Antimicrobial peptides (AMPs), also called host defense peptides (HDPs), which commonly content 5-40 amino acids, are natural antibiotics produced by various organisms.
The characteristics of Antimicrobial peptides
The first AMP was found by Dubos when extracted an antimicrobial agent from a soil bacillus strain in 1939. This extract was demonstrated to protect mice from pneumococci infection. Since then nearly 5000 of AMPs have been found or synthesis. They usually have the common characteristics: small peptide (30–60 aa), strong cationic (pI 8.9–10.7), heat-stable (100 ℃, 15 min), no drug fastness and no effect on eukaryotic cell. Almost all of them have a positively charged surface. The selective interaction with the membrane of microorganisms generally depends on the cationic nature of AMPs. According to their charge they can be divided into cationic peptides and non-cationic peptides.
The largest group corresponds to cationic peptides, which is divided in three classes
- The first category composed by linear cationic α-helical peptides, such as Magainin and Cecropins that are linear before their interaction with the cell membrane, and then adopt an amphipathic α-helical secondary structure.
- The second category comprises cationic peptides enriched for specific amino acids like proline, arginine, and other residues. These peptides are linear, although some may display extended coils.
- The third category includes cationic peptides that contain cysteine residues and form disulphide bonds and stable β-sheets. For example defensins have six cysteine residues and are divided according to the alignment of their disulphide bridges.
Other groups of AMPs are described as non-cationic peptides and are also an important part of the innate immune system.
Main action mechanism of AMPs
Currently, several complex and controversial mechanisms of action exist as to how AMPs disrupt the bacterial cell membrane. The three popular models are the carpet model, the toroidal pore model, and the barrel-stave model (Fig1).
Fig 1 The three popular models of AMP-membrane interactions.
Most AMPs act by provoking an increase in plasma membrane permeability. First step in the mechanism of membrane permeabilization is the electrostatic interaction between the positively charged AMP with the negatively charged membrane surface of the microorganism. Subsequent disruption of the membrane by creation of pores within the microbial membrane ultimately results in cell death of the organism due to leakage of ions, metabolites, cessation of membrane-coupled respiration, and biosynthesis. The mechanics of AMPs represent a unusual source for the targeted exploration of new applications in the therapy of microbial and viral infection, cancer, and sepsis.
Nowadays AMPs are gaining much attention either in Pharmaceuticals industry or in research area. AMPs damage microorganisms mostly by membrane disruption, making it difficult for microorganisms to develop resistance and also make itself more popular in pharmacology. Hence, acquired resistance did not become an issue with antimicrobial peptides. While modern synthetic methods will allow the relatively cheap and accurate production of peptide candidates.
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