**2. Classification of plants secondary metabolite**

Plants secondary metabolites can be classfied into three groups namely


#### **2.1 Terpenes**

Terpenoids constitutes the largest class of secondary products; they comprise of more than 40,000 different structures and are the largest natural products in plants [9, 10]. Terpenes consists of five-carbon isoprene units, and classified into hemiterpenes (C5), monoterpenes (C10), sesquiterpenes (C15), diterpenes (C20), triterpenes (C30), tetraterpenes (C40), and polyterpenes [11, 12]. Terpenoids originate from two different biosynthetic routes: plastid-located deoxyxylulose phosphate (DXP) pathway (also called methylerythritol phosphate or MEP pathway) and the cytosolic mevalonic acid (MVA) pathway (**Figure 1**) [13–15].

#### *2.1.1 Hemiterpenes*

This is a volatile compound synthesized from DMAPP, isoprene is the most abundant true hemiterpene from plants (**Figure 2**). The species that synthesize isoprene are found among ferns, mosses, angiosperms, and gymnosperms. The emission and production of isoprene are distributed very widely in the plant kingdom. Isoprene is emitted into the atmosphere and protects leaves to survive short periods of high temperature. Moreover, it increases the plant's tolerance towards ozone and reactive oxygen species [16]. Hemiterpenes may also act as signaling molecules. The highly volatile hemiterpene methacrolein are emitted in the leaves of sagebrush (*Artemisia tridentata*) (**Figure 2**) in addition to other volatile compounds like hexenal, monoterpenes, and methyl jasmonate when the plant is damaged, this

**53**

**Figure 3.**

*Mono- and bicyclic monoterpenes derived from geranyl diphosphate (GPP).*

*Biosynthesis of Natural Products*

*2.1.2 Monoterpenes*

**Figure 2.** *Hemiterpenes.*

*DOI: http://dx.doi.org/10.5772/intechopen.97660*

is perceived by plants and enables them to react faster to a possible attack. A plant that is prepared in this manner is less likely to be damaged by herbivores [17]. The C-5 units derived from DMAPP are found in natural products of mixed biosynthetic

Monoterpenes originate from one molecule DMAPP and one molecule IPP that are joined in most cases head-to-tail, yielding all- trans geranyl diphosphate (GPP) (**Figure 3**). Several plant families, e.g., the Lamiaceae and Asteraceae, have glandular trichomes with secretory cells that produce terpenes and secrete them into a shared subcuticular storage cavity [18]. Conifers accumulate a complex mixture of mono-, sesqui-, and diterpenes, oleoresin, in resin blisters or ducts, which are covered by a layer of epithelial cells that secrete and synthesize the terpenes into the lumen [19]. As in the case of the conifers, many other plants accumulate monoterpenes in mixtures containing the larger sesqui- and diterpenes, rather than monoterpenes alone. The physiological function of monoterpenes is defense, the attraction of pollinators, and plant–plant communication [20]. The plant-insect interactions role of terpenes has been well-studied in conifers and the bark beetle. The oleoresin is secreted from the ducts or produced newly upon tissue damage by the beetle [21]. Ingested monoterpenes are converted by the beetles to pheromones that either attract more beetles or serve as anti-aggregation signals. Besides, conifer monoterpenes take part in tritrophic interactions and attract insect predators that feed on bark beetles [19]. Most aromatherapy, insecticides, perfumes and pharmaceutical products are made from monoterpenes [18]. The essential oil of corn mint (*Mentha arvensis* var. piperascens) produce more than 7000 tons of menthol every year either by total synthesis or from the steam-distilled. The cooling sensation stimulated by menthol is caused by the excitation of cation channels that serve as thermal receptors [22]. Two monoterpenes with promising anticancer effects are perrillyl alcohol and (+)-(R)- limonene [23], these two compounds

origin, e.g., hop bitter acids, prenylated flavonoids, and hyperforin.

