Defensin

Defensin
Example defensins with alpha helix in red, beta strands in blue, disulphide bonds in yellow (PDB: 1MR4, 2KOZ, 1FJN, 2LXZ, 1IJV, 2RNG​)
Identifiers
Symbol	Defensin
Pfam clan	CL0075
OPM superfamily	54
OPM protein	6cs9

Defensins are small cysteine-rich cationic proteins across cellular life, including vertebrate^[1] and invertebrate^[2] animals, plants,^[3][4] and fungi.^[5] They are host defense peptides, with members displaying either direct antimicrobial activity, immune signaling activities, or both. They are variously active against bacteria, fungi and many enveloped and nonenveloped viruses. They are typically 18-45 amino acids in length, with three or four highly conserved disulphide bonds.

In animals, they are produced by cells of the innate immune system and epithelial cells, whereas in plants and fungi they are produced by a wide variety of tissues. An organism usually produces many different defensins, some of which are stored inside the cells (e.g. in neutrophil granulocytes to kill phagocytosed bacteria), and others are secreted into the extracellular medium. For those that directly kill microbes, their mechanism of action varies from disruption of the microbial cell membrane to metabolic disruption.