Tryptophan- and arginine-rich antimicrobial peptides: structures and mechanisms of action

Antimicrobial peptides encompass a number of different classes, including those that are rich in a particular amino acid. An important subset are peptides rich in Arg and Trp residues, such as indolicidin and tritrpticin, that have broad and potent antimicrobial activity. The importance of these two amino acids for antimicrobial activity was highlighted through the screening of a complete combinatorial library of hexapeptides. These residues possess some crucial chemical properties that make them suitable components of antimicrobial peptides. Trp has a distinct preference for the interfacial region of lipid bilayers, while Arg residues endow the peptides with cationic charges and hydrogen bonding properties necessary for interaction with the abundant anionic components of bacterial membranes. In combination, these two residues are capable of participating in cation-pi interactions, thereby facilitating enhanced peptide-membrane interactions. Trp sidechains are also implicated in peptide and protein folding in aqueous solution, where they contribute by maintaining native and nonnative hydrophobic contacts. This has been observed for the antimicrobial peptide from human lactoferrin, possibly restraining the peptide structure in a suitable conformation to interact with the bacterial membrane. These unique properties make the Arg- and Trp-rich antimicrobial peptides highly active even at very short peptide lengths. Moreover, they lead to structures for membrane-mimetic bound peptides that go far beyond regular alpha-helices and beta-sheet structures. In this review, the structures of a number of different Trp- and Arg-rich antimicrobial peptides are examined and some of the major mechanistic studies are presented.