Alkaloids: Structure, Biosynthesis, Classification and Uses

The alkaloids they are molecules that contain atoms of nitrogen and carbon in their molecular structure, which generally forms complex rings. The term alkaloid, which was first proposed by the pharmacist W. Meissner in 1819, means"similar to alkali."

The word alkali refers to the ability of a molecule to absorb hydrogen ions (protons) from an acid. The alkaloids are found as individual molecules, so they are small and can absorb hydrogen ions, turning them into a base.

Alkaloid biosynthesis

Some common bases include milk, calcium carbonate in antacids or ammonia in cleaning products. Alkaloids are produced by some living beings, particularly plants. However, the role of these molecules in vegetables is not clear.

Regardless of their role in plants, many alkaloids have uses in medicine for humans. Analgesics derived from the opium poppy plant, such as morphine, have existed since 1805. Another example is antimalarial quinine, which has been used by the Amazon tribes for more than 400 years.

Papaver somniferum, or poppy plant

Index

  • 1 Structure
  • 2 Biosynthesis
    • 2.1 Biosynthesis of tropic and nicotinic alkaloids
    • 2.2 Biosynthesis of benzylisoquinoline alkaloids
    • 2.3 Biosynthesis of terpene indolic alkaloids
  • 3 Classification
    • 3.1 According to its biosynthetic origin
    • 3.2 According to its biogenetic predecessor
    • 3.3 According to its chemical structure or basic nucleus
  • 4 Applications
    • 4.1 The alkaloids can be used as medicines
    • 4.2 The alkaloids can be used as narcotics
    • 4.3 The alkaloids can be used as pesticides and repellents
    • 4.4 The alkaloids can be used in scientific research
  • 5 References

Structure

The chemical structures of alkaloids are extremely variable. Generally, an alkaloid contains at least one nitrogen atom in an amine type structure; that is, an ammonia derivative replacing hydrogen atoms with hydrogen-carbon groups called hydrocarbons.

This or another nitrogen atom can be active as a base in acid-base reactions. The name alkaloid was originally applied to these substances because, like inorganic alkalis, they react with acids to form salts.

Most alkaloids have one or more of their nitrogen atoms as part of a ring of atoms, often called a cyclic system. The names of alkaloids usually end in the suffix"-ina", a reference to their chemical classification as amines.

Biosynthesis

The biosynthesis of alkaloids in plants involves many metabolic steps, catalyzed by enzymes belonging to a wide range of protein families; for this reason, the routes of alkaloid biosynthesis is considerably complex.

However, it is possible to comment some generalities. There are some main branches in the synthesis of alkaloids that include:

Biosynthesis of tropic and nicotinic alkaloids

In this group of alkaloids the biosynthesis is carried out from the compounds L-Arginine and Ornithine. They undergo a decarboxylation process mediated by their respective enzymes: arginine decarboxylase and ornithine decarboxylase.

The product of these reactions are putrecine molecules. After other steps, which include the transfer of methyl groups, nicotinic derivatives (such as nicotine) and tropics (such as atropine and scopolamine) are produced.

Biosynthesis of benzylisoquinoline alkaloids

The synthesis of benzylisoquinoline alkaloids starts from L-tyrosine molecules, which are decarboxylated by the enzyme tyrosine decarboxylase to give rise to tyramine molecules.

The enzyme norcoclaurin synthase uses the tyramine produced in the previous step and the L-DOPA to form norcoclaurin molecules; they undergo another series of complex reactions to give rise to the alkaloids berberine, morphine and codeine.

Biosynthesis of terpene indolic alkaloids

This group of alkaloids is synthesized from two routes: one that starts from L-tryptophan and another from geraniol. The products of these routes are tryptamine and secolaganin, these molecules are the substrate of the enzyme estrectosidina synthase, which catalyzes the synthesis of strictosidine.

From the estrectosidina the different terpénicos indolic alkaloids are produced, like the ajmalicina, the catarantina, the serpentina and the vinblastina; the latter has been used in the treatment of Hodgkin's disease.

In the fields of structural biochemistry, molecular and cellular biology, and biotechnological applications, the characterization of new biosynthetic alkaloid enzymes has been the focus of research in recent years.

Classification

Due to their diversity and structural complexity, alkaloids can be classified in different ways:

According to its biosynthetic origin

According to their biosynthetic origin, alkaloids are classified into three major groups:

True alkaloids

They are those that are derived from amino acids and have the nitrogen atom as part of the heterocyclic ring. For example: hygrin, cocaine and physostigmine.

Protoalkaloids

They are also derived from amino acids, but nitrogen does not form part of the heterocyclic ring. For example: ephedrine and colchicine.