#### **Figure 1.**

*Schematic overview of terpene biosynthesis in plants.*

*Biosynthesis of Natural Products DOI: http://dx.doi.org/10.5772/intechopen.97660*

**Figure 2.** *Hemiterpenes.*

*Bioactive Compounds - Biosynthesis, Characterization and Applications*

Plants secondary metabolites can be classfied into three groups namely

Terpenoids constitutes the largest class of secondary products; they comprise of more than 40,000 different structures and are the largest natural products in plants [9, 10]. Terpenes consists of five-carbon isoprene units, and classified into hemiterpenes (C5), monoterpenes (C10), sesquiterpenes (C15), diterpenes (C20), triterpenes (C30), tetraterpenes (C40), and polyterpenes [11, 12]. Terpenoids originate from two different biosynthetic routes: plastid-located deoxyxylulose phosphate (DXP) pathway (also called methylerythritol phosphate or MEP pathway) and the cyto-

This is a volatile compound synthesized from DMAPP, isoprene is the most abundant true hemiterpene from plants (**Figure 2**). The species that synthesize isoprene are found among ferns, mosses, angiosperms, and gymnosperms. The emission and production of isoprene are distributed very widely in the plant kingdom. Isoprene is emitted into the atmosphere and protects leaves to survive short periods of high temperature. Moreover, it increases the plant's tolerance towards ozone and reactive oxygen species [16]. Hemiterpenes may also act as signaling molecules. The highly volatile hemiterpene methacrolein are emitted in the leaves of sagebrush (*Artemisia tridentata*) (**Figure 2**) in addition to other volatile compounds like hexenal, monoterpenes, and methyl jasmonate when the plant is damaged, this

**2. Classification of plants secondary metabolite**

solic mevalonic acid (MVA) pathway (**Figure 1**) [13–15].

• Terpenes

**2.1 Terpenes**

*2.1.1 Hemiterpenes*

• Phenolic compounds

• Nitrogen-containing compounds [8]

**52**

**Figure 1.**

*Schematic overview of terpene biosynthesis in plants.*

is perceived by plants and enables them to react faster to a possible attack. A plant that is prepared in this manner is less likely to be damaged by herbivores [17]. The C-5 units derived from DMAPP are found in natural products of mixed biosynthetic origin, e.g., hop bitter acids, prenylated flavonoids, and hyperforin.

#### *2.1.2 Monoterpenes*

Monoterpenes originate from one molecule DMAPP and one molecule IPP that are joined in most cases head-to-tail, yielding all- trans geranyl diphosphate (GPP) (**Figure 3**). Several plant families, e.g., the Lamiaceae and Asteraceae, have glandular trichomes with secretory cells that produce terpenes and secrete them into a shared subcuticular storage cavity [18]. Conifers accumulate a complex mixture of mono-, sesqui-, and diterpenes, oleoresin, in resin blisters or ducts, which are covered by a layer of epithelial cells that secrete and synthesize the terpenes into the lumen [19]. As in the case of the conifers, many other plants accumulate monoterpenes in mixtures containing the larger sesqui- and diterpenes, rather than monoterpenes alone. The physiological function of monoterpenes is defense, the attraction of pollinators, and plant–plant communication [20]. The plant-insect interactions role of terpenes has been well-studied in conifers and the bark beetle. The oleoresin is secreted from the ducts or produced newly upon tissue damage by the beetle [21]. Ingested monoterpenes are converted by the beetles to pheromones that either attract more beetles or serve as anti-aggregation signals. Besides, conifer monoterpenes take part in tritrophic interactions and attract insect predators that feed on bark beetles [19]. Most aromatherapy, insecticides, perfumes and pharmaceutical products are made from monoterpenes [18]. The essential oil of corn mint (*Mentha arvensis* var. piperascens) produce more than 7000 tons of menthol every year either by total synthesis or from the steam-distilled. The cooling sensation stimulated by menthol is caused by the excitation of cation channels that serve as thermal receptors [22]. Two monoterpenes with promising anticancer effects are perrillyl alcohol and (+)-(R)- limonene [23], these two compounds

