Defensins are one of the largest and most studied families of antimicrobial peptides.
More than 300 defensins have been identified so far and they are represented in a range of organisms including mammals, birds, invertebrates, plants and recently in the ebony-cup fungus.
Defensin peptides are essentially ancient natural antibiotics with antimicrobial activity against a range of microorganisms: gram-positive and gram-negative bacteria, fungi and some viruses.
In addition to their antimicrobial activity, emerging evidence suggest that they can also assume fundamental roles in both innate and adaptive immunity.
Almost all defensins discovered so far are multiple-disulphide bonded with a cationic charge and an amphipathic design.
The three dimensional structure of defensin, an example of a small cysteine-rich proteins.
As a class, small cysteine-rich proteins are characterized by the presence of an unusually high number of cysteine residues that form disulfide bonds (see the yellow areas of the defensin protein) and thus make the structures of them very compact.
Definition and structure
Defensine are a family of antimicrobial peptides widely distributed in nature with a characteristic beta-sheet-rich fold and a framework of six disulphide-linked cysteines.
The two main defensin subfamilies, alfa- and beta-defensins, differ in the length of peptides segments between the six cysteines and the pairing of the cysteines thet are connected by disulphide bonds.
Both alfa- and beta-defensins consist of a triple-stranded beta-sheet with a distinctive “defensin” fold. Recently, another structurally distinct subfamily of teta-defensins has been identified in rhesus macaque monkey leukocytes.
The mature teta-defensin peptides arise by an as-yet-uncharacterized process that generates a cyclic peptide by splicing and cyclization from two of the nine-amino-acid segments of alfa-defensin-like precursor peptides.
The teta-defensins apparently evolved in primates, but are inactivated in humans owing to mutations that encode premature stop codons.
Antimicrobial peptides from invertebrates and plants that contain six or eight cysteines in disulphide linkage are also known as defensins. Their evolutionary relationship to vertebrate defensins is uncertain. Insect defensins contain an alfa-helix
that is disulphide –linked to a beta-sheet with cysteine linkage (1-4,2-5,3-6) that differs from vertebrate defensins.
Defensins are abundant in cells and tissues thet are involved in host defence against microbial infections.
In many animals, the highest concentrations of defensins are found in granules, the storage organelles of leukocytes.
When leukocytes ingest microorganisms into phagocytic vacuoles, the granules fuse to these vacuoles and deliver their contents onto the terget microrganism.
Paneth cells, specialized host defence cells of the small intestine, are another site of high defensin concentartion in many animal species. Paneth cells contain defensin-rich secretory granules that are released into narrow intestinal pits, known as crypts. Various barrier and secretory epithelial cells produce defensins, in some cases constitutively and in other cases in response to infection.
Patterns of tissue distribution vary even when closely related species are compared.
It is probable that these peculiarities in the pattern of expression of defensins in certain animal species are related to evolutionary pressures from species-specific pathogens.
Most defensins show antimicrobial activity against bacteria and fungi,especially when tested under low ionic strength conditions and with low concentrations of divalent cations, plasma proteins or other interfering sustances. Increasing the concentration of salts and plasma proteins competitively inhibits the antimicrobial activity of defensins, in a manner that depends on both the specific defensin and its microbial target. In general, the activity of defensins was diminished in the presence of increased salt concentrations, supporting the importance of electrostatic forces between the anionic phospholipid headgroups and the cationic defensins.
Recently, human neutrophil defensin have been implicated among the molecules that are responsible for the antiviral activity secreted by CD8+ T cells of HIV-non-progressors.
Mechanisms of antimicrobial activity
Permeabilization of target membranes is the crucial step in defensin-mediated antimicrobial activity and cytotoxicity. In bacteria, permeabilization coincided with the inhibition of RNA, DNA and protein synthesis and decreased bacterial viability as assessed by the colony forming assay, indicating that permeabilization is essential for bacterial killing (Defensins: antimicrobial peptides of innate immunity, 2003).