Secondary metabolism as a defense of plants
Introduction
Metabolism consists of chemical reactions that take place in an organism. Most of the carbon, nitrogen and energy ends in molecules common to all cells, necessary for their operation and that of organisms. (Ávalos García & Pérez-Urria Carri, 2009). Primary metabolites are amino acids, nucleotides, sugars and lipids, present in all plants. Secondary metabolism is constituted by processes that lead to the formation of peculiar compounds of certain taxonomic groups and their products are called secondary metabolites, which biosyntically derive from certain primary compounds, both kinds of metabolism are interconnected.
The biosynthesis of the secondary metabolites is usually restricted to specific phases of development, both of the organism and of the specialized cells, and to periods of stress, for example, by the deficiency of nutrients, by environmental factors or by the attack of microorganisms. The expression of secondary metabolism is based on a differentiation process since it depends on phase and its corresponding enzymes. Proteins formed as a result of differentiation processes can be classified, according to their biological significance and their function in the producing cell secondary metabolism. Secondary metabolism can be defined as biosynthesis, transformation and degradation of endogenous compounds through specialization proteins. It can be important for the producing body as a whole.
Many secondary metabolites are involved in ecological relationships, that is, of the producing plant with the other organisms of its natural environment. Some examples are the pigments of the flowers that attract pollinating insects, and compounds that inhibit the growth of other plant organisms (allelopathic substances), or that protect the infections -producing plant (phytoalexins) or predators (nutritious dysjasiosor food). Secondary metabolites constitute chemical signals through which plants are related to their surroundings.
Other compounds formed in the secondary metabolism routes have physiological importance or serve as signs that integrate cell differentiation and metabolism in different parts of the multicellular plant organism. The psychimic acid, by the metabolic route that bears its name, gives rise to many aromatic compounds, including aromatic amino acids, cinamic acids and certain polyphenols. Acetate is the precursor of fatty acids and polycetid.
The terpenes of the plastics are synthesized from other primary metabolites such as pyruvate and glyceraldehyde-3-P. Amino acids are precursors of alkaloids and peptide antibiotics. There are secondary metabolites in whose formation several metabolic routes are involved. These compounds of mixed synthesis include polyphenols such as stylbenos or flavonoids, formed by the path of psychimic acid and acetate-mallonato. Certain variations on primary metabolism routes can lead to the formation of secondary metabolites.
Developing
Vegetable secondary metabolism is an important source of valuable chemicals, for example, medicinal plants owe their healing abilities to these compounds, as well as drugs and other chemical compounds, such as essences and dyes. In addition, some secondary metabolites of food, such as polyphenols and carotenoids, have recently recognized therapeutic and preventive properties for certain diseases, and therefore for these food. (Azcón-Bieto & Talón, 2013) The main routes of secondary metabolites biosynthesis derive from the primary metabolism of carbon. Secondary metabolites are grouped into four main classes:
- Terpenes: hormones, pigments or essential oils.
- Phenolic compounds: coumarins, flavonoids, lignin and tannins.
- Glycosides: saponins, cardiac glycosides, cyanogenic glycosides and glucosinolates.
- Alcaloids
Terpenes
The terpenes, or isoprenoids, constitute the most numerous group of secondary metabolites (more than 40.000 different molecules). The biosynthetic route of these compounds gives rise to both primary and secondary metabolites of great importance for the growth and survival of plants. The terpenes are classified by the number of isoprene units (C5) containing: monoterpenes (2 units);Sesquititerpenos (3 units);diterpenes (4 units);Triterpenes (6 units);tetraterpenes (8 units) and polyterpenos (more than 8 units are synthesized from the compound C5 ispenil diphosphate by two routes: that of the mevalonic acid, active in the cytosol, in which three molecules of acetyl-Coa are condensed to form mevalonic acid thatreacts to form isopennil diphosphate (IPP), or the methyleritritol phosphate (MEP) route that works in chloroplasts and also generates IPP.
The biosynthesis of the terpenes in the plant cells is compartmentalized, that is, the formation of IPPP and the subsequent steps run differently depending on the biosynthesis of cytoplasmic or chloroplastic terpenes. Many terpenoids are commercially interesting for their use as aromas and fragrances in food and cosmetics, or for their importance in the quality of agricultural products. Other terpenoid compounds have medicinal importance due to their anticinogenic, antiulcery, antimalarial, antimicrobial, etc. Many plants (lemon, mint, eucalyptus or thyme) produce mixtures of alcohols, aldehydes, ketones and terpenoids called essential oils, responsible for the smells and flavors characteristic of these plants, some of which act as insect or insecticide repellent.
There are steroids and sterols derived from the Escualeno, a 30 C linear chain molecule from which all cyclic triterpenes derive. Steroids that contain an alcohol group, and is the case of almost all vegetable steroids, are called sterols. The most abundant in plants are stigmasterol and sitosterol, which only differs from stigmasterol in the absence of the double bond between C 22 and C 23. They are part of the cell membranes and determine their viscosity and stability. Among the triterpenes are some glycoside steroids. These steroid glycosides, with important functions in medicine and in the industry (Cardenolipids and saponins).
