Antimicrobial peptides with unusual amino acid compositions and unusual structures

Antimicrobial peptides, which constitute an important component of innate immunity in animal and plant kingdom, are ubiquitously distributed in nature. However, they differ widely in their sizes, sequences and structures. On the basis of their structure they have been broadly classified into three classes: a) linear peptides with propensity for amphiphilic alpha-helical structure, b) peptides with beta or alphabeta structure stabilized by different number of disulfide bridges and c) peptides with over-representation of certain amino acids or unusual structures. Although considerable amount of work has been done on peptides of all the three classes, recent reviews have emphasized on peptides belonging to the first two classes. The present review focuses on the peptides belonging to the third group. The antimicrobial peptides discussed in this article include aromatic amino acid-rich peptides, (Pro-Arg)-rich peptides, unusual defensins and defensin-like molecules, unusual antimicrobial peptides from amphibians, bacteriocins with unusual structure and anionic antimicrobial peptides.