Secondary Metabolites:- These are organic compounds produced by any lifeform, e.g. bacteria, fungi, animals, or plants, which are not directly involved in the metabolism for normal growth, development, or reproduction of the organism.
Classification:-
1. Terpenes:-
> These are the largest single class of compounds found in essential oils.
> They are responsible for the aromas, flavors, and colours.
> These are polymer of isoprene molecules. Hence also called as isoprenoids. Each isoprene molecule contains five carbon atoms with double bonds.
> Examples:- Geraniol (Monoterpene), Cafestol (Diterpene), Lycopene (Tetraterpene), Gutta-percha (Polyterpene)
2. Phenolics:-
> Most abundant secondary metabolites of plants.
> They have potent antioxidant properties.
> They are polyphenols which are ubiquitous means found in all plant organs.
> Examples:-
i. Lignin (Structural polymer)
ii. Flavonoids and Carotenoids (Attractants)
iii. Flavonoids (UV screens)
iv. Salicylic acid and Flavonoids (Signal compounds)
v. Tannins and Phytoalexins (Defense response chemicals)
vi. Coumarins
3. Alkaloids:-
> These are N - containing cyclic organic compound.
> The term ‘alkaloid’ was coined by W. Meibner, a German pharmacist, meaning ‘alkali like’. Latter it was demonstrated that the alkalinity was due to the presence of a basic nitrogen atom.
> Morphine was the first alkaloid which was isolated by F.W. Serturner in 1806 from the latex of the opium poppy.
Types of Alkaloids:-
a. Based upon type of nitrogen and the biochemical origin:-
i. True alkaloids:- They are derived from amino acids and have nitrogen in a heterocyclic ring. Eg.- Cocaine, Morphine, Quinine etc.
ii. Proto alkaloids:- They are derived from amino acids and do not have nitrogen in a heterocyclic ring. Eg.- Yohimbine, Mescaline, Hordenine etc.
iii. Pseudo alkaloids:- They are not derived from amino acids but have nitrogen in a heterocyclic ring. Eg.- Capsaicin, Caffeine, Ephedrine etc.
b. Based upon ring structure:-
i. Tropane alkaloids:- They are abundantly found in the Solanaceae family. They are derived from ornithine and acetoacetate. Eg.- Hyoscyamine, Cocaine, Atropine, Scopolamine
ii. Pyrrolizidine alkaloids:- They occur in the plants from Asteraceae and Fabaceae family. Eg.- Senecionine
iii. Piperidine alkaloids:- Presence of odor is the common feature. Eg.- Trigonelline, Coniine, Arecoline, Lobeline, Pelletierine, Nicotine, Anabasine, Piperine and Ricinine.
iv. Quinolines alkaloids:- These alkaloids are obtained exclusively from the bark of the Cinchona plant. Eg.- Cinchonine, Cinchonidine, Quinine, and Quinidine
v. Isoquinoline alkaloids:- These groups of alkaloids have huge types of medicinal properties like antiviral, antifungal, anticancer, antioxidant, antispasmodic, and an enzyme inhibitor. Eg.- Morphine, Codeine, Papaverine, Narcotine, Hydrastine, Berberine, Emetine, Cephaëline, Tubocurarine, Corydaline.
vi. Indole alkaloids:- This is the largest and most interesting alkaloid group derived from tryptophan. Polyhalogenation is a common feature of these alkaloids. Eg.- Ergometrine, Ergotamine, Physostigmine, Ajmaline, Serpentine, Reserpine, Yohimbine, Aspidospermine, Vincablastine, Strychnine, Bruceine.
vii. Steroidal alkaloids:- 1,2-Cyclopentane phenanthrene ring system is the characteristic of this type of alkaloids. Eg.- Solanine, Veratrum, Funtamine, Conessine.
viii. Imidazole alkaloids:- The imidazole ring structure is the characteristic of this type of alkaloids. Eg.- Pilocarpine.
ix. Purine alkaloids:- They contain purine ring. Purine is the nitrogenous base. Eg.- Caffeine, Theobromine, Theophylline.
x. Pyrrolidine alkaloids:- Eg.- Hygrine, Stachydrine, Ficine, Brevicolline.
Stratagies to increase production of secondary metabolites in tissue culture:-
1. Origin of plant tissue:- The genetics of the plants have a high influence on the production of secondary metabolites. For example, high fielding cultivars produce high nicotine content compared to low fielding cultivars.
2. Culture Media:- The composition of the culture medium influences both, biomass production and the synthesis of secondary metabolites. So, ideal conditions are required to be maintained to keep a balance between these two.
3. Growth Regulators:- They have a significant effect on the production of secondary metabolites, as they can either be synergistic or antagonists in their actions. For example, the addition of gibberellic acid to the Taxus cuspidate’s medium increases the taxol production.
4. Carbon source in the medium:- Sucrose is one the most widely used carbon source in the culture media. However, there are also some others, which include glucose, fructose, and galactose.
5. Temperature:- A higher temperature is reported to enhance the production of secondary metabolites.
6. pH:- A perfect pH between 5-6 is required to be maintained for the production of secondary metabolites.
7. Light intensity:- Apart from stimulating the production of secondary metabolites, light intensity also has a role in determining the amount of the production.
8. High-cell Density culture:- A high-density culture increases the production of secondary metabolites. The case is reported in Japonic cells where they have grown up to 759 \ L of cell mass to increase productivity.
9. Elicitation:- Elicitors are compounds of biological origin involved in plant microbe interaction. Elicitors are considered as mediator compounds which induce secondary metabolites formation in cells cultures. Varieties of elicitors have been used for production of secondary metabolites. Elicitation improves the efficiency of Sec. Product accumulation in plant cell culture by:
i. Minimizing up on time
ii. Avoiding change of media
iii. Induction of enzymes involved in biosynthetic pathway
iv. Inducing excretion of metabolites into the medium
Some of the biotic elicitor- induced products
10. Precursor feeding:- Under in vitro circumstances, often the precursors of metabolite die and can’t carry out the reaction. This can be overcome by supplying enough precursors to cells, which can be done by two methods:
a. Add precussors:- By adding precursors in the medium so that their availability is not limited.
b. Prevent resistant precursor analogs:- By preventing resistant precursor analogs and increasing the intracellular levels of precursors for the metabolite production. Eg.-
i. Production of shikonin increased three-fold after the addition of L-Phenylalanine to the medium in Lithospermum species.
ii. Datura sp. cell suspension cultures are supplemented with hydroquinone, in traces, the arbutin synthesis increase considerably.
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