Pseudoalkaloids

They are the alkaloids that do not derive from amino acids and nitrogen is part of the heterocyclic structure. For example: aconitine (terpene alkaloid) and solanidine (steroidal alkaloid).

According to its biogenetic predecessor

In this classification the alkaloids are grouped depending on the molecule from which their synthesis starts. Thus, alkaloids derived from:

- L-Phenylalanine.

- L-Tyrosine.

- L-Tryptophan.

- L-Ornithine.

- L-Lysine.

- L-Histidine.

- Nicotinic acid.

- Anthranilic acid.

- Pyrrhic bases.

- Terpene metabolism.

According to its chemical structure or basic nucleus

- Pyrrolidine.

- Pyridine-pyrrolidine.

- Isoquinoline.

- Imidazole.

- Piperidine.

- Pyridine-Piperidine.

- Quinoline.

- Purina.

- Tropane.

- Indole .

Applications

The alkaloids have multiple uses and applications, both in nature and in society. In medicine, the use of alkaloids is based on the physiological effects they cause on the body, which is a measure of the compound's toxicity.

Being organic molecules produced by living beings , alkaloids have the structural capacity to interact with biological systems and directly affect the physiology of an organism. This property may seem dangerous, but the use of alkaloids in a controlled manner is very useful.

Despite its toxicity, some alkaloids are useful when used in the correct doses. An excess of dose could cause damage and be considered poisonous to the organism.

The alkaloids are obtained mainly from shrubs and herbs. They can be found in different parts of the plant, such as leaves, stem, roots, etc.

The alkaloids can be used as medicines

Some alkaloids have significant pharmacological activity. These physiological effects make them valuable as medicines to cure some serious disorders.

For example: the vincristine of Vinca roseus is used as an anticancer drug, and ephedrine Ephedra distachya It is used to regulate blood pressure.

Other examples include curarina, which is found in curare and is a powerful muscle relaxant; atropine, which is used to dilate the pupils; codeine, which is used as a cough suppressant; and the ergot alkaloids, which are used to relieve migraine, among many others.

The alkaloids can be used as narcotics

Many psychotropic substances, which act on the central nervous system, are alkaloids. For example, opium morphine ( Papaver somniferum ) is considered a drug and an analgesic. Lysergic acid diethylamide, better known as LSD, is an alkaloid and a psychedelic drug.

These narcotics have been used since antiquity as instruments for mental excitement and euphoria, although they are considered harmful according to modern medicine.

The alkaloids can be used as pesticides and repellents

Most pesticides and natural repellents are derived from plants, where they exert their function as part of the plant's own defense system against insects, fungi or bacteria that affect them. These compounds are generally alkaloids.

As mentioned above, these alkaloids are toxic by nature, although this property depends to a large extent on the concentration.

For example, pyrethrin is used as an insect repellent, at a concentration that is deadly to mosquitoes but not to humans.

The alkaloids can be used in scientific research

Due to its specific effects on the body, alkaloids are widely used in scientific studies. For example, the alkaloid atropine can cause dilation of the pupil.

Then, to evaluate whether a new substance has similar effects or opposite effects, it is compared with the effect of atropine.

Some alkaloids are studied with great interest due to their antitumor properties, such as vinblastine and vincristine.

Other important alkaloids in scientific research include quinine, codeine, nicotine, morphine, scopolamine and reserpine, among others.

References

  1. Cordell, G. (2003). The Alkaloids: Chemistry and Biology, Volume 60 (1st ed.). Elsevier
  2. De Luca, V., & St Pierre, B. (2000). The cell and developmental biology of alkaloid biosynthesis. Trends in Plant Science , 5 (4), 168-173.
  3. Facchini, P. J. P. J. (2001). Alkaloid biosynthesis in plants: biochemistry, cell biology, molecular regulation, and metabolic engineering applications. Annual Review of Plant Biology , 52 (1), 29-66.
  4. Kutchan, T. M. (1995). Alkaloid Biosynthesis [mdash] The Basis for Metabolic Engineering of Medicinal Plants. The Plant Cell Online , 7 (7), 1059-1070.
  5. Pelletier, S. (1983). Alkaloids: Chemical and Biological Perspectives, Volume 11 (1 st ). Wiley
  6. Roberts, M. (2013). Alkaloids: Biochemistry, Ecology, and Medicinal Applications. Springer Science & Business Media.
  7. Wenkert, E. (1959). Alkaloid Biosynthesis. EXPERIENTIA , XV (5), 165-204.
  8. Ziegler, J., & Facchini, P. J. (2008). Alkaloid Biosynthesis: Metabolism and Trafficking. Annual Review of Plant Biology , 59 (1), 735-769.


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