Browsing by Author "Chen, Yen-Fei"

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    Interaction of Daptomycin with Lipid Bilayers Correlated to Its Action on Cell Membranes
    (2013-05-31) Chen, Yen-Fei; Huang, Huey W.; Toffoletto, Frank R.; Killian, Thomas C.
    Daptomycin is a lipopeptide antibiotic notably against multidrug-resistant, gram-positive pathogens. Evidence shows that the antibiotic acts neither on DNA nor on the proteins. It has been shown to insert and aggregate in the bacteria membrane; nevertheless, how the molecular interaction leads to cell death is unknown. In this work, the physical properties of interactions between daptomycin and model membranes are studied in order to understand the underlying mechanism of daptomycin. First, daptomycin’s binding affinity to membranes was found to be proportional to Ca++ concentration. The effect of Ca++ cannot be replaced by other divalent ions such as Mg++. After binding, daptomycin was found to form lipid-peptide aggregations on phosphatidylglycerol(PG)-containing vesicles. PG is required for the formation of lipid-peptide aggregates, which eventually lead to membrane ruptures. Cardiolipin, another main component in bacterial membrane, cannot substitute PG to induce the same membrane defect. In addition, with a fixed concentration of Ca++, it requires a minimum concentration of daptomycin to trigger the membrane defect process. The membrane defect results mainly from lipid-peptide aggregations rather than from pores forming in the membrane. Finally, x-ray data imply that daptomycin binds to the headgroup region of the bilayer, which causes membrane thinning. The elastic energy of membrane thinning elevates the energy level of the daptomycin binding state, which explains the transition to the aggregation state.
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    Interaction of Daptomycin with Lipid Bilayers: A Lipid Extracting Effect
    (American Chemical Society, 2014) Chen, Yen-Fei; Sun, Tzu-Lin; Sun, Yen; Huang, Huey W.
    Daptomycin is the first approved member of a new structural class of antibiotics, the cyclic lipopeptides. The peptide interacts with the lipid matrix of cell membranes, inducing permeability of the membrane to ions, but its molecular mechanism has been a puzzle. Unlike the ubiquitous membrane-acting host-defense antimicrobial peptides, daptomycin does not induce pores in the cell membranes. Thus, how it affects the permeability of a membrane to ions is not clear. We studied its interaction with giant unilamellar vesicles (GUVs) and discovered a lipid-extracting phenomenon that correlates with the direct action of daptomycin on bacterial membranes observed in a recent fluorescence microscopy study. Lipid extraction occurred only when the GUV lipid composition included phosphatidylglycerol and in the presence of Ca2+ ions, the same condition found to be necessary for daptomycin to be effective against bacteria. Furthermore, it occurred only when the peptide/lipid ratio exceeded a threshold value, which could be the basis of the minimal inhibitory concentration of daptomycin. In this first publication on the lipid extracting effect, we characterize its dependence on ions and lipid compositions. We also discuss possibilities for connecting the lipid extracting effect to the antibacterial activity of daptomycin.