Defensin

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Defensin
Example defensins with alpha helix in red, beta strands in blue, disulphide bonds in yellow (PDB: 1MR4, 2KOZ, 1FJN, 2LXZ, 1IJV, 2RNG​)
Identifiers
SymbolDefensin
Pfam clanCL0075
OPM superfamily54
OPM protein6cs9

Defensins are small

disulphide bonds
.

In animals, they are produced by cells of the

microbial cell membrane
to metabolic disruption.

Varieties

Characteristic disulphide linkages
Trans-defensin superfamily: In yellow, the two most conserved disulphides link a beta strand to two different secondary structure elements (motif = CC). On the right, an example structure (PDB: 1IJV​).
Cis-defensin superfamily: In yellow, the two most conserved disulphides link a beta strand to the same alpha helix (motif = CxC...CxxxC). On the right, an example structure (PDB: 1MRR4​).

The name 'defensin' was coined in the mid-1980s, though the proteins have been called 'Cationic Antimicrobial Proteins,' 'Neutrophil peptides,' 'Gamma thionins' amongst others.[6]

Proteins called 'defensins' are not all evolutionarily related to one another.[7] Instead fall into two broad superfamilies, each of which contains multiple families.[7][8] One superfamily, the trans-defensins, contains the defensins found in humans and other vertebrates,[9][10] as well as some invertebrates.[11][12] The other superfamily, cis-defensins, contains the defensins found in invertebrates, plants, and fungi.[13][14][15] The superfamilies and families are determined by the overall tertiary structure, and each family usually has a conserved pattern of disulphide bonds.[9][16] All defensins form small and compact folded structures, typically with a high positive charge, that are highly stable due to the multiple disulphide bonds. In all families, the underlying genes responsible for defensin production are highly polymorphic.[citation needed]

Trans-defensins

Vertebrate defensins are primarily α-defensins and β-defensins. Some primates additionally have the much smaller θ-defensins. In general, both α- and β-defensins are encoded by two-exon genes, where the first exon encodes for a hydrophobic leader sequence (removed after translation) and the cysteine-rich sequence (the mature peptide). The disulfide linkages formed by the cysteines have been suggested to be essential for activities related to innate immunity in mammals, but are not necessarily required for antimicrobial activity.[17][18] Theta defensins form a single beta-hairpin structure and represent a distinct group. Only alpha and beta-defensins are expressed in humans.[19]

Table of human defensins
Type Gene Symbol Gene Name Protein Name Description
α-defensins DEFA1 Defensin, alpha 1 Neutrophil defensin 1 Are expressed primarily in
Paneth cells
of the small intestine, where they may regulate and maintain microbial balance in the intestinal lumen.
DEFA1B Defensin, alpha 1B Defensin, alpha 1
DEFA3 Defensin, alpha 3, neutrophil-specific Neutrophil defensin 3
DEFA4 Defensin, alpha 4, corticostatin Neutrophil defensin 4
DEFA5 Defensin, alpha 5, Paneth cell-specific Defensin-5
DEFA6 Defensin, alpha 6, Paneth cell-specific Defensin-6
β-defensins
DEFB1
 Defensin, beta 1 Beta-defensin 1 Are the most widely distributed, being secreted by
epithelial cells of many kinds. For example, they can be found on the tongue, skin, cornea, salivary glands, kidneys, esophagus, and respiratory tract. It has been suggested (but also challenged) that some of the pathology of cystic fibrosis
arises from the inhibition of β-defensin activity on the epithelial surfaces of the lungs and trachea due to higher salt content.
DEFB2 Defensin, beta 2 Beta-defensin 2
DEFB3 Defensin, beta 3 Beta-defensin 3
DEFB103A Defensin, beta 103B Beta-defensin 103
... ... ...
DEFB106A Defensin, beta 106A Beta-defensin 106A
DEFB106
B 		Defensin, beta 106B Beta-defensin 106B
DEFB107B Defensin, beta 107A Beta-defensin 107
DEFB110 Defensin, beta 110 Beta-defensin 110
... ... ...
DEFB136 Defensin, beta 136 Beta-defensin 136
θ-defensins DEFT1P Defensin, theta 1 pseudogene not expressed in humans Are rare, and thus far have been found only in the leukocytes of the
Papio anubis, the gene coding for it is corrupted in humans and other primates.[21][22]

