Packing behaviour of two predominant anionic phospholipids of bacterial cytoplasmic membranes

In Bacterial Membranes Lipid II Changes the Stability of Pores Formed by the Antimicrobial Peptide Nisin

Resistance to available antibiotics poses a growing challenge to modern medicine, as this often disallows infections to be controlled. This problem can only be alleviated by the development of new drugs. Nisin, a natural lantibiotic with broad antimicrobial activity, has shown promise as a potential candidate for combating antibiotic-resistant bacteria. However, nisin is poorly soluble and barely stable at physiological pH, which despite attempts to address these issues through mutant design has restricted its use as an antibacterial drug. Therefore, gaining a deeper understanding of the antimicrobial effectiveness, which relies in part on its ability to form pores, is crucial for finding innovative ways to manage infections caused by resistant bacteria. Using large-scale molecular dynamics simulations, we find that the bacterial membrane-specific lipid II increases the stability of pores formed by nisin and that the interplay of nisin and lipid II reduces the overall integrity of bacterial membranes by changing the local thickness and viscosity.

Interaction of detergent with complex mimics of bacterial membranes

Detergents are valuable tools to extract membrane proteins for biophysical, biochemical, and structural scrutiny. The detergent-driven solubilization of bilayers made from a single lipid species is commonly described in terms of pseudo-phase diagrams and a three-stage model accounting for three ranges comprising (i) intact vesicles, (ii) vesicle/micelle co-existence, or (iii) mixed micelles. Moreover, the pseudo-phase boundaries thus determined can often be quantitatively rationalized in terms of the molecular shapes of the lipid and the detergent used. Yet, it has remained unclear to what extent this approach can be applied to multi-component lipid membranes that more closely mimic the compositional complexity of cellular membranes. Here, we studied how lipid mixtures composed of palmitoyl oleoyl phosphatidylethanolamine (POPE), palmitoyl oleoyl phosphatidylglycerol (POPG), and tetraoleoyl cardiolipin (TOCL) are solubilized by the commonly used zwitterionic detergent lauryldimethylamine N-oxide using isothermal titration calorimetry. While phase diagrams of the diverse lipid mixtures showed the typical ranges of the three-stage model, we found that POPG-rich POPE/POPG bilayers are more difficult to solubilize than POPG-poor POPE/POPG bilayers. In turn, POPE/POPG/TOCL bilayers became increasingly resistant to detergent with increasing TOCL content. Since POPG is nearly cylindrically shaped and TOCL adopts inverted cone-like shapes under current buffer conditions, our solubilization data do not align with shape-based arguments. Instead, additional electrostatic interactions between lipids and detergents lead to non-additive mixing behavior affecting the resilience of complex lipid bilayers against solubilization.

Specific Binding of the α-Component of the Lantibiotic Lichenicidin to the Peptidoglycan Precursor Lipid II Predetermines Its Antimicrobial Activity

To date, a number of lantibiotics have been shown to use lipid II-a highly conserved peptidoglycan precursor in the cytoplasmic membrane of bacteria-as their molecular target. The α-component (Lchα) of the two-component lantibiotic lichenicidin, previously isolated from the Bacillus licheniformis VK21 strain, seems to contain two putative lipid II binding sites in its N-terminal and C-terminal domains. Using NMR spectroscopy in DPC micelles, we obtained convincing evidence that the C-terminal mersacidin-like site is involved in the interaction with lipid II. These data were confirmed by the MD simulations. The contact area of lipid II includes pyrophosphate and disaccharide residues along with the first isoprene units of bactoprenol. MD also showed the potential for the formation of a stable N-terminal nisin-like complex; however, the conditions necessary for its implementation in vitro remain unknown. Overall, our results clarify the picture of two component lantibiotics mechanism of antimicrobial action.

