Secondary metabolites are the most important physiologically active compounds in the plant world. Their number, studied by science, is increasing every year. Currently, about 15% of all plant species have been studied for the presence of these substances. They also have high biological activity against animals and humans, which determines their potential as pharmaceuticals.
What are secondary metabolites?
A distinctive feature of all living organisms is that they metabolize - metabolism. It is a combination of chemical reactions that produce primary and secondary metabolites.
The difference between them is that the former are characteristic of all creatures (synthesis of proteins, aminocarboxylic and nucleic acids, carbohydrates, purines, vitamins), and the latter are characteristic of certain types of organisms and do not participate in the growth and reproduction process. However, they perform certain functions.
In the animal world, secondary compounds are rarely produced, more often they enter the body along with plant foods. These substances are synthesized mainly in plants, fungi, sponges and unicellular bacteria.
Signs and Features
In biochemistry, the following main signs of secondary metabolites of plants are distinguished:
high biological activity;
small molecular weight (2-3 kDa);
production of a small amount of starting materials (5-6 amino acids for 7 alkaloids);
synthesis is inherent in certain plant species;
education in the later stages of the development of a living organism.
Any of these features is optional. Thus, secondary phenolic metabolites are produced in all plant species, and natural rubber has a high molecular weight. The production of secondary metabolites in plants occurs only on the basis of proteins, lipids and carbohydrates under the influence of various enzymes. These compounds do not have their own synthesis route.
They are also characterized by the following features:
the presence in different parts of the plant;
uneven distribution in tissues;
localization in specific cell compartments to neutralize the biological activity of secondary metabolites;
the presence of a basic structure (most often hydroxyl, methyl, methoxy groups play its role), on the basis of which other variants of compounds are formed;
different types of structural changes;
the ability to transition to an inactive, "spare" form;
lack of direct participation in the metabolism.
Secondary metabolism is often considered as the ability of a living organism to interact with its own enzymes and genetic material. The main process, as a result of which secondary compounds are formed, is dissimilation (decomposition of the products of primary synthesis). In this case, a certain amount of energy is released, which is involved in the production of secondary compounds.
Functions
Initially, these substances were considered unnecessary waste products of living organisms. It has now been established that they play a role in metabolic processes:
phenols - participation in photosynthesis, respiration, electron transfer, phytohormone production, root system development; attraction of pollinating insects, antimicrobial effect; coloring of individual parts of the plant;
tannins - development of resistance to fungal diseases;
carotenoids - participation in photosynthesis, protection against photooxidation;
alkaloids - regulation of growth;
isoprenoids - protection against insects, bacteria, animals;
sterols - regulation of cell membrane permeability.
The main function of secondary compounds in plants is environmental: protection from pests, pathogenic microorganisms, adaptation to external conditions. Since environmental factors differ significantly for different species of flora, the spectrum of these compounds is almost unlimited.
Classifications
There are several fundamentally different classifications of secondary metabolites:
Trivial. Substances are divided into groups in accordance with their specific properties (saponins form a foam, bitterness has an appropriate taste and so on).
Chemical. Based on the characteristics of the chemical structure of the compounds. It is currently the most common. The disadvantage of this classification is that the substances of one group may differ in the production method and properties.
Biochemical At the head of this type of systematization is the method of biosynthesis. It is the most scientifically sound, but due to the poor knowledge of plant biochemistry, the use of this classification is limited.
Functional. Based on certain functions of substances in a living organism. In one group, there may be secondary metabolites having a different chemical structure.
The complexity of the classification lies in the fact that each group of secondary metabolites is closely related to the rest. So, bitterness (a class of terpenes) are glycosides, and carotenoids (derivatives of tetraterpenes) are vitamins.
Main groups
The following types of substances are referred to secondary metabolites of plant cells:
alkaloids (pyridine, imidazole, purine, betalain, glycoalkaloids, protoalkaloids and others);
anthracene derivatives (derivatives of chrysacin, anthron, alizarin and other compounds);
phytosteroids (vitanolides);
glycosides (monosides, biosides and oligosides, cyanogenic glycosides and thioglycosides);
isoprenoids (terpenes and their derivatives - terpenoids and steroids);
phenolic compounds and others.
Many of these substances have unique properties. So, curare alkaloids are the strongest poison, and some groups of glycosides have a pronounced therapeutic effect and are used for the manufacture of drugs used in the treatment of heart failure.
Application
Secondary metabolites have an active effect on the organs and systems of humans and animals; therefore, they are widely used in pharmacology and veterinary medicine, and are used as enhancers of taste and aroma in food products. Some plants that accumulate these substances in significant quantities are used as raw materials in the production of technical materials.
Abroad, in countries with a developed chemical industry, about a quarter of all compounds used in pharmacy are of plant origin. The valuable therapeutic effect of secondary metabolites is associated with their properties such as:
Since these compounds are still poorly understood, their further study may lead to the creation of fundamentally new pharmaceutical preparations.