**Figure 3.** *Mono- and bicyclic monoterpenes derived from geranyl diphosphate (GPP).* suppress translation of 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) reductase, an enzyme of the MVA pathway and induce apoptosis [24]. The 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) reductase enzyme is a promising target for anti-tumor compounds because tumor cells have elevated HMG-CoA reductase levels and many proteins involved in cell growth are phenylated. The decreased terpene biosynthesis in humans leads to suppression of HMG-CoA reductase [25]. The secoiridoids yields from the cleavage of the cyclopentane ring of the iridoid skeleton, which are monoterpene indole alkaloids and the biosynthetic building units of the Ipecac alkaloids [25]. Many iridoids have an intense bitter-taste and therefore act as feeding deterrents (**Figure 4**) [26].

#### *2.1.3 Sesquiterpenes*

In general, sesquiterpenes are less volatile than monoterpenes; they contain three isoprene units and are formed by condensation of DMAPP with two molecules IPP, the central C15 intermediate farnesyl diphosphate (FPP) can be folded into mono-, bi- or tricyclic systems [9]. Initially, it was assumed that all sesquiterpenes are produced via the cytosolic MVA pathway. Recent studies, however, revealed that certain sesquiterpenes originate from isoprene units provided by the DXP pathway [12, 13] or by both biosynthetic routes [27]. This can be explained by the transport of isoprenoid precursors from the plastids to the cytosol [28]. Abscisic acid is a sesquiterpene phytohormone that is induced by drought and promotes stomatal closure and seed dormancy. Other sesquiterpenes take part in tritrophic plant-herbivore-parasite interactions [13]. The sesquiterpenes (E)-b-farnesene and the (E)-a-bergamotene attract the parasitic wasp *Cotesia marginiventris*, in maize infested with lepidopteran larvae [29]. Maize roots release (E)-b-caryophyllene (**Figure 5**) upon an attack of larvae of the beetle *Diabrotica virgifera* to attract the parasitic nematode *Heterorhabditis megidis* [30]. Many sesquiterpenes (sesquiterpene lactones) contain a pentacyclic lactone group, these compounds occur abundantly in the family Asteraceae, because of their bitter taste sesquiterpene lactones presumably serve as feeding deterrents of herbivores [31]. Pharmacologically active sesquiterpene lactones often show anti-inflammatory effects due to inhibition of the transcription factor NF- kB that mediates immunological responses and inflammation [32]. One of the most popular medicinal plants, chamomile (*Matricaria recutita*) is a sesquiterpenes with such activities. Antimigraine action of some sesquiterpene lactones, e.g., parthenolide from feverfew (*Tanacetum parthenium*), is mediated by inhibition of platelet aggregation and serotonin secretion [9]. The reason for the cytotoxicity

**55**

*coli* or yeast [36, 37].