Investigating lipid headgroup composition within epithelial membranes: a systematic review

Membrane lipid composition is often quoted within the literature, but with very little insight into how or why these compositions vary when compared to other biological membranes. One prominent area that lacks understanding in terms of rationale for lipid variability is the human gastro-intestinal tract (GIT). We have carried out a comprehensive systematic literature search to ascertain the key lipid components of epithelial membranes, with a particular focus on addressing the human GIT and to use compositional data to understand structural aspects of biological membranes. Both bacterial outer membranes and the human erythrocyte membrane were used as a comparison for the mammalian [epithelial] membranes and to understand variations in lipid presence. We show that phosphatidylcholine (PC) lipid types tend to dominate (33%) with phosphatidylethanolamines (PE) and cholesterol having very similar abundances (25 and 23% respectively). This systematic review presents a detailed insight into lipid headgroup composition and roles in various membrane types, with a summary of the distinction between the major lipid bilayer forming lipids and how peripheral lipids regulate charge and fluidity. The variety of lipids present in biological membranes is discussed and rationalised in terms function as well as cellular position.

Structural Organization of Cardiolipin-Containing Vesicles as Models of the Bacterial Cytoplasmic Membrane

The bacterial cytoplasmic membrane is the innermost bacterial membrane and is mainly composed of three different phospholipid species, i.e., phosphoethanolamine (PE), phosphoglycerol (PG), and cardiolipin (CL). In particular, PG and CL are responsible for the negative charge of the membrane and are often the targets of cationic antimicrobial agents. The growing resistance of bacteria toward the available antibiotics requires the development of new and more efficient antibacterial drugs. In this context, studying the physicochemical properties of the bacterial cytoplasmic membrane is pivotal for understanding drug-membrane interactions at the molecular level as well as for designing drug-testing platforms. Here, we discuss the preparation and characterization of PE/PG/CL vesicle suspensions, which contain all of the main lipid components of the bacterial cytoplasmic membrane. The vesicle suspensions were characterized by means of small-angle neutron scattering, dynamic light scattering, and electron paramagnetic spectroscopy. By combining solution scattering and spectroscopy techniques, we propose a detailed description of the impact of different CL concentrations on the structure and dynamics of the PE/PG bilayer. CL induces the formation of thicker bilayers, which exhibit higher curvature and are overall more fluid. The experimental results contribute to shed light on the structure and dynamics of relevant model systems of the bacterial cytoplasmic membrane.

Mixed liposomes containing gram-positive bacteria lipids: Lipoteichoic acid (LTA) induced structural changes

Lipoteichoic acid (LTA), a surface associated polymer amphiphile tethered directly to the Gram-positive bacterial cytoplasmic membrane, is a key structural and functional membrane component. Its composition in the membrane is regulated by bacteria under different physiological conditions. How such LTA compositional variations modulate the membrane structural stability and integrity is poorly understood. Here, we have investigated structural changes in mixed liposomes mimicking the lipid composition of Gram-positive bacteria membranes, in which the concentration of Bacillus Subtilis LTA was varied between 0-15 mol%. Small-angle neutron scattering (SANS) and dynamic light scattering (DLS) measurements indicated formation of mixed unilamellar vesicles, presumably stabilized by the negatively charged LTA polyphosphates. The vesicle size increased with the LTA molar concentration up to ∼6.5 mol%, accompanied by a broadened size distribution, and further increasing the LTA concentration led to a decrease in the vesicle size. At 80 °C, SANS analyses showed the formation of larger vesicles with thinner shells. Complementary Cryo-TEM imaging confirmed the vesicle formation and the size increase with LTA addition, as well as the presence of interconnected spherical aggregates of smaller size at higher LTA concentrations. The results are discussed in light of the steric and electrostatic interactions of the bulky LTA molecules with increased chain fluidity at the higher temperature, which affect the molecular packing and interactions, and thus depend on the LTA composition, in the membrane.

Amphiphilic Aminoglycosides as Medicinal Agents

The conjugation of hydrophobic group(s) to the polycationic hydrophilic core of the antibiotic drugs aminoglycosides (AGs), targeting ribosomal RNA, has led to the development of amphiphilic aminoglycosides (AAGs). These drugs exhibit numerous biological effects, including good antibacterial effects against susceptible and multidrug-resistant bacteria due to the targeting of bacterial membranes. In the first part of this review, we summarize our work in identifying and developing broad-spectrum antibacterial AAGs that constitute a new class of antibiotic agents acting on bacterial membranes. The target-shift strongly improves antibiotic activity against bacterial strains that are resistant to the parent AG drugs and to antibiotic drugs of other classes, and renders the emergence of resistant Pseudomonas aeruginosa strains highly difficult. Structure–activity and structure–eukaryotic cytotoxicity relationships, specificity and barriers that need to be crossed in their development as antibacterial agents are delineated, with a focus on their targets in membranes, lipopolysaccharides (LPS) and cardiolipin (CL), and the corresponding mode of action against Gram-negative bacteria. At the end of the first part, we summarize the other recent advances in the field of antibacterial AAGs, mainly published since 2016, with an emphasis on the emerging AAGs which are made of an AG core conjugated to an adjuvant or an antibiotic drug of another class (antibiotic hybrids). In the second part, we briefly illustrate other biological and biochemical effects of AAGs, i.e., their antifungal activity, their use as delivery vehicles of nucleic acids, of short peptide (polyamide) nucleic acids (PNAs) and of drugs, as well as their ability to cleave DNA at abasic sites and to inhibit the functioning of connexin hemichannels. Finally, we discuss some aspects of structure–activity relationships in order to explain and improve the target selectivity of AAGs.