Phenolic compounds
Phenolic compounds are secondary products that contain a phenol group, an aromatic ring with a hydroxyl group. It is a group that comprises from simple molecules such as phenolic acids to complex polymers such as tannins and lignin. Flavonoid pigments are also found. Many of these products are involved in plant-Herbivorous interactions. There are two basic routes involved in the biosynthesis of phenolic compounds, the path of psychimic acid and the malonic acid route.
The Malonic Acid Route is an important source of phenols in fungi and bacteria, but is little used in upper plants. The path of psychimic acid is responsible for the biosynthesis of most plant phenolic compounds. From erythrous-4-P and phosphoenolpirúvic acid a sequence of reactions that leads to the synthemical acid synthesis and, derived from it, aromatic amino acids (phenylalanine, tryptophan and tyrosine) begins). Most phenolic compounds derive from phenylalanine. This route is present in plants, fungi and bacteria, but not in animals. Phenylalanine and tryptophan are among the essential amino acids for animals that are incorporated into the diet. The enzyme phenylalanine ammonium Liasa (PAL) catalyzes the formation of cinamic acid by eliminating a ammonium ammonium molecule. This enzyme is located at a branch point between primary and secondary metabolism, so the reaction that catalyzes is an important regulatory stage in the formation of many phenolic compounds.
Among the phenolic compounds are also the derivatives of benzoic acid that have a skeleton formed by phenylpropanoids that have lost a two -carbon fragment of the side chain. Examples of these derivatives are vanillin and salicylic acid, which acts as a plant growth regulator, involved in the resistance of the plant in front of pathogens. Lignin is a highly branched polymer polymer. After cellulose, it is the most abundant organic substance in plants. There are also flavonoids (15 carbons ordered in two aromatic rings united by a three -carbon bridge). They are classified according to the degree of oxidation of the three carbon bridge. The main anthocyanins (pigments), flavones, flavonoles and isoflavones. Among its functions is defense and pigmentation.
Tannins are polymeric phenolic compounds that bind to proteins denaturing them. There are two types, condensate tannins and hydrolyzable tannins. Condensate tannins are polymers of flavonoid units linked by C-C bonds, which cannot be hydrolyzed but oxidized by a strong acid to perform anthocyanidines. Hydrolyzable tannins are heterogeneous polymers that contain phenolic acids, especially gallic acid and simple sugars;They are smaller than condensate and hydrolyzed more easily. They are generally toxins due to their ability to join proteins. They also act as food repellent of many animals that avoid, in the case of mammals, plants or parts of plants that contain high concentrations.
Glycosides
Glycosides are plant metabolites of great importance, it is formed when a sugar molecule condenses with another containing a hydroxyl group. There are three groups of glycosides of particular interest: saponins, cardiac glycosides and cyanogenic glycosides. Saponins are found as steroid glycosides, alkaloid steroid glycosides or triterpean glycosides. They are triterpenoides or steroids that contain one or more sugar molecules in their structure. They can be presented without sugar (agliconas) in which case they are called toadogenins. Cardiac glycosides or are similar to saponins but their structure contains a lactone. Perhaps the best known is digitoxin, or its analogue digoxin, isolated from digitalis glitter and used as a medicine in the treatment of congestive heart failure. Cyanogenic glycosides are nitrogen compounds, which are not toxic by themselves but degrade when the plant is crushed releasing toxic volatile substances such as hydrogen cyanide.
Alcaloids
Alcaloids are a great family of more than 15.000 secondary metabolites that have three characteristics in common: they are soluble in water, they contain at least one nitrogen atom in the molecule, and exhibit biological activity. The majority are heterocyclic although some are nitrogen compounds aliphatic (non -cyclic) are found in approximately 20% of vascular plants, most herbaceous dicotyledons. In humans, alkaloids generate physiological and psychological responses most of them consequence of their interaction with neurotransmitters. At high doses, almost all alkaloids are very toxic. However, at low doses they have a high therapeutic value such as muscle relaxants, tranquilizers, antitusive or analgesics. They are normally synthesized from lysine, tyrosine and tryptophan (Ávalos García & Pérez-Urria Carri, 2009).
Conclusions
Secondary metabolites are of great importance for the development and survival of the plant, since it provides it with defense mechanisms against the attack of bacteria, viruses and fungi. Some compounds synthesized during secondary metabolism are essential for the life and development of plants, such as lignin (structural) or sterols (being part of the membranes). His study is very important to understand the response of these organisms in various stress situations.
Since primary and secondary metabolites are synthesized from the same compounds, both metabolisms are united, so it is difficultYou already have of the plants and their mechanisms of action. The diversity of metabolites is very wide, for hundreds of years humans have taken advantage of them to manufacture useful products or substances, many times with high economic, cultural and social value, which have delimited intellectual development and today they are very important for developmentof new technologies, especially now, that it is necessary to seek sustainable and natural origin alternatives to stop the increase in toxic pollutants.
Bibliography
- Ávalos García, to., & Pérez-Uria Carri, and. (2009). Secondary metabolism of plants. (D. d. I, ed.)
- Reduce (biology), 2 (3). Retrieved in 2019
- Azcón-Bieto, J., & Heel, M. (2013). Plant physiology. Madrid: McGraw-Hill. Retrieved in 2019
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