File Name: primary and secondary metabolites in plants .zip
Advances and challenges on the in vitro production of secondary metabolites from medicinal plants. The production of secondary metabolites from medicinal plants, also called Plant-Derived Medicinal Compounds PDMC , is gaining ground in the last decade. Concomitant to the increase in the knowledge about pharmacological properties of these compounds, horticultural plants are becoming the most important, sustainable and low-cost biomass source to obtain high-complex PDMCs to be used as medicaments. Biotechnological tools, including plant cell and tissue culture and plant genetic transformation, are increasingly being employed to produce high quality and rare PDMC under in vitro conditions. The proper use of these technologies requires studies in organogenesis to allow for better control of in vitro plant development and, thus, to the production of specific tissues and activation of biochemical routes that result in the biosynthesis of the target PDMCs.
Primary and secondary metabolisms are coextensive; they can occur at the same time in the same cell and draw carbon-containing intermediates from the same sources. Generally, though, as nutrients are depleted the rate of growth slows and eventually stops. The progress of metabolism is correspondingly altered and a number of special biochemical mechanisms appear or become amplified to establish what is called secondary metabolism.
The resultant secondary metabolites seem to be unnecessary for normal growth of the organism, but rather appear to be aimed at functions such as intercellular communication and defence and competition with other organisms. Secondary metabolism is a common feature of fungi.
These later stages of secondary metabolism are so varied that individual secondary metabolites can have a narrow species distribution. The majority of secondary metabolites have their origins in a small number of the intermediates in pathways dealt with above:.
Terpenes , carotenoids and steroids are widely distributed in nature and though the fungal products are not unusual in that sense, many of the end-products have chemical structures which are unique to fungal metabolism. All of these compounds are related by having five carbon atoms arranged as in the hydrocarbon isoprene. The terpenes are the simplest of the naturally occurring isoprenoid compounds and are derived by condensation of a precursor isoprene unit to form an isoprenoid chain Fig.
Strictly, terpene is the name of the hydrocarbon with the formula C 9 H 16 , but the term is used generally to refer to the ten-carbon isoprenoids which may be open-chain that is, acyclic or cyclic compounds. Synthesis of acyclic terpenes starts when dimethylallylpyrophosphate condenses with a molecule of isopentenyl-pyrophosphate to form the 9-carbon monoterpene geranylpyrophosphate Fig.
As many as 24 isoprene units may condense in this way, though secondary metabolites usually have between two and five. Not all such molecules are secondary metabolites; the ubiquinones of the respiratory chain are polyprenoid quinones.
Sesquiterpenes are the largest group of terpenes isolated from fungi. Most of the fungal sesquiterpenes are based on carbon skeletons which can be derived by cyclisation of farnesylpyrophosphate as shown in the schemes in Fig. Diterpenes are derived by cyclisation of geranylgeranyl pyrophosphate Fig. The major significance of the triterpenes C 30 is that the acyclic triterpene squalene is the precursor of sterols. Squalene is formed by a head-to-tail condensation of two sesquiterpene farnesylpyrophosphate units; it is so called because it is found in high concentration in shark liver oil Squalus is a genus of dogfish sharks , but in fungi as well as in other organisms sterols and cyclic triterpenes are derived by cyclisation of squalene oxide Fig.
Cholesterol is the quantitatively predominant sterol in animals where it serves to control membrane fluidity. Ergosterol , so named because it was first isolated from the ergot fungus Claviceps purpurea , probably fulfils a similar role in fungi, influencing permeability characteristics of the membrane and as it is unique to fungi; ergosterol synthesis is a potential target for antifungal agents.
However, other sterols, even cholesterol, occur commonly and a very wide range of sterols has been detected in fungi which have scope for characterising strains. Lipid, sterol and phospholipid contents differ between yeast and mycelial forms of Candida albicans and Mucor lusitanicus , so there is a morphogenetic connection, too.
The sequential order of steps in sterol synthesis is well established. In much the same manner as squalene is formed by head-to-tail condensation of sesquiterpenes, the C40 carotenoid pigments are formed by head-to-tail condensation of two diterpenes. The carotenes and some keto-derivatives are widely synthesised and carotenoids are good taxonomic markers for some fungi, though not all fungi produce them; the biosynthetic pathway is well established.
Continued condensation of isoprene units can produce terpenes with very long carbon chains. Natural rubber is a highly polymerised isoprene compound, containing of the order of isoprene units. The latex of the major commercial source, the rubber palm Hevea brasiliensis , is an aqueous colloidal suspension of particles of this hydrocarbon. The latter is a modified fatty acid 6-oxo-octadecanoic acid, also known as 6-ketostearic acid. More secondary metabolites are synthesised through the polyketide pathway in fungi than by any other pathway.
Polyketides are characteristically found as secondary metabolites among Ascomycota; they are rarely encountered in Basidiomycota and are produced by only a few organisms other than fungi. As might be expected from the chemical nature of the repeating unit, synthesis of polyketides involves transfer of acetyl groups which are ultimately derived from acetyl-CoA. In fact the synthesis of polyketides has a lot in common with fatty acid biosynthesis.