Environmental and dynamic effects explain how nisin captures membrane-bound lipid II

Antibiotics (AB) resistance is a major threat to global health, thus the development of novel AB classes is urgently needed. Lantibiotics (i.e. nisin) are natural compounds that effectively control bacterial populations, yet their clinical potential is very limited. Nisin targets membrane-embedded cell wall precursor - lipid II - via capturing its pyrophosphate group (PPi), which is unlikely to evolve, and thus represents a promising pharmaceutical target. Understanding of exact molecular mechanism of initial stages of membrane-bound lipid II recognition by water-soluble nisin is indispensable. Here, using molecular simulations, we demonstrate that the structure of lipid II is determined to a large extent by the surrounding water-lipid milieu. In contrast to the bulk solvent, in the bilayer only two conformational states remain capable of nisin binding. In these states PPi manifests a unique arrangement of hydrogen bond acceptors on the bilayer surface. Such a "pyrophosphate pharmacophore" cannot be formed by phospholipids, which explains high selectivity of nisin/lipid II recognition. Similarly, the "recognition module" of nisin, being rather flexible in water, adopts the only stable conformation in the presence of PPi analogue (which mimics the lipid II molecule). We establish the "energy of the pyrophosphate pharmacophore" approach, which effectively distinguishes nisin conformations that can form a complex with PPi. Finally, we propose a molecular model of nisin recognition module/lipid II complex in the bacterial membrane. These results will be employed for further study of lipid II targeting by antimicrobial (poly)cyclic peptides and for design of novel AB prototypes.

Interaction of the antibiotic peptide nisin with anionic membranes in different phase-states: a vibrational study

Interactions between the antibiotic peptide nisin and multilamellar vesicles of phosphoglycerol lipids in different phase-states were studied using vibrational spectroscopy. The infrared amide I' band of nisin, both in solution and in the membrane-bound state, was analyzed in the temperature range comprised between 20 and 60 °C in order to study its conformational behavior. Nisin presented mainly unordered and β-turns conformations. Their relative populations varied according to the environment and as the temperature increased: β turns were more favored in the membrane-bound state than in solution, but at higher temperatures the disordered conformation was dominant in both states. Spectral changes of specific infrared bands belonging to the hydrocarbon and polar moieties of lipids were also analyzed to evaluate the perturbation of the lipid membrane order. Nisin interactions with the membrane polar region induced a high restriction to water incorporation, promoting a small increase in the temperature of the lipid phase transition. Raman spectra of nisin/phosphoglycerol systems at ambient temperature were also analyzed. They revealed that the peptide incorporation to a membrane in the fluid phase caused drastic structural modifications in the hydrophobic region of the bilayer. Although nisin may be able to disrupt the hydrophobic portion of the bilayer in the gel phase, the most of the peptide molecule remained at the membrane surface interacting with the polar headgroups. This work provides evidence of a differential effect of nisin on anionic membranes, depending on the phase-state of the lipid.

The role of cardiolipin concentration and acyl chain composition on mitochondrial inner membrane molecular organization and function

Cardiolipin (CL) is a key phospholipid of the mitochondria. A loss of CL content and remodeling of CL's acyl chains is observed in several pathologies. Strong shifts in CL concentration and acyl chain composition would presumably disrupt mitochondrial inner membrane biophysical organization. However, it remains unclear in the literature as to which is the key regulator of mitochondrial membrane biophysical properties. We review the literature to discriminate the effects of CL concentration and acyl chain composition on mitochondrial membrane organization. A widely applicable theme emerges across several pathologies, including cardiovascular diseases, diabetes, Barth syndrome, and neurodegenerative ailments. The loss of CL, often accompanied by increased levels of lyso-CLs, impairs mitochondrial inner membrane organization. Modest remodeling of CL acyl chains is not a major driver of impairments and only in cases of extreme remodeling is there an influence on membrane properties.