Although the most well-studied defensins are from vertebrates, a family of trans-defensins called 'big defensins' are found in

arthropods and lancelets.[7][8]

Cis-defensins

molluscs, annelids and cnidaria.[24]

Plant defensins were discovered in 1990 and have subsequently been found in most plant tissues with antimicrobial activities, with both antifungal and antibacterial examples.[25] They have been identified in all major groups of vascular plants, but not in ferns, mosses or algae.[25]

divisions of fungi (Ascomycota and Basidiomycota), as well as in the more basal groups of Zygomycota and Glomeromycota.[27]

Bacterial defensins have also been identified, but are by far the least studied. They include variants with only four cysteines, whereas defensins from eukaryote defensins almost all have six or eight.[28]

Related defensin-like proteins

In addition to the defensins involved in host defence, there are a number of related Defensin-Like Peptides (DLPs) that have evolved to have other activities.

Toxins

There appear to have been multiple evolutionary recruitments of defensins to be toxin proteins used in the venoms of animals;

nerve signals. Examples include the crotamine toxin in snake venom,[30] many scorpion toxins,[31] some sea anemone toxins,[10] and one of the toxins in platypus venom.[29] Indeed, an insect defensin has been experimentally converted into a toxin by deletion of a small loop that otherwise sterically hindered interactions with the ion channels.[32]

Signalling

In vertebrates, some α- and β-defensins are involved in signalling between the

self-incompatibility to prevent inbreeding.[35]

Enzyme inhibitors

Some antimicrobial defensins also have enzyme inhibitory activity, and some DLPs function primarily as enzyme inhibitors, acting as antifeedants (discouraging animals from eating them).[36][37][38]

Function

In immature

pathogens. [citation needed
] They are produced in the milk of the mother as well as by the young marsupial in question.

In human breast milk, defensins play a central role in neonate immunity.[39]

The human genome contains theta-defensin genes, but they have a premature

influenza A
. They act primarily by preventing these viruses from entering their target cells.

Also interesting is the effect of alpha-defensins on the

MAPKK, is vulnerable to human neutrophil protein-1 (HNP-1). This group showed HNP-1 to behave as a reversible noncompetitive inhibitor of LF.[42]

They have generally been considered to contribute to mucosal health; however, it is possible that these peptides can be considered biological factors that can be upregulated by bioactive compounds present in human breast milk. In this sense, the intestinal production of antimicrobial peptides as hBD2 and hBD4 by trefoil from milk might play an important role on neonate colonization, thereby enhancing the immune response of newborns against pathogens with which they may come in contact.[39][43]

Pathology

The alpha defensin peptides are increased in chronic inflammatory conditions.

Alpha defensin are increased in several cancers, including colorectal cancer.[44]

An imbalance of defensins in the skin may contribute to acne.[45]

A reduction of ileal defensins may predispose to Crohn's disease.[46][47]

In one small study, a significant increase in alpha defensin levels was detected in T cell lysates of schizophrenia patients; in discordant twin pairs, unaffected twins also had an increase, although not as high as that of their ill siblings. The authors suggested that alpha-defensin levels might prove a useful marker for schizophrenia risk.[48]

Defensins are found in the human skin during inflammatory conditions like psoriasis[49] and also during wound healing.

Applications

Defensins

At present, the widespread spread of antibiotic resistance requires the search and development of new antimicrobial drugs. From this point of view, defensins (as well as antimicrobial peptides in general) are of great interest. It was shown that defensins have pronounced antibacterial activity against a wide range of pathogens.[50] In addition, defensins can enhance the effectiveness of conventional antibiotics.[50]

Defensin-mimetics

Defensin

antifungals, especially for candidiasis.[55][56][57]

See also

  • Host defense peptides
    , to which defensins belong

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