Synthesis of fatty acids is carried out by a complex of enzymes called the fatty acid synthetase system. For chain lengthening, the butyryl residue is condensed with another malonyl group followed by repetition of the reduction, dehydration, reduction cycle. This brief description of fatty acid synthesis can be echoed by a description of polyketide synthesis which is initiated by condensation of an acetyl unit with malonyl units, requires the respective CoA derivatives, and seems to occur on the enzymes involved.
In both pathways, free precursors are not found in the cytoplasm. The number of these can be used to designate the chain formed as, for example:. A great variety of cyclisations thus become possible and this, in part, accounts for the wide range of polyketides which are encountered; the number is further increased by subsequent chemical modifications.
The primary role of the shikimate-chorismate pathway is the synthesis of the aromatic amino acids phenylalanine, tyrosine and tryptophan Fig. Plants synthesise a particularly large variety of compounds by this route, though it is not so widely used in fungi, where the polyketide pathway is used more to make aromatic ring compounds. Nevertheless, numerous shikimate-chorismate derivatives that commonly occur in plants have been isolated from fungi, such as gallic acid, pyrogallol, and methyl p -methoxycinnamate Fig.
All of these are essentially tryptophan derivatives. Non-aromatic amino acids may also be modified: the muscarines and muscaridines Fig. Volatile amines formed by decarboxylation of neutral amino acids form part of the distinctive odours of some fungi; the fungus causing stinking smut of wheat Tilletia tritici produces large quantities of trimethylamine, and the aroma of Camembert cheese depends on the formation of N -dimethyl methioninol by Penicillium camemberti.
A variety of fungal secondary metabolites are derived from peptides ; two or more amino acids linked through a peptide bond. Among those derived from dipeptides are the penicillins and cephalosporins. These could rate as the most important compounds ever to be isolated from fungi as they gave rise to a new era in medicine and a new branch of biotechnology.
The basic structure of both antibiotics is derived from cysteine and valine Fig. Instead, amino-acid activating domains of peptide synthetases determine the number and order of the amino acid constituents of the peptide secondary metabolites.
In vitro reconstruction of the gene sequences of these multifunctional enzymes produces hybrid genes that encode peptide synthetases with altered amino acid specificities able to synthesise peptides with modified amino acid sequences. Because of the resemblance between Amanita phalloides and edible field mushrooms, these toxins are involved in the majority of cases of mushroom poisoning. Many fungi produce siderophores for acquiring iron which, though an essential nutrient, is not readily available in aquatic or terrestrial environments or in animal hosts.
These iron-binding compounds also originate as peptides formed from modified amino acids. Chemical modification of fatty acids produces a variety of secondary metabolites, most particularly the polyacetylenes, many of which have been obtained from basidiomycetes.
These compounds have straight carbon chains varying between C 6 and C 18 , though C 9 and C 10 are most common in fungi. They are conjugated acetylenes i. Polyacetylenes are derived from fatty acids by a series of dehydrogenation reactions Negri, Also formed from fatty acids are cyclopentanes like brefeldin, which has been extracted from Penicillium , Nectria and Curvularia species Fig.
As must be clear from the above discussion, fungi are prolific producers of secondary metabolites and these show a variety of biological activities. Advances in genome sequencing have shown that fungal genomes harbour far more gene clusters able to synthesise secondary metabolites than are expressed under normal laboratory conditions.
Filamentous fungi have already played such an important role in the history of drug discovery and development; examples being antibiotics such as penicillin, immunosuppressants such as cyclosporine, antifungals such as griseofulvin and the echinocandins, and antihypercholesterolemic drugs such as lovastatin.
Robson and Anthony P. Table of Contents Chapter 9: Continuing the diversity theme: cell and tissue differentiation Chapter Fungi in ecosystems Contributions of fungi to ecosystems Breakdown of polysaccharide: cellulose Breakdown of polysaccharide: hemicellulose Breakdown of polysaccharide: pectins Breakdown of polysaccharide: chitin Breakdown of polysaccharide: starch and glycogen Lignin degradation Digestion of protein Lipases and esterases Phosphatases and sulfatases The flow of nutrients: transport and translocation Primary intermediary metabolism Secondary metabolism Chapter 10 References and further reading Buy a PDF of Chapter 10 Chapter Exploiting fungi for food.
The majority of secondary metabolites have their origins in a small number of the intermediates in pathways dealt with above: acetyl CoA is, perhaps, the most important, being a precursor for terpenes, steroids, fatty acids and polyketides; phosphoenolpyruvate and erythrose 4-phosphate initiate synthesis of aromatic secondary metabolites through the shikimic acid pathway by which aromatic amino acids are synthesised; further secondary metabolites are derived from other non-aromatic amino acids Fig.
Metabolic route map summarising the relationships between primary metabolism and the major pathways for synthesis of secondary metabolites. Compare with Figs 9 and Modified from Moore, Derivation of terpenes from isoprene units. The basic isoprenoid repeat structure is shown at the top; the lower panel shows the synthesis of mevalonate from two molecules of acetyl-coenzyme A. Growth of the terpene chain is continued in Fig. Synthesis of acyclic that is, straight chain terpenes by successive condensations.