Could Cardiolipin Protect Membranes against the Action of Certain Antimicrobial Peptides? Aurein 1.2, a Case Study

The activity of a host of antimicrobial peptides has been examined against a range of lipid bilayers mimicking bacterial and eukaryotic membranes. Despite this, the molecular mechanisms and the nature of the physicochemical properties underlying the peptide-lipid interactions that lead to membrane disruption are yet to be fully elucidated. In this study, the interaction of the short antimicrobial peptide aurein 1.2 was examined in the presence of an anionic cardiolipin-containing lipid bilayer using molecular dynamics simulations. Aurein 1.2 is known to interact strongly with anionic lipid membranes. In the simulations, the binding of aurein 1.2 was associated with buckling of the lipid bilayer, the degree of which varied with the peptide concentration. The simulations suggest that the intrinsic properties of cardiolipin, especially the fact that it promotes negative membrane curvature, may help protect membranes against the action of peptides such as aurein 1.2 by counteracting the tendency of the peptide to induce positive curvature in target membranes.

Structure of Lung-Mimetic Multilamellar Bodies with Lipid Compositions Relevant in Pneumonia

The hierarchical assembly of lipids, as modulated by composition and environment, plays a significant role in the function of biological membranes and a myriad of diseases. Elevated concentrations of calcium ions and cardiolipin (CL), an anionic tetra-alkyl lipid found in mitochondria and some bacterial cell membranes, have been implicated in pneumonia recently. However, their impact on the physicochemical properties of lipid assemblies in lungs and how it impairs alveoli function is still unknown. We use small- and wide-angle X-ray scattering (S/WAXS) and solid-state nuclear magnetic resonance (ssNMR) to probe the structure and dynamics of lung-mimetic multilamellar bodies (MLBs) in the presence of Ca²⁺ and CL. We conjecture that CL overexpressed in the hypophase of alveoli strongly affects the structure of lung-lipid bilayers and their stacking in the MLBs. Specifically, S/WAXS data revealed that CL induces significant shrinkage of the water-layer separating the concentric bilayers in multilamellar aggregates. ssNMR measurements indicate that this interbilayer tightening is due to undulation repulsion damping as CL renders the glycerol backbone of the membranes significantly more static. In addition to MLB dehydration, CL promotes intrabilayer phase separation into saturated-rich and unsaturated-rich lipid domains that couple across multiple layers. Expectedly, addition of Ca²⁺ screens the electrostatic repulsion between negatively charged lung membranes. However, when CL is present, addition of Ca²⁺ results in an apparent interbilayer expansion likely due to local structural defects. Combining S/WAXS and ssNMR on systems with compositions pertinent to healthy and unhealthy lung membranes, we propose how alteration of the physiochemical properties of MLBs can critically impact the breathing cycle.

Antimicrobial Peptides Targeting Gram-Positive Bacteria

Antimicrobial peptides (AMPs) have remarkably different structures as well as biological activity profiles, whereupon most of these peptides are supposed to kill bacteria via membrane damage. In order to understand their molecular mechanism and target cell specificity for Gram-positive bacteria, it is essential to consider the architecture of their cell envelopes. Before AMPs can interact with the cytoplasmic membrane of Gram-positive bacteria, they have to traverse the cell wall composed of wall- and lipoteichoic acids and peptidoglycan. While interaction of AMPs with peptidoglycan might rather facilitate penetration, interaction with anionic teichoic acids may act as either a trap for AMPs or a ladder for a route to the cytoplasmic membrane. Interaction with the cytoplasmic membrane frequently leads to lipid segregation affecting membrane domain organization, which affects membrane permeability, inhibits cell division processes or leads to delocalization of essential peripheral membrane proteins. Further, precursors of cell wall components, especially the highly conserved lipid II, are directly targeted by AMPs. Thereby, the peptides do not inhibit peptidoglycan synthesis via binding to proteins like common antibiotics, but form a complex with the precursor molecule, which in addition can promote pore formation and membrane disruption. Thus, the multifaceted mode of actions will make AMPs superior to antibiotics that act only on one specific target.

