Many antimicrobial peptides bear arginine (R)- and tryptophan (W)-rich sequence motifs. Based on the sequence Ac-RRWWRF-NH2, sets of linear and cyclic peptides were generated by changes in the amino acid sequence, L-D-amino acid exchange and naphthylalanine substituted for tryptophan. Linear RW-peptides displayed moderate activity towards Gram-positive Bacillus subtilis (15 < MIC < 31 microm) and were inactive against Gram-negative Escherichia coli at peptide concentrations < 100 microm. Cyclization induced high antimicrobial activity. The effect of cyclization was most pronounced for peptides with three adjacent aromatic residues. Incorporation of d-amino acid residues had minor influence on the biological activity. The haemolytic activity of all RW-peptides at 100 microm concentration was low (< 7% lysis for linear R/W-rich peptides and < 28% for the cyclic analogues). Introduction of naphthylalanine enhanced the biological activities of both the linear and cyclic peptides. All peptides induced permeabilization of large unilamellar vesicles (LUVs) composed of lipids of the membrane of B. subtilis and erythrocytes, but surprisingly had no effect on LUVs composed of lipids of the E. coli inner membrane. The profiles of peptide activity against B. subtilis and red blood cells correlated with the permeabilizing effects on the corresponding model membranes and were related to hydrophobicity parameters as derived from reversed phase high-performance liquid chromatography (HPLC). The results underlined the importance of amphipathicity as a driving force for cell lytic activity and suggest that conformational constraints and an appropriate position of aromatic residues allowing the formation of hydrophobic clusters are highly favourable for antimicrobial activity and selectivity.
Antimicrobial, cationic peptides are abundant throughout nature as part of many organisms' defence against microorganisms. They exhibit a large variety of sequences and structural motifs and are thought to act by rupturing the bacterial membrane. Several models based on biophysical experiments have been proposed for their mechanism of action. Here we present the NMR-determined structure of the cyclic, cationic antimicrobial peptide cyclo(RRWWRF) both free in aqueous solution and bound to detergent micelles. The peptide has a rather flexible but ordered structure in water. A distinct structure is formed when the peptide is bound to a detergent micelle. The structures in neutral and negatively charged micelles are nearly identical but differ from that in aqueous solution. The orientation of the amino acid side chains creates an amphipathic molecule with the peptide backbone forming the hydrophilic part. The orientation of the peptide in the micelle was determined by using NOEs and paramagnetic agents. The peptide is oriented mainly parallel to the micelle surface in both detergents. Substitution of the arginine and tryptophan residues is known to influence the antimicrobial activity. Therefore the structure of the micelle-bound analogues cyclo(RRYYRF), cyclo(KKWWKF) and cyclo(RRNalNalRF) were also determined. They exhibit remarkable similarities in backbone conformation and side-chain orientation. The structure of these peptides allows the side-chain properties to be correlated to biological activity.
Cyclization of R-and W-rich hexapeptides has been found to enhance specifically the antimicrobial activity against Gram-negative Escherichia coli. To gain insight into the role of the bacterial outer membrane in mediating selectivity, we assayed the activity of cyclic hexapeptides derived from the parent sequence c-(RRWWRF) against several E. coli strains and Bacillus subtilis, L-form bacteria, and E. coli lipopolysaccharide (LPS) mutant strains, and we also investigated the peptide-induced permeabilization of the outer and inner membrane of E. coli. Wall-deficient L-form bacteria were distinctly less susceptible than the wild type strain. The patterns of peptide-induced permeabilization of the outer and inner E. coli membranes correlated well with the antimicrobial activity, confirming that membrane permeabilization is a detrimental effect of the peptides upon bacteria. Truncation of LPS had no influence on the activity of the cyclic parent peptide, but the highly active c-(RRWFWR), with three adjacent aromatic residues, required the complete LPS for maximal activity. Furthermore, differences in the activity of the parent peptide and its all-D sequence indicated stereospecific interactions with the LPS mutant strains. We suggest that, depending on the primary sequence of the peptides, either hydrophobic interactions with the fatty acid chains of lipid A, or electrostatic interactions disturbing the polar core region and interference with saccharide-saccharide interactions prevail in the barrier-disturbing effect upon the outer membrane and thereby provide peptide accessibility to the inner membrane. The results underline the importance of tryptophan and arginine residues and their relative location for a high antimicrobial effect, and the activity-modulating function of the outer membrane of E. coli. In addition to membrane permeabilization, the data provided evidence for the involvement of other mechanisms in growth inhibition and killing of bacteria.
New antimicrobial compounds are of major importance because of the growing problem of bacterial resistance. In this context, antimicrobial peptides have received a lot of attention. Their mechanism of action, however, is often obscure. Here, the structures of two cyclic, antimicrobial peptides from the family of arginine-and tryptophan-rich peptides determined in a membrane-mimicking environment are described. The sequence of the peptides has been obtained from a cyclic parent peptide by scrambling the amino acids. While the activity of the peptides is similar to that of the parent peptide, the structures are not. The peptides do, however, all adopt an amphiphilic structure. A comparison between the structures helps to define the requirements for the activity of these peptides.
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