Defensin

Defensins are small cysteine-rich cationic proteins found in both vertebrates and invertebrates. They have also been reported in plants. They are, and function as, host defense peptides. They are active against bacteria, fungi and many enveloped and nonenveloped viruses. They consist of 18-45 amino acids including six (in vertebrates) to eight conserved cysteine residues. Cells of the immune system contain these peptides to assist in killing phagocytosed bacteria, for example in neutrophil granulocytes and almost all epithelial cells. Most defensins function by binding to the microbial cell membrane, and, once embedded, forming pore-like membrane defects that allow efflux of essential ions and nutrients.

Defensins are antimicrobial peptides that act mainly by disrupting the structure of bacterial cell membranes and are found in many compartments of the body. Evidence is accumulating that defensins play a central role in defense against pathogens, and they are considered as a part of the innate immune response. 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 presents 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 newborn against pathogens with which they may come in contact.

The name 'defensin' was coined in the mid1980s, though the proteins had been studied as 'Cationic Antimicrobial Proteins'. The underlying genes responsible for defensin production are highly polymorphic. Some aspects are conserved, however; the hallmarks of a β-defensin are its small size, high density of cationic charge, and six-cysteine-residue motif. In general, they are encoded by two-exon genes, wherein the first exon encodes for a hydrophobic leader sequence and the second for a peptide containing the cysteine motif. All defensins have disulfide linkages. The disulfide linkages have been suggested to be essential for activities related to innate immunity in mammals, but are not necessarily required for antimicrobial activity.

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