Interaction of the Antimicrobial Peptides Rhesus θ-Defensin and Porcine Protegrin-1 with Anionic Phospholipid Monolayers

A combination of Langmuir isotherm, Brewster angle microscopy (BAM), and neutron reflectivity studies have been performed to gain insight into the effects on model bacterial cell membranes of the antimicrobial peptides, Rhesus θ-defensin 1 (RTD-1), and porcine protegrin 1 (PG-1). The peptides were interacted with monolayers spread at the air-water interface and prepared from a 3:1 molar mixture of phosphatidylethanolamine and phosphatidylglycerol used to approximate the cell membranes of Gram positive bacteria. The Langmuir film balance measurements show that both peptides perturb the lipid monolayers causing an increase in surface pressure, and the BAM studies show that each results in the formation of small domains within the lipid films, around 5 μm diameter. The overall change in monolayer surface pressure caused by PG-1, however, is a little more pronounced than that due to RTD-1 (+8.5 mN·m(-1) vs +5.5 mN·m(-1)), and the rate of its initial interaction with the monolayer is a little more rapid than that for RTD-1. The neutron reflectivity studies also show differences for PG-1 and RTD-1, with the model fits to these data showing that the more amphiphilic PG-1 becomes fully embedded within the lipid film-causing an extension of the lipid acyl chains but leaving the thickness of the lipid headgroup layer unaffected-while RTD-1 is seen to insert less deeply-causing the same extension of the lipid acyl chains as PG-1 but also causing a significant increase in thickness of the lipid headgroup layer. The various differing effects of the two peptides on anionic lipid monolayers are discussed in the context of their differing hemolytic activities, and their proposed differing propensities to form transmembrane pores.

Effects of elevated growth temperature and heat shock on the lipid composition of the inner and outer membranes of Yersinia pseudotuberculosis

Differences in the distribution of individual phospholipids between the inner (IM) and outer membranes (OM) of gram-negative bacteria have been detected in mesophilic Escherichia, Erwinia and Salmonella species but have never been investigated in the psychrotrophic Yersinia genus. Therefore, the influence of an elevated growth temperature and heat shock on the phospholipid and fatty acid (FA) compositions of the fractionated Yersinia pseudotuberculosis envelope was investigated. The shift of the growth temperature from 8 °C to 37 °C to mimic the switch from saprophytic to parasitic growth of this bacteria and the exposure of the cells to heat shock, which was induced by a sharp increase in the temperature from 8 °C to 45 °C, increased the lysophosphatidylethanolamine content from zero and 1% to 6% and 10% in the IM and OM, respectively. These changes were accompanied by a decrease in the phosphatidylethanolamine (PE) content and a drastic increase (up to 3-fold higher) in the phosphatidylglycerol (PG) level in the OM of the bacteria, which increases the net negative charge of the cell envelope. The levels of the predominant saturated palmitic (16:0) and cyclopropane FAs were approximately 1.5- and 7.5-fold higher, respectively, but the content of the predominant unsaturated palmitoleic (16:1n-7) and cis-vaccenic (18:1n-7) FAs was approximately 10-30-fold lower in both membranes that were isolated from the cells grown at elevated temperatures. Due to these changes, reflecting the process of "homeoviscous adaptation", the ratio between the unsaturated and saturated FAs decreased but remained higher in the IM than that in the OM. Simultaneously, no significant changes were observed in the FA composition of cells subjected to heat shock, demonstrating a difference between the responses of the heat-shocked and heat-adapted Y. pseudotuberculosis. The unique ability of Y. pseudotuberculosis to reciprocally regulate the ratio of anionic PG and net neutral PE and therefore adjust the negative charge of the OM may be a common strategy used by pathogenic bacteria to promote the barrier function of the OM.