*2.1.4 Diterpenes*

*Biosynthesis of Natural Products*

**Figure 5.**

*Linear and cyclic sesquiterpenes.*

*DOI: http://dx.doi.org/10.5772/intechopen.97660*

and allergenicity of sesquiterpene lactones with a, b-unsaturated lactone is the alkylation of proteins. Artemisinin is a novel promising agent against malaria. Structurally, it is a tetracyclic sesquiterpene with a six-membered lactone ring and an unusual 1, 2, 4-trioxane ring (**Figure 5**). The president of North Vietnam, to the Chinese government (Ho Chi Minh) discovered artemisinin for a cure against malaria to support his troops in the malaria-infested jungles during the American/ Vietnamese war [33]. The ether extract from *A. annua*, and artemisinin (qinghaosu) revealed the antimalarial activity and the mode of action of artemisinin are still being investigated. Most likely, it interferes with sarco-endoplasmic reticulum calcium ATPase (SERCA) of *Plasmodium falciparum*, but other mechanisms, for example, alkylation of biological macromolecules or the production of reactive oxygen species [34]. The structural feature required for antimalarial activity is the peroxide bridge. Artemether and artesunate (two semisynthetic analogs), in efficiency comparison to artemisinin were developed and are now used as firstline therapy in the treatment of malaria, in combination with other antimalarial drugs like the lumefantrine and quinine analogs mefloquine. This combination tends to prevent resistance to Plasmodium. The artemisinin and its analogs success has triggered by extraction of the sesquiterpene from the plant because *A. annua* contains only 0.01–1.5% of artemisinin [35]. Therefore, an powerful and affordable drug for the people in malaria-endemic areas are necessary, either by breeding of *A. annua* plants with elevated artemisinin levels or biotechnological production of the artemisinin precursor artemisinic acid by cloning the biosynthetic genes from *A. annua* [35] and engineering the pathway into the bacterium *Escherichia* 

Diterpenes originate from the Plasmid DXP pathway and are synthesized from

DMAPP and three molecules IPP yielding the C20 metabolite geranyl geranyl diphosphate (GGPP). GGPP is a smaller terpene; it can undergo rearrangements and cyclization to many different structures and also a precursor of the lipophilic phytyl side chain of chlorophyll and plastoquinone. Gibberellins are tetracyclic diterpenes that act as phytohormones and promote shoot elongation, flowering and seed germination [38]. Diterpenes like abietic- and levopimaric acid (**Figure 6**) are constituents of conifer oleoresin and function as a defense against herbivores and pathogens. After mono- and sesquiterpenes (turpentine) are removed from oleoresin by distillation, the solid diterpene fraction (rosin) is called colophonium. The mono and sesquiterpene containing distillate are used as oil of turpentine for the

**Figure 4.** *Iridoids.*

*Biosynthesis of Natural Products DOI: http://dx.doi.org/10.5772/intechopen.97660*

**Figure 5.** *Linear and cyclic sesquiterpenes.*

*Bioactive Compounds - Biosynthesis, Characterization and Applications*

therefore act as feeding deterrents (**Figure 4**) [26].

*2.1.3 Sesquiterpenes*

suppress translation of 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) reductase, an enzyme of the MVA pathway and induce apoptosis [24]. The 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) reductase enzyme is a promising target for anti-tumor compounds because tumor cells have elevated HMG-CoA reductase levels and many proteins involved in cell growth are phenylated. The decreased terpene biosynthesis in humans leads to suppression of HMG-CoA reductase [25]. The secoiridoids yields from the cleavage of the cyclopentane ring of the iridoid skeleton, which are monoterpene indole alkaloids and the biosynthetic building units of the Ipecac alkaloids [25]. Many iridoids have an intense bitter-taste and

In general, sesquiterpenes are less volatile than monoterpenes; they contain three isoprene units and are formed by condensation of DMAPP with two molecules IPP, the central C15 intermediate farnesyl diphosphate (FPP) can be folded into mono-, bi- or tricyclic systems [9]. Initially, it was assumed that all sesquiterpenes are produced via the cytosolic MVA pathway. Recent studies, however, revealed that certain sesquiterpenes originate from isoprene units provided by the DXP pathway [12, 13] or by both biosynthetic routes [27]. This can be explained by the transport of isoprenoid precursors from the plastids to the cytosol [28]. Abscisic acid is a sesquiterpene phytohormone that is induced by drought and promotes stomatal closure and seed dormancy. Other sesquiterpenes take part in tritrophic plant-herbivore-parasite interactions [13]. The sesquiterpenes (E)-b-farnesene and the (E)-a-bergamotene attract the parasitic wasp *Cotesia marginiventris*, in maize infested with lepidopteran larvae [29]. Maize roots release (E)-b-caryophyllene (**Figure 5**) upon an attack of larvae of the beetle *Diabrotica virgifera* to attract the parasitic nematode *Heterorhabditis megidis* [30]. Many sesquiterpenes (sesquiterpene lactones) contain a pentacyclic lactone group, these compounds occur abundantly in the family Asteraceae, because of their bitter taste sesquiterpene lactones presumably serve as feeding deterrents of herbivores [31]. Pharmacologically active sesquiterpene lactones often show anti-inflammatory effects due to inhibition of the transcription factor NF- kB that mediates immunological responses and inflammation [32]. One of the most popular medicinal plants, chamomile (*Matricaria recutita*) is a sesquiterpenes with such activities. Antimigraine action of some sesquiterpene lactones, e.g., parthenolide from feverfew (*Tanacetum parthenium*), is mediated by inhibition of platelet aggregation and serotonin secretion [9]. The reason for the cytotoxicity

