*2.1.6 Saponins*

*Bioactive Compounds - Biosynthesis, Characterization and Applications*

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*

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

**56**

(**Figure 6**).

**Figure 6.** *Diterpenes.*

*2.1.5 Triterpenes and steroids*

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].

## *2.1.7 Tetraterpenes*

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,

**Figure 7.** *Sterols derived from 2, 3-oxidosqualene.*

carotenoids attract pollinators and seed dispersers [48]. Carotenoids (a-carotene, b-carotene) are essential for human health (**Figure 9**), b-cryptoxanthine is precursors of vitamin A. They serve as the pigment of the light receptors of the human eyes, and converted in the liver to vitamin A. To overcome vitamin A deficiency in areas with malnutrition, transgenic rice termed, golden rice "was developed that expresses high levels of carotenoid biosynthetic enzymes in the endosperm and accumulates elevated levels of carotenoids [49–51].

#### **2.2 Phenolic compounds**

Phenolic compounds (phenolic acids and polyphenols) are derivatives of the shikimic, pentose phosphate, and phenylpropanoid pathways in plants [52].

**59**

**Figure 10.**

*(blue) [57].*

*Biosynthesis of Natural Products*

**Figure 9.** β*-Carotene.*

nin include Gallic acid [56].

*2.2.1 Phenol derivatives, especially flavonoids*

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

Polyphenols are aromatic ring which contains a phenyl group and a hydroxyl functional group [53]. Plant phenolic compounds are lignin, flavonoids, carotenoids, tannins, and phytoalexins; they are responsible for antioxidant, antiaging, antiproliferation and anti-inflammatory activities. Vegetables, fruits and beverages are major sources of phenolics [54, 55]. Tannins significantly reduce the growth of many herbivores when added to their diets because they are generally toxic. Tannins can be seen in fruits like apples, blackberries, tea and red wine [56]. Tannins are mainly constituent of woody plants especially heartwood. Some derivatives of tan-

The biosynthetic pathways are derived from the shikimate pathway (**Figure 10**), which is shared by indoles, and by several alkaloids and betalains. The phenylalanine

*Schematic overview for the biosynthetic pathways of selected phenols from phenylalanine as a precursor (bold). A key enzyme, phenylalanine ammonia-lyase (PAL), is shown (red). Some example structures are depicted* 

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

**Figure 9.** β*-Carotene.*

*Bioactive Compounds - Biosynthesis, Characterization and Applications*

carotenoids attract pollinators and seed dispersers [48]. Carotenoids (a-carotene, b-carotene) are essential for human health (**Figure 9**), b-cryptoxanthine is precursors of vitamin A. They serve as the pigment of the light receptors of the human eyes, and converted in the liver to vitamin A. To overcome vitamin A deficiency in areas with malnutrition, transgenic rice termed, golden rice "was developed that expresses high levels of carotenoid biosynthetic enzymes in the endosperm and

Phenolic compounds (phenolic acids and polyphenols) are derivatives of the shikimic, pentose phosphate, and phenylpropanoid pathways in plants [52].

accumulates elevated levels of carotenoids [49–51].

**2.2 Phenolic compounds**

*Triterpene and steroid saponins.*

**58**

**Figure 8.**

Polyphenols are aromatic ring which contains a phenyl group and a hydroxyl functional group [53]. Plant phenolic compounds are lignin, flavonoids, carotenoids, tannins, and phytoalexins; they are responsible for antioxidant, antiaging, antiproliferation and anti-inflammatory activities. Vegetables, fruits and beverages are major sources of phenolics [54, 55]. Tannins significantly reduce the growth of many herbivores when added to their diets because they are generally toxic. Tannins can be seen in fruits like apples, blackberries, tea and red wine [56]. Tannins are mainly constituent of woody plants especially heartwood. Some derivatives of tannin include Gallic acid [56].

#### *2.2.1 Phenol derivatives, especially flavonoids*

The biosynthetic pathways are derived from the shikimate pathway (**Figure 10**), which is shared by indoles, and by several alkaloids and betalains. The phenylalanine

**Figure 10.**

*Schematic overview for the biosynthetic pathways of selected phenols from phenylalanine as a precursor (bold). A key enzyme, phenylalanine ammonia-lyase (PAL), is shown (red). Some example structures are depicted (blue) [57].*

is the precursor for the cinnamic acid derivatives and flavonoids, and it is converted by an enzyme, phenylalanine ammonia-lyase (PAL) to cinnamic acid. Rosmarinic acid has high antioxidative potential and also good aromatic qualities. The cinnamic acid derivatives serve as precursors for polymers (lignin), which is synthesized via cinnamaldehydes and monolignols. Much information is also available from maize and a legume, the latter also contains isoflavonoids (**Figure 11**). Other mutants in the pathway of, for example, the next enzyme encoding chalcone isomerase (which is responsible for the synthesis of naringenin), also show this phenotype, and consequently, the mutations were numbered consecutively, starting with "1." Mutations in the transcription factors that control the synthesis of flavonoids have similar phenotypes [57].

