Abstract
SB056 is a novel semi-synthetic antimicrobial peptide with a dimeric dendrimer scaffold. Active against both Gram-negative and -positive bacteria, its mechanism has been attributed to a disruption of bacterial membranes. The branched peptide was shown to assume a β-stranded conformation in a lipidic environment. Here, we report on a rational modification of the original, empirically derived linear peptide sequence [WKKIRVRLSA-NH2, SB056-lin]. We interchanged the first two residues [KWKIRVRLSA-NH2, β-SB056-lin] to enhance the amphipathic profile, in the hope that a more regular β-strand would lead to a better antimicrobial performance. MIC values confirmed that an enhanced amphiphilic profile indeed significantly increases activity against both Gram-positive and -negative strains. The membrane binding affinity of both peptides, measured by tryptophan fluorescence, increased with an increasing ratio of negatively charged/zwitterionic lipids. Remarkably, β-SB056-lin showed considerable binding even to purely zwitterionic membranes, unlike the original sequence, indicating that besides electrostatic attraction also the amphipathicity of the peptide structure plays a fundamental role in binding, by stabilizing the bound state. Synchrotron radiation circular dichroism and solid-state 19F-NMR were used to characterize and compare the conformation and mobility of the membrane bound peptides. Both SB056-lin and β-SB056-lin adopt a β-stranded conformation upon binding POPC vesicles, but the former maintains an intrinsic structural disorder that also affects its aggregation tendency. Upon introducing some anionic POPG into the POPC matrix, the sequence-optimized β-SB056-lin forms well-ordered β-strands once electro-neutrality is approached, and it aggregates into more extended β-sheets as the concentration of anionic lipids in the bilayer is raised. The enhanced antimicrobial activity of the analogue correlates with the formation of these extended β-sheets, which also leads to a dramatic alteration of membrane integrity as shown by 31P-NMR. These findings are generally relevant for the design and optimization of other membrane-active antimicrobial peptides that can fold into amphipathic β-strands.
Highlights
Nowadays, many important pathogens have developed multi-drug resistance and are able to evade treatment with multiple antimicrobial classes, covering most, sometimes all, clinically usable antibiotics [1], [2]
Before investigating any effects in the context of the dimeric dendrimer scaffold, here we have focused on a comparison of the two linear analogues (SB056-lin and β-SB056-lin) in order to find out whether the strategy of improving the amphiphilic profile would really enhance the formation of β-stranded structures in lipid bilayers, and whether this feature could be used to improve the antimicrobial activity
The original SB056-lin sequence [WKKIRVRLSA-NH2] has been optimized by inverting the first two residues to give a perfectly regular amphiphilic sequence [KWKIRVRLSA-NH2]. This strategy was based on the expectation that better membrane binding and a more favorable β-strand assembly would lead to enhanced disruption of the lipid bilayer, as has been reported recently for various antimicrobial peptides (AMP) in a vesicle fusion assay combined with circular dichroism (CD) analysis [31]
Summary
Many important pathogens have developed multi-drug resistance and are able to evade treatment with multiple antimicrobial classes, covering most, sometimes all, clinically usable antibiotics [1], [2]. Over the last two decades, much interest has focused on natural compounds known as antimicrobial peptides (AMP) or host defence peptides This is a wide group of molecules expressed by multicellular organisms as effectors of the innate immune system. While conventional antibiotics interact with specific bacterial targets (e.g., enzymes), allowing the pathogens to develop resistance relatively AMPs usually act by physically destroying or permeabilizing the microbial plasma membrane through interactions with the lipids. This makes AMPs and their derivatives suitable as novel antimicrobial drugs, because target substitution/modification, and resistance, is less likely to occur [7], [8]
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