol alkaloids (Terpenoid alkaloids):
These alkaloids occur in many plants as glycosides. For example, the aglycone of tomatin is tomatidine and of solanine is solanidine.
(vi) Steroid hormones:
Many steroids which occur in animals as hormones are also widespread in plants, but their role in plants is not yet clear. For example, the hormone progesterone (from placenta and corpus in animals) is also present in Holarrhena floribunda. Similarly, deoxycorticosterone (from adrenal cortex in animals) is also found in Digitalis lanata.
Structures of some steroids in plants are given in Fig. 24.7.
5. Polyterpenes [C5]n:
Many high molecular weight polyterpenes are found in plants as natural products. Of these, rubber is best known. Other examples are gutta and chicle. Their function in plants is to provide defense against herbivores and to help in wound healing.
Rubber:
It is high molecular weight polyisoprene or polyterpene compound produced in latex of over 300 genera of angiospcrms. However, the most important commercial source of rubber is Hevea brasiliensis. Rubber is found as particles suspended in milky latex in long vessels called laticifers or laticiferous ducts in plants.
Rubber consists of a large number (1500 – 60,000) isopentenyl units in which carbon- carbon double bonds have cis (Z) configuration, (Fig. 24.8). Accordingly, the molecular weight of rubber ranges from ~ 1 x 105 – ~ 4 × 106.
Gutta:
It is a polymer of isoprene residues with a low molecular weight than rubber and in which carbon-carbon double bonds have trans (E) configuration (Fig. 24.9). It is obtained from various trees of the genus Palaquium (Family Sapotaceae). It is also obtained commercially on a small scale from desert shrub guayule (Parthenium argenatum) of the family Asteraceae.
Chicle:
It consists of a mixture of comparatively low molecular weight cis and trans polyisopentenyl units along with resins that are soluble in acetone. It is obtained from Sapodilla (Manilkara zapota) of family Sapotaceae and is used commercially as original chewing gum base.
Many terpenoids are well known to have a primary role in growth and development of plants and are therefore considered as primary metabolites rather than secondary plant products.
Examples are:
i. Phytohormones gibberellins are diterpenes.
ii. Absisic acid (ABA) is a sesquiterpene and degradation product of carotenoid precursor.
iii. Sterols are derivatives of triterpenes and are essential components of cell membranes.
iv. Carotenoids (red, orange and yellow) are tetraterpenes. Their roles as accessory pigment in photosynthesis and to protect photosynthetic tissues from photo-oxidation are well known.
v. Phytol side chains of chlorophylls are diterpene derivatives. Bacteriochlorophylls also have terpenoid side chains.