On the miscibility of cardiolipin with 1,2-diacyl phosphoglycerides: Binary mixtures of dimyristoylphosphatidylglycerol and tetramyristoylcardiolipin

The thermotropic phase behavior and organization of model membranes composed of binary mixtures of the quadruple-chained, nominally dianionic phospholipid tetramyristoylcardiolipin (TMCL) with the double-chained, monoanionic phospholipid dimyristoylphosphatidylglycerol (DMPG) were examined by differential scanning calorimetry (DSC) and Fourier-transform infrared (FTIR) spectroscopy. The gel/liquid-crystalline phase transitions observed in these mixtures by DSC are generally rather broad and exhibit complex endotherms over a range of compositions. However, the phase transition temperatures and enthalpies exhibit nearly ideal behavior. Also, FTIR spectroscopic detection of the formation of stable and metastable DMPG-like lamellar crystalline (Lc) phases only at high DMPG levels upon low temperature annealing, and stable TMCL-like Lc phases at all higher TMCL concentrations, indicates that at low temperatures, laterally segregated domains of these two phospholipids must form, from which these different Lc phases nucleate and grow. Comparison of these results with those of a previous study of DMPE/TMCL mixtures (Frias et al., 2011) indicates that DMPG mixes slightly less well with TMCL than DMPE, perhaps because of the negative charge of the latter. However, in both binary mixtures, TMCL inhibits the formation of the Lc phase by DMPE even more strongly than for DMPG. Overall, our data suggest that TMCL and DMPG actually mix well across a broad temperature and composition range when the fatty acid chains of the two components are identical and only a modest (~17°C) difference between their Lβ/Lα phase transition temperatures exists. A recent DSC and X-ray diffraction study of DPPG/TMCL mixtures report similar results (Prossnigg et al., 2010).

Response mechanisms of bacterial degraders to environmental contaminants on the level of cell walls and cytoplasmic membrane

Bacterial strains living in the environment must cope with the toxic compounds originating from humans production. Surface bacterial structures, cell wall and cytoplasmic membrane, surround each bacterial cell and create selective barriers between the cell interior and the outside world. They are a first site of contact between the cell and toxic compounds. Organic pollutants are able to penetrate into cytoplasmic membrane and affect membrane physiological functions. Bacteria had to evolve adaptation mechanisms to counteract the damage originated from toxic contaminants and to prevent their accumulation in cell. This review deals with various adaptation mechanisms of bacterial cell concerning primarily the changes in cytoplasmic membrane and cell wall. Cell adaptation maintains the membrane fluidity status and ratio between bilayer/nonbilayer phospholipids as well as the efflux of toxic compounds, protein repair mechanisms, and degradation of contaminants. Low energy consumption of cell adaptation is required to provide other physiological functions. Bacteria able to survive in toxic environment could help us to clean contaminated areas when they are used in bioremediation technologies.

N-acylated peptides derived from human lactoferricin perturb organization of cardiolipin and phosphatidylethanolamine in cell membranes and induce defects in Escherichia coli cell division

Two types of recently described antibacterial peptides derived from human lactoferricin, either nonacylated or N-acylated, were studied for their different interaction with membranes of Escherichia coli in vivo and in model systems. Electron microscopy revealed striking effects on the bacterial membrane as both peptide types induced formation of large membrane blebs. Electron and fluorescence microscopy, however demonstrated that only the N-acylated peptides partially induced the generation of oversized cells, which might reflect defects in cell-division. Further a different distribution of cardiolipin domains on the E. coli membrane was shown only in the presence of the N-acylated peptides. The lipid was distributed over the whole bacterial cell surface, whereas cardiolipin in untreated and nonacylated peptide-treated cells was mainly located at the septum and poles. Studies with bacterial membrane mimics, such as cardiolipin or phosphatidylethanolamine revealed that both types of peptides interacted with the negatively charged lipid cardiolipin. The nonacylated peptides however induced segregation of cardiolipin into peptide-enriched and peptide-poor lipid domains, while the N-acylated peptides promoted formation of many small heterogeneous domains. Only N-acylated peptides caused additional severe effects on the main phase transition of liposomes composed of pure phosphatidylethanolamine, while both peptide types inhibited the lamellar to hexagonal phase transition. Lipid mixtures of phosphatidylethanolamine and cardiolipin revealed anionic clustering by all peptide types. However additional strong perturbation of the neutral lipids was only seen with the N-acylated peptides. Nuclear magnetic resonance demonstrated different conformational arrangement of the N-acylated peptide in anionic and zwitterionic micelles revealing possible mechanistic differences in their action on different membrane lipids. We hypothesized that both peptides kill bacteria by interacting with bacterial membrane lipids but only N-acylated peptides interact with both charged cardiolipin and zwitterionic phosphatidylethanolamine resulting in remodeling of the natural phospholipid domains in the E. coli membrane that leads to defects in cell division.

