ReviewDiversity of antimicrobial peptides and their mechanisms of actionRichard M. Epanda, Hans J. VogelbaDepartment of Biochemistry, McMaster University Health Sciences Centre, Hamilton, Ont. L8N 3Z5, CanadabStructural Biology Research Group, Department of Biological Sciences, University of Calgary, Calgary, Alta. T2N 1N4, CanadaAccepted 5 October 1999AbstractAntimicrobial peptides encompass a wide variety of structural motifs. Many peptides have K-helical structures. Themajority of these peptides are cationic and amphipathic but there are also hydrophobic K-helical peptides which possessantimicrobial activity. In addition, some L-sheet peptides have antimicrobial activity and even antimicrobial K-helicalpeptides which have been modified to possess a L-structure retain part of their antimicrobial activity. There are alsoantimicrobial peptides which are rich in a certain specific amino acid such as Trp or His. In addition, antimicrobial peptidesexist with thio-ether rings, which are lipopeptides or which have macrocyclic Cys knots. In spite of the structural diversity, acommon feature of the cationic antimicrobial peptides is that they all have an amphipathic structure which allows them tobind to the membrane interface. Indeed, most antimicrobial peptides interact with membranes and may be cytotoxic as aresult of disturbance of the bacterial inner or outer membranes. Alternatively, a necessary but not sufficient property of thesepeptides may be to be able to pass through the membrane to reach a target inside the cell. The interaction of these peptideswith biological membranes is not just a function of the peptide but is also modulated by the lipid components of themembrane. It is not likely that this diverse group of peptides has a single mechanism of action, but interaction of the peptideswith membranes is an important requirement for most, if not all, antimicrobial peptides. ß 1999 Elsevier Science B.V. Allrights reserved.Keywords: Cytotoxic peptide; Peptide-lipid interaction; Membrane permeability ; Peptide conformation ; LipopolysaccharideContents1. Introduction .......................................................... 122. Amphipathic and hydrophobic K-helices ...................................... 133. L-Sheet peptides and small proteins . . ....................................... 144. Peptides with irregular amino acid composition ................................ 165. Peptides with thio-ether rings . . ............................................ 166. Peptaibols . ........................................................... 177. Macrocyclic cystine knot peptides . . . ....................................... 170005-2736 / 99 / $ ^ see front matter ß 1999 Elsevier Science B.V. All rights reserved.PII: S0005-2736(99)00198-4BBAMEM 77742 25-11-99 Cyaan Magenta Geel ZwartBiochimica et Biophysica Acta 1462 (1999) 11^28www.elsevier.com/locate/bba8. Role of membrane lipids ................................................. 189. The potential role of lipopolysaccharide (LPS) and the bacterial outer membrane ....... 1810. Alternative mechanisms of action ........................................... 2011. Biosynthesis of antimicrobial peptides ....................................... 2112. Spectroscopic techniques to study peptide membrane interactions . .................. 2313. Summary . ........................................................... 25Acknowledgements . . . ...................................................... 25References ............................................................... 251. IntroductionOrganisms from throughout the phylogenetic tree,including animals [1], produce substances for protec-tion against microbes. Many of these substances arepeptides. With the growing problem of pathogenicorganisms which are resistant to conventional anti-biotics, there is increased interest in the pharmaco-logical application of antimicrobial peptides to treatinfection. E¡orts are currently underway to increasethe potency and speci¢city of these peptides so thatthey are toxic to microbes and not to mammals. Inorder to achieve this in an e¤cient manner, it isimportant to understand the mechanism of actionof these agents and the reason for their selectivityagainst microbes.Even limiting consideration of antimicrobialagents to peptides, there is still a large variety ofstructures known. This makes the task for designingimproved agents more complex, but at the sametime, it provides a range of opportunities for furtherdevelopment. The classi¢cation of antimicrobial pep-tides is somewhat arbitrary and there exist analogswith similar sequences but di¡erent conformationalmotifs which would fall into di¡erent classes, despitethe similarity of their chemical structure and possiblyalso of their mechanism of action. Nevertheless, tosimplify the problem and to illustrate the range ofstructures of peptides with antimicrobial activities,we have divided these peptides into groups. Thesegroups include linear peptides which form amphi-pathic and hydrophobic helices, cyclic peptides andsmall proteins which form L-sheet structures, pep-tides with unique amino acid compositions, cyclicpeptides with thio-ether groups in the ring, lipopep-tides terminating in an amino alcohol and macro-cyclic knotted peptides. We will discuss what isknown about the structure and the mechanism ofaction of each of these classes of antimicrobial pep-tides individually.In general, the mechanism of action of any ofthese agents is not very well established. For manyof these peptides, there is evidence that one of thetargets for the peptide is the lipid bilayer of the mem-brane. This is because these peptides can often in-crease the rate of leakage of the internal aqueouscontents of liposomes. In addition, most of the anti-microbial peptides are cationic and their interactionwith anionic phospholipids would provide a readyexplanation for their speci¢city for bacterial mem-branes. In Gram-negative bacteria, both the outerlea£et of the plasma membrane as well as the outermembrane contain anionic molecules oriented to-wards the exterior of the cell. This is not the casefor mammalian membranes. Hence, the cationic anti-microbial peptides will preferentially bind to the ex-posed negative charges of bacterial membranes, butnot to the zwitterionic amphiphiles present in theextracellular monolayer of mammalian plasma mem-branes. This speci¢city for anionic membrane com-ponents is also mimicked in model liposome