1.C.19 The Defensin (Defensin) Family
Many organisms synthesize proteins (or peptides) which are degraded to relatively small hydrophobic or amphipathic, bioactive peptides. These peptides exhibit antibiotic, fungicidal, virucidal, hemolytic and/or tumoricidal activities by interacting with membranes and forming transmembrane channels that allow the free flow of electrolytes, metabolites and water across the phospholipid bilayers. Most of these peptides appear to function in biological warfare. There are many designations given to these bioactive peptides. They include the magainins, cecropins, melittins, defensins, bacteriocidins, etc. The proteins in each family within this functional superfamily are homologous, but they exhibit little or no significant sequence similarity with members of the other families. Thus, each family may have evolved independently. However, certain common structural features observed between members of distinct families suggest that at least some of these families share a common ancestry.
Several families of eukaryotic channel-forming amphipathic peptides, each from a different group of organisms, are recognized. These families comprise the defensin superfamily. They have been shown to be homologous using statistical means and have been described from phylogenetic standpoints (Chen et al. 2011). However, Shafee et al. 2017; Shafee et al. 2016 have suggested that defensins and small defensin-like proteins fall into two superfamilies, which they call the cis-defensins (broadly distributed in living organisms) and the trans-defensins (narrowly distrubuted). They suggest that these two groups of proteins converged to show similar sequences, secondary and tertiary structures, and disulfide connectivities, with overlapping organismal sources and functions, in spite of their independent origins. The functions of these short proteins vary tremendously including pore formation, bacterial and fungal toxicity, lipid targeting, toxic receptor and channel interactions, fertilization, protease inhibiton and stress adaptation. However, as noted by the authors, alternative pathways involving divergent evolution from a common evolutionary source could have also occurred although they consider this possibility less likely (Shafee et al. 2016; Shafee et al. 2017).
Defensins are often produced by mammals. Their precursors vary in size (35-95 amino acyl residues). The three-dimensional structures of defensin-1 have been solved both by x-ray crystallography (1.9 Å resolution) and by NMR. An α-defensin, HNP-1, a β-stranded toxin, forms a dimeric pore (Zhang et al., 2010). The amino acid composition, amphipathicity, cationic charge and size of a defensin allow it to attach to and insert into membranes to form pores by 'barrel-stave', 'carpet' or 'toroidal-pore' mechanisms. Transmembrane pore formation is not the only mechanism of microbial killing. Translocated peptides can alter cytoplasmic membrane septum formation, inhibit cell-wall synthesis, inhibit nucleic-acid synthesis, inhibit protein synthesis or inhibit enzymatic activities (Brogden 2005).
Defensins contribute to innate immunity, including protection of mucosal tissues. Human α-defensin 6 (HD6) is highly expressed by secretory Paneth cells of the small intestine. However, in contrast to the other defensins, it lacks appreciable bactericidal activity. Chu et al. (2012) reported that HD6 affords protection against invasion by enteric bacterial pathogens in vitro and in vivo. After stochastic binding to bacterial surface proteins, HD6 undergoes ordered self-assembly to form fibrils and nanonets that surround and entangle bacteria. This self-assembly mechanism occurs in vivo, requires histidine-27, and is consistent with x-ray crystallography data. These findings support a key role for HD6 in protecting the small intestine against invasion by diverse enteric pathogens and may explain the conservation of HD6 throughout Hominidae evolution (Chu et al., 2012).
The generalized transport reaction catalyzed by channel-forming amphipathic peptides is:
Small solutes, electrolytes and water (in) small solutes, electrolytes and water (out)