Antimicrobial peptides have been isolated and characterized from tissues and organisms representing virtually every kingdom and phylum, ranging from prokaryotes to humans. Yet, recurrent structural and functional themes in mechanisms of action and resistance are observed among peptides of widely diverse source and composition. Biochemical distinctions among the peptides themselves, target versus host cells, and the microenvironments in which these counterparts convene, likely provide for varying degrees of selective toxicity among diverse antimicrobial peptide types. Moreover, many antimicrobial peptides employ sophisticated and dynamic mechanisms of action to effect rapid and potent activities consistent with their likely roles in antimicrobial host defense. In balance, successful microbial pathogens have evolved multifaceted and effective countermeasures to avoid exposure to and subvert mechanisms of antimicrobial peptides. A clearer recognition of these opposing themes will significantly advance our understanding of how antimicrobial peptides function in defense against infection. Furthermore, this understanding may provide new models and strategies for developing novel antimicrobial agents, that may also augment immunity, restore potency or amplify the mechanisms of conventional antibiotics, and minimize antimicrobial resistance mechanisms among pathogens. From these perspectives, the intention of this review is to illustrate the contemporary structural and functional themes among mechanisms of antimicrobial peptide action and resistance.

Mechanisms of Antimicrobial Peptide Action and Resistance

Mechanism of the binding, insertion and destabilization of phospholipid bilayer membranes by alpha-helical antimicrobial and cell non-selective membrane-lytic peptides.

http://web.stanford.edu/class/mi211/problemsets/Tan-Innate/Matsuzaki-BBA-99.pdf

Why and how are peptide-lipid interactions utilized for self-defense? Magainins and tachyplesins as archetypes.

The increasing resistance of bacteria to conventional antibiotics resulted in a strong effort to develop antimicrobial compounds with new mechanisms of action. Antimicrobial peptides seem to be a promising solution to this problem. Many studies aimed at understanding their mode of action were described in the past few years. The most studied group includes the linear, mostly alpha-helical peptides. Although the exact mechanism by which they kill bacteria is not clearly understood, it has been shown that peptide-lipid interactions leading to membrane permeation play a role in their activity. Membrane permeation by amphipathic alpha-helical peptides can proceed via either one of the two mechanisms: (a) transmembrane pore formation via a "barrel-stave" mechanism; and (b) membrane destruction/solubilization via a "carpet-like" mechanism. The purpose of this review is to summarize recent studies aimed at understanding the mode of action of linear alpha-helical antimicrobial peptides. This review, which is focused on magainins, cecropins, and dermaseptins as representatives of the amphipathic alpha-helical antimicrobial peptides, supports the carpet-like rather the barrel-stave mechanism. That these peptides vary with regard to their length, amino acid composition, and next positive charge, but act via a common mechanism, may imply that other linear antimicrobial peptides that share the same properties also share the same mechanism.

Mode of action of linear amphipathic α-helical antimicrobial peptides

Peptides with antimicrobial properties are present in most if not all plant species All plant antimicrobial peptides isolated so far contain even numbers of cysteines (4, 6, or 8), which are all pairwise connected by disulfide bridges, thus providing high stability to the peptides Based on homologies at the primary structure level, plant antimicrobial peptides can be classified into distinct families including thionins, plant defensins, lipid transfer proteins, and he vein- and knottin-type antimicrobial peptides Detailed three-dimensional structure information has been obtained for one or more members of these peptide families All antimicrobial peptides studied thus far appear to exert their antimicrobial effect at the level of the plasma membrane of the target microorganism, but the different peptide types are likely to act via different mechanisms Antimicrobial peptides can occur in all plant organs In unstressed organs, antimicrobial peptides are usually most abundant in the outer cell 

Antimicrobial Peptides from Plants

Magainins, positively charged peptides present in the skin of Xenopus laevis, are known to permeabilize free-energy transducing membranes. Structural studies in otherwise protein-free model systems show alpha-helical magainins parallel to the membrane water interface. However, functional studies in biological membranes suggest that magainins operate as oligomeric complexes. Here we investigate whether magainins function as oligomers in protein-free liposomes also. We report that they do exhibit strong positive heterocooperativity. The magainins, magainin 2 and PGLa, act synergistically. Both activity and cooperativity are enhanced by net negative charge of the liposomal membranes. A transmembrane electric potential, negative inside, enhanced the activity of the peptides. We propose a model in which (i) binding to the surface of the membrane, mainly guided by electrostatic interactions, occurs and (ii) the bound form is in equilibrium with an n-meric complex of magainins spanning the membrane.

Electric potentiation, cooperativity, and synergism of magainin peptides in protein-free liposomes.

The synthesis is described of the photoaffinity label N-(4-azido-2-nitrophenyl)glycyl-(Pro-Pro-Gly)5 for the peptide binding site of prolyl 4-hydroxylase. The photoaffinity label is a good substrate and is capable of light-induced inactivation of prolyl 4-hydroxylase activity. Inactivation depends on the concentration of photoaffinity label and is prevented by competition with excess (Pro-Pro-Gly)5. Two moles of photoaffinity label per mole of enzyme is needed for 100% inactivation of enzymic activity. Oxidative decarboxylation of 2-oxoglutarate measured in the absence of added peptide substrate is not affected by labeling. We conclude that the covalently bound nitreno derivative of N-(4-azido-2-nitrophenyl)glycyl-(Pro-Pro-Gly)5 acts by preventing the binding of peptide substrate to the catalytic site without interfering with the binding of the other substrates and cofactors 2-oxoglutarate, O2, Fe2+, and ascorbate. Labeling is specific for the alpha subunit of the tetrameric alpha 2 beta 2 enzyme. In addition to two catalytic binding sites that are blocked by the photoaffinity label, the enzyme contains binding subsites for peptide substrates, as judged from the capability of photoinactivated enzyme to bind to a poly(L-proline) affinity column. These binding subsites may account for the rapidly increasing affinity for peptide substrates with increasing chain length.

Photoaffinity labeling of peptide binding sites of prolyl 4-hydroxylase with N-(4-azido-2-nitrophenyl)glycyl-(Pro-Pro-Gly)5.

https://pure.uva.nl/ws/files/2904062/960_11105y.pdf

Energy, control and DNA structure in the living cell.

Magainins are 23 or 21 residues-long peptides, initially detected in the skin of Xenopus laevis 2: Magainin 2: GIGKFLHSAKKFGKAFVGEIMNS-CONH2 , PGLa: GMASKAGAIAGKIAKVALKAL-CONH2.

A. de Waal

Papers

Electric potentiation, cooperativity, and synergism of magainin peptides in protein-free liposomes.

Photoaffinity labeling of peptide binding sites of prolyl 4-hydroxylase with N-(4-azido-2-nitrophenyl)glycyl-(Pro-Pro-Gly)5.

Energy, control and DNA structure in the living cell.

Structure-function relationship of magainins. Studies in liposomal model system.