X-ray structure, thermodynamics, elastic properties and MD simulations of cardiolipin/dimyristoylphosphatidylcholine mixed membranes

Cardiolipins (CLs) are important biologically for their unique role in biomembranes that couple phosphorylation and electron transport like bacterial plasma membranes, chromatophores, chloroplasts and mitochondria. CLs are often tightly coupled to proteins involved in oxidative phosphorylation. The first step in understanding the interaction of CL with proteins is to obtain the pure CL structure, and the structure of mixtures of CL with other lipids. In this work we use a variety of techniques to characterize the fluid phase structure, material properties and thermodynamics of mixtures of dimyristoylphosphatidylcholine (DMPC) with tetramyristoylcardiolipin (TMCL), both with 14-carbon chains, at several mole percentages. X-ray diffuse scattering was used to determine structure, including bilayer thickness and area/lipid, the bending modulus, KC, and SXray, a measure of chain orientational order. Our results reveal that TMCL thickens DMPC bilayers at all mole percentages, with a total increase of ∼6 Å in pure TMCL, and increases AL from 64 Å(2) (DMPC at 35 °C) to 109 Å(2) (TMCL at 50 °C). KC increases by ∼50%, indicating that TMCL stiffens DMPC membranes. TMCL also orders DMPC chains by a factor of ∼2 for pure TMCL. Coarse grain molecular dynamics simulations confirm the experimental thickening of 2 Å for 20mol% TMCL and locate the TMCL headgroups near the glycerol-carbonyl region of DMPC; i.e., they are sequestered below the DMPC phosphocholine headgroup. Our results suggest that TMCL plays a role similar to cholesterol in that it thickens and stiffens DMPC membranes, orders chains, and is positioned under the umbrella of the PC headgroup. CL may be necessary for hydrophobic matching to inner mitochondrial membrane proteins. Differential scanning calorimetry, SXray and CGMD simulations all suggest that TMCL does not form domains within the DMPC bilayers. We also determined the gel phase structure of TMCL, which surprisingly displays diffuse X-ray scattering, like a fluid phase lipid. AL=40.8 Å(2) for the ½TMCL gel phase, smaller than the DMPC gel phase with AL=47.2 Å(2), but similar to AL of DLPE=41 Å(2), consistent with untilted chains in gel phase TMCL.

Lipid-II forms potential "landing terrain" for lantibiotics in simulated bacterial membrane

Bacterial cell wall is targeted by many antibiotics. Among them are lantibiotics, which realize their function via interaction with plasma membrane lipid-II molecule — a chemically conserved part of the cell wall synthesis pathway. To investigate structural and dynamic properties of this molecule, we have performed a series of nearly microsecond-long molecular dynamics simulations of lipid-II and some of its analogs in zwitterionic single component and charged mixed simulated phospholipid bilayers (the reference and the mimic of the bacterial plasma membrane, respectively). Extensive analysis revealed that lipid-II forms a unique “amphiphilic pattern” exclusively on the surface of the simulated bacterial membrane (and not in the reference one). We hypothesize that many lantibiotics exploit the conserved features of lipid-II along with characteristic modulation of the bacterial membrane as the “landing site”. This putative recognition mechanism opens new opportunities for studies on lantibiotics action and design of novel armament against resistant bacterial strains.

Molecular structures of fluid phase phosphatidylglycerol bilayers as determined by small angle neutron and X-ray scattering

We have determined the molecular structures of commonly used phosphatidylglycerols (PGs) in the commonly accepted biologically relevant fluid phase. This was done by simultaneously analyzing small angle neutron and X-ray scattering data, with the constraint of measured lipid volumes. We report the temperature dependence of bilayer parameters obtained using the one-dimensional scattering density profile model - which was derived from molecular dynamics simulations - including the area per lipid, the overall bilayer thickness, as well as other intrabilayer parameters (e.g., hydrocarbon thickness). Lipid areas are found to be larger than their phosphatidylcholine (PC) counterparts, a result likely due to repulsive electrostatic interactions taking place between the charged PG headgroups even in the presence of sodium counterions. In general, PG and PC bilayers show a similar response to changes in temperature and chain length, but