The Role and Therapeutic Potential of Host Defense Peptides — Classification of Cationic Peptides

The Role and Therapeutic Potential of Host Defense Peptides — Classification of Cationic Peptides

Hancock, R. E. W., & Sahl, H.-G. (2006). Cationic host defence peptides: novel antimicrobial strategies against old and new pathogens. Nature Reviews Drug Discovery, *5*(2), 123–130. https://doi.org/10.1038/nrd2201

Despite their structural diversity, cationic peptides often fold into amphipathic or amphiphilic structures, either in solution or upon interaction with biological membranes. Many cationic peptides can be broadly classified based on their primary structural classes. Although these categories are used to describe the folded structures of given peptides, they can still reflect relationships with their biological activities.

The most common and well-characterized class of cationic peptides is the amphipathic α-helical peptides. These peptides can adopt an α-helical conformation when interacting with membranes, where one surface of the helix is rich in polar amino acids, while the opposite face contains a large proportion of hydrophobic residues. Members of the α-helical class include frog skin secretions (magainins), bee venom (melittin), and insect dipteran defensins (cecropins). These three types of peptides are potent antimicrobial agents and also exhibit selective toxicity toward mammalian cells. Today, thousands of synthetic α-helical peptides have been developed to study the contributions of charge, hydrophobicity, and helicity to their antibacterial and hemolytic activities.

Mice, rats, and humans all possess members of the α-helical cathelicidin family of peptides. In mice, this peptide is known as cathelicidin-related antimicrobial peptide (CRAMP), while the rat homolog is named rCRAMP. Humans also express a homolog of this peptide called hCAP18/LL-37. Although LL-37 has been demonstrated to act as an antimicrobial in vitro, its activity is antagonized under physiological salt concentrations, particularly by divalent cations, and its antibacterial effects may have been overestimated in the past.

The second major class of cationic peptides is the β-sheet family. This is a large group consisting of peptides composed of two or more β-strands stabilized by one or more disulfide bonds. This class includes several subfamilies of antimicrobial and host defense peptides, notably the α-defensins and β-defensins in mammals. Recently, another defensin subclass, θ-defensins, which are cyclic, was discovered in rhesus macaque neutrophils. Humans possess a homologous θ-defensin-like pseudogene, but a premature stop codon mutation prevents its expression. α-Defensins contain 29–35 residues and are stabilized by three disulfide bonds forming a three-stranded β-sheet. These disulfide bonds connect C1→C6, C2→C4, and C3→C5, distinguishing them from β-defensins, which consist of 34–47 residues with disulfide linkages at C1→C5, C2→C4, and C3→C6.

A particularly effective group of β-sheet cationic peptides features a hairpin structure (anti-parallel β-strands) interconnected by a type II β-turn, stabilized by one or two disulfide bonds between the β-strands. The prototypes of disulfide-stabilized peptides are tachyplesins and polyphemusins, key molecules in the innate immune systems of Asian and American horseshoe crabs, respectively. These peptides exhibit salt-insensitive antimicrobial activity against both Gram-negative and Gram-positive bacteria, as well as antifungal activity. Another emerging area of research on such peptides stems from the finding that a homolog of polyphemusin II has been identified as a specific inhibitor of CXCR4, a receptor required for HIV viral entry. This group also includes β-hairpin peptides stabilized by a single disulfide bond, such as the cyclic peptide bactenecin or bovine neutrophil dodecapeptide.

There is also a class of cationic peptide derivatives that lack typical secondary structures. This category includes bovine neutrophil peptide (indolicidin) and a porcine peptide fragment (tritrpticin). These peptides feature tryptophan residues at 3 (tritrpticin) or 5 (indolicidin) out of 13 amino acid positions. Indolicidin and tritrpticin exhibit moderately broad activity against Gram-positive and Gram-negative bacteria. The tertiary structures of both peptides have been solved by 2D-NMR, revealing an active boat-like extended structure when interacting with diphosphatidylcholine (DPC). The central tryptophan-rich region of the peptide interacts with a bilayer membrane, positioning and spanning the aqueous/membrane interface, while the N- and C-terminal containing cationic lysine and arginine residues orient toward the aqueous environment.

Another class consists of histidine-rich peptides produced by human salivary glands. These contain a very high proportion of histidine residues (18–29%), carry a high cationic charge (+7 to +8), and display antifungal activity. Other members of this group include a range of proline-rich peptides found in many species such as insects. Yet another group comprises glycine-rich peptides, present in skin secretions of some amphibians and widely identified in numerous insect species. Their structures may resemble those of other peptide classes such as melittin, cecropin, or attacin, and they may also undergo glycosylation in vivo.