**54**

**Figure 4.** *Iridoids.*

and allergenicity of sesquiterpene lactones with a, b-unsaturated lactone is the alkylation of proteins. Artemisinin is a novel promising agent against malaria. Structurally, it is a tetracyclic sesquiterpene with a six-membered lactone ring and an unusual 1, 2, 4-trioxane ring (**Figure 5**). The president of North Vietnam, to the Chinese government (Ho Chi Minh) discovered artemisinin for a cure against malaria to support his troops in the malaria-infested jungles during the American/ Vietnamese war [33]. The ether extract from *A. annua*, and artemisinin (qinghaosu) revealed the antimalarial activity and the mode of action of artemisinin are still being investigated. Most likely, it interferes with sarco-endoplasmic reticulum calcium ATPase (SERCA) of *Plasmodium falciparum*, but other mechanisms, for example, alkylation of biological macromolecules or the production of reactive oxygen species [34]. The structural feature required for antimalarial activity is the peroxide bridge. Artemether and artesunate (two semisynthetic analogs), in efficiency comparison to artemisinin were developed and are now used as firstline therapy in the treatment of malaria, in combination with other antimalarial drugs like the lumefantrine and quinine analogs mefloquine. This combination tends to prevent resistance to Plasmodium. The artemisinin and its analogs success has triggered by extraction of the sesquiterpene from the plant because *A. annua* contains only 0.01–1.5% of artemisinin [35]. Therefore, an powerful and affordable drug for the people in malaria-endemic areas are necessary, either by breeding of *A. annua* plants with elevated artemisinin levels or biotechnological production of the artemisinin precursor artemisinic acid by cloning the biosynthetic genes from *A. annua* [35] and engineering the pathway into the bacterium *Escherichia coli* or yeast [36, 37].

#### *2.1.4 Diterpenes*

Diterpenes originate from the Plasmid DXP pathway and are synthesized from DMAPP and three molecules IPP yielding the C20 metabolite geranyl geranyl diphosphate (GGPP). GGPP is a smaller terpene; it can undergo rearrangements and cyclization to many different structures and also a precursor of the lipophilic phytyl side chain of chlorophyll and plastoquinone. Gibberellins are tetracyclic diterpenes that act as phytohormones and promote shoot elongation, flowering and seed germination [38]. Diterpenes like abietic- and levopimaric acid (**Figure 6**) are constituents of conifer oleoresin and function as a defense against herbivores and pathogens. After mono- and sesquiterpenes (turpentine) are removed from oleoresin by distillation, the solid diterpene fraction (rosin) is called colophonium. The mono and sesquiterpene containing distillate are used as oil of turpentine for the

#### **Figure 6.** *Diterpenes.*

thinning of paints and varnishes [39]. Paclitaxel binds to microtubules, stabilizes them against depolymerisation and thus blocks cell proliferation [40]. Paclitaxel is used in the therapy of cancers (breast, ovarian, lung, head and neck and Kaposi's sarcoma). In the bark of *T. brevifolia* (0.01–0.02%), paclitaxel occurs only in relatively low amounts and the trees grow slowly, other sources had to be found to supply enough of the diterpene for industrial production. Paclitaxel is obtained either by semisynthesis from baccatin III and 10-deacetylbaccatin III or from tissue cultures of various Taxus species, which can be extracted from the leaves and twigs of the common yew (*T. baccata*), a tree that grows much faster than *T. brevifolia* (**Figure 6**).

## *2.1.5 Triterpenes and steroids*

Triterpenes are synthesized from two molecules of FPP that are joined by tailto-tail condensation to squalene via the MVA pathway. Various structures, mostly tetra- or pentacyclic yields from cyclization of its metabolite 2, 3-oxidosqualene

**57**

**Figure 7.**

*Sterols derived from 2, 3-oxidosqualene.*

*Biosynthesis of Natural Products*

xylem differentiation [38, 41].

*2.1.6 Saponins*

*2.1.7 Tetraterpenes*

*DOI: http://dx.doi.org/10.5772/intechopen.97660*

followed by rearrangements and methyl shifts. The precursor of plant steroids is 2, 3-Oxidosqualene (**Figure 7**). In this case, it is cyclized to the triterpene cycloartenol, which is then converted to the C-27 compound cholesterol with the loss of three methyl groups. In both triterpenes and steroids the oxygen of 2, 3-oxidosqualene is usually retained as hydroxy group at C-3. Phytosterols are lipophilic and are readily incorporated into the micelles involved in fat digestion. Esters of phytosterols are therefore used as cholesterol-lowering food additives [9]. A group of plant hormones (Brassinosteroids) is derived from campesterol. They regulate various biological processes, e.g., stem elongation, leaf expansion, seed germination, and

Monocots preferably accumulate steroidal saponins, which are abundant in the Agavaceae, Dioscoraeceae, and Yuccaceae. Triterpenoid saponins contain the lupane skeleton, tetracyclic dammarane backbone as aglycone and the pentacyclic a-amyrin (ursane), b-amyrin (oleanane). This aglycone is linked with one to three carbohydrate chains containing up to six sugar molecules or uronic acids [9, 42]. In the triterpene backbone, the first sugar chain is attached to the hydroxy group at C-3. When two or more carbohydrate chains are present, they are connected with carboxy or hydroxy groups at C-30 or C-28. Spirostanols and furostanols are two groups of steroid saponins. A tetrahydrofuran ring in furostanols is formed from the side chain of cholesterol, and the hydroxy group at C-26 is glycosylated. Upon cleavage of this sugar moiety, a second oxygen-containing heterocycle is formed, thus yielding a spirostanol (**Figure 8**). As in the case of the triterpene saponins, steroidal saponins carry a sugar chain at the C-3 hydroxy group [41, 43–47].

Tetraterpenes are synthesized from two molecules GGPP by tail-to-tail addition

and comprise only one group of compounds, the carotenoids. The tetraterpene chain is cyclized to a six-membered ring at either one or both ends. Carotenoids with hydroxy or epoxy functions are classified as xanthophylls [9]. The important physiological functions of carotenoids in plants, is that it act as accessory pigments of chlorophyll, since they are part of the light-harvesting complex. Besides, they quench triplet oxygen and singlet oxygen in case of excess light energy and thus protect the plant from photo-oxidative damage. As pigments of flowers and fruits,

#### *Biosynthesis of Natural Products DOI: http://dx.doi.org/10.5772/intechopen.97660*

followed by rearrangements and methyl shifts. The precursor of plant steroids is 2, 3-Oxidosqualene (**Figure 7**). In this case, it is cyclized to the triterpene cycloartenol, which is then converted to the C-27 compound cholesterol with the loss of three methyl groups. In both triterpenes and steroids the oxygen of 2, 3-oxidosqualene is usually retained as hydroxy group at C-3. Phytosterols are lipophilic and are readily incorporated into the micelles involved in fat digestion. Esters of phytosterols are therefore used as cholesterol-lowering food additives [9]. A group of plant hormones (Brassinosteroids) is derived from campesterol. They regulate various biological processes, e.g., stem elongation, leaf expansion, seed germination, and xylem differentiation [38, 41].