#### **2.3 Nitrogen-containing compounds**

Alkaloids are heterocyclic nitrogen compounds biosynthesized from amino acids. Alkaloids represent one of the biggest groups of natural products, with currently more than 12,000 known structures. In addition to alkaloids, benzoxazinoids, glucosinolates, and cyanogenic glucosides will be represented. Like alkaloids, these

#### **Figure 11.**

*The main pathways for flavonoid synthesis derived from different plant species. CHS: chalcone synthase; CHI: chalcone isomerase; IFS: isoflavonoid synthase; FNS: flavone synthase; F3H: flavanone-3- hydroxylase; FLS: flavonol synthase; DFR: dihdroflavonol reductase; ANS: anthocyanidin synthase; UGT: glycosyltransferase; ANR: anthocyanidin reductase [57].*

**61**

**Table 1.**

*Biosynthesis of Natural Products*

*2.3.1 Alkaloids*

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

metabolites contain nitrogen and are derived from amino acids. Morphine is an alkaloid isolated in 1805 from opium poppy *Papaver somniferum* [58, 59]. The role of alkaloids in the plant has been a subject of speculation for at least 100 years. Most alkaloids are now believed to function as a defense against especially mammals, because of the general toxicity and deterrence capacity [60]. One group of alkaloids, the pyrolizidine alkaloid illustrates how herbivores can become adapted to tolerate

Alkaloid was introduced by a German Chemist, Carl F.W Meissnerin in 1815. Alkaloids are Alkali-like and derived from the word Alkali. They are a group of naturally occurring organic compounds which are basic, contain one or more nitrogen atoms normally of Heterocyclic nature. They also possess specific physiological actions on the human and animal bodies and are abundant in higher plants (Angiosperm). Major types of alkaloids and their examples are represented in **Table 1**. Families rich in alkaloids are- Apocynaceae, Rubiaceae, Solanaceae, Papaveraceae, Berberidaceae, etc. Alkaloids are present in many parts of the plant- Aerial part (Ephedra – Ephedrine),

Entire plant (Vinca- Vincristine, Vinblastine), Leaves (Tea- Caffeine), Root

**Type Plant source Example Uses**

Pyrrolidine Leaves of *Peruvian coca* shrub

Piperidine Bark of bomegranate,

Pyramidinepyridine

Pyridinepiperidine

*Major types of alkaloids [53].*

*maculatum*

*tabacum*

Isoquinoline *Papaver somniferum* Seed

Tropine *Atropa belladonna* Atropine,

Oil of hemlock. *Conium* 

Tobacco leaf *Nicotina* 

of nuxvomica Strychnos

Indole *Claviceps purpurea* Strychnine,

(Rauwolfia- Reserpine), Bark (Cinchona- Quinine), Seed (Nuxvomica), Fruit (Black pepper- Piperine), Latex (Opium- Morphine, Codeine). Pharmacological uses include; Anagelsic, Antimalarial, Antispasmodic, Hypertension, Mental disorder, Anticancer etc. Alkaloids occur mainly in plants as Salts of organic acid (oxalic acid, citric acid, acetic acid, maleic acid, tartaric acid, fumaric, benzoic, etc). Functions in plants include; protective against insects and herbivores (bitterness and toxicity), a product of detoxification (a waste product) in a certain case, a reservoir for protein synthesis, and a source of nitrogen in case of deficiency. Many precursors are involved in various pathways, such as aromatic amino acids (tryptophan, tyrosine and phenylalanine), and also aspartate, glutamine, lysine, glycine and valine (**Figure 12**). Besides, the nonproteinogenic amino acid ornithine is an important precursor for various alkaloids.

Cocaine

Codeine, morphine

Reserpine, Psilocybin

*Anabasis aphylla* Anabasine Antimicrobial, antioxidant

Quinoline Cinchona tree Quinine Treatment of malaria

Hygrine Stimulants, Depressant

Coniine Poison (paralyzes of motor neuron)

Nicotine Respiratory stimulation

Analgesic

Antidote of poison

Treatment of cough and

Treatment of hypertension, uterine atonia, postpartum bleeding, hallucination

plant defensive substances and even use them in their defense [60].

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

metabolites contain nitrogen and are derived from amino acids. Morphine is an alkaloid isolated in 1805 from opium poppy *Papaver somniferum* [58, 59]. The role of alkaloids in the plant has been a subject of speculation for at least 100 years. Most alkaloids are now believed to function as a defense against especially mammals, because of the general toxicity and deterrence capacity [60]. One group of alkaloids, the pyrolizidine alkaloid illustrates how herbivores can become adapted to tolerate plant defensive substances and even use them in their defense [60].