Dimethylallyl-pyrophosphate condenses with isopentenyl-pyrophosphate to form geranylpyrophosphate. Squalene is the precursor of sterols see Fig.
Squalene is formed by head-to-tail condensation of two sesquiterpene farnesylpyrophosphate units. Check the numbering on the carbon atoms to understand how these molecules are assembled.
Sesquiterpenes: possible cyclisations of farnesylpyrophosphate to produce the trichothecane nucleus. Cyclisation of the diterpene geranyl-geranyl pyrophosphate producing gibbane and kaurane structures. Gibberellic acid is a plant growth hormone that was discovered only after it had been isolated as a secondary metabolite of a pathogenic fungus that caused growth abnormalities in its plant host.
Sterols and cyclic triterpenes are derived by cyclisation of squalene oxide. This is illustrated in the upper panel and structural formulae of some sterols are shown below.
More details about the clinical importance of differences between cholesterol and ergosterol can be found in the first section of Chapter Fungal carotenoid pigments. Polyketide chains can fold in a variety of ways and internal aldol condensations form closed aromatic rings. To illustrate this, the alternative cyclisations of a tetraketide are shown at the top, together with the potential products orsellinic acid and acetylphloroglucinol.
The bottom section shows the structures of a small selection of polyketides discussed in the text. The shikimate-chorismate pathway produces a variety of aromatic compounds as well as the amino acids phenylalanine, tyrosine and tryptophan. Gallic acid, pyrogallol, and methyl p -methoxycinnamate are relatively simple compounds produced by many plants as well as fungi.
Gyrocyanin is a product of Gyroporus cyanescens which oxidises to a blue pigment in injured fruit bodies, and hispidin is the precursor of a polymer which seems to be responsible for toughening the fruit bodies of Polyporus hispidus.
Non-aromatic amino acids may also be modified and accumulated as secondary metabolites. Agaritine may account for up to 0. N-dimethylmethioninol is a volatile amine formed by decarboxylation of methionine by Penicillium camemberti. It is responsible for the aroma of Camembert cheese. The penicillin antibiotics are representative of secondary metabolites which are derivatives of peptides.
This figure shows the structural formulae of two penicillins. The aminoadipoyl, cysteine and valine residues of this compound are identified by colour coding and the double-headed arrows show the bonds which have to be made to create the penicillin nucleus. See also Figs 20 and 21 in Chapter 17 for more details.
Chemical modification of fatty acids produces a variety of secondary metabolites including polyacetylenes and cyclopentanes.
Maquis species play a central role in the maintenance of coastal ecosystems thanks to anatomical, physiological and biochemical features evolved to cope with severe stress conditions. Because the seasonal and daily dynamics of physiological and biochemical traits of maquis species are not fully addressed, we performed a field study on three coexisting Mediterranean shrubs Pistacia lentiscus L. These species differed in their physiological and biochemical responses especially on a seasonal level. Temperature resulted the most important climatic factor controlling the physiological and biochemical status of these coexisting shrubs and, thus, in determining plant performances in this Mediterranean coastal habitat. Evergreen sclerophylls face drought conditions with a high specific leaf area, thick cuticle and deep root system Karavatas and Manetas,
synthesis. Primary Vs Secondary Metabolites: Primary. metabolites are found in all plants and execute. vital metabolic responsibilities, by participating in.
Raja and Ramasamy Vijayakumar. Reviewed: June 26th Published: September 5th Secondary Metabolites - Sources and Applications.
Primary and secondary metabolites are often used in industrial microbiology for the production of food, amino acids, and antibiotics. Bacterial metabolism can be classified into three major categories: the kind of energy used for growth, the carbon source, and the electron donors used for growth. Pathogenic bacteria are capable of exhibiting various types of metabolism. Metabolites, the intermediates and products of metabolism, are typically characterized by small molecules with various functions. Metabolites can be categorized into both primary and secondary metabolites.
Secondary metabolites are chemicals produced by plants for which no role has yet been found in growth, photosynthesis, reproduction, or other "primary" functions. These chemicals are extremely diverse; many thousands have been identified in several major classes. Each plant family, genus, and species produces a characteristic mix of these chemicals, and they can sometimes be used as taxonomic characters in classifying plants. Humans use some of these compounds as medicines, flavorings, or recreational drugs. Secondary metabolites can be classified on the basis of chemical structure for example, having rings, containing a sugar , composition containing nitrogen or not , their solubility in various solvents, or the pathway by which they are synthesized e.
In contrast to the primary metabolism, responsible for essential synthesis mechanisms and mass balance in plants, the secondary metabolism is not of particular importance for each cell but for the plant organism as its whole. Most of these metabolites show antioxidant properties and are beneficial for human health. In order to affect accumulation of those metabolites, light is an essential factor. It is possible to select various combinations of light intensity and light quality to address corresponding photoreceptors and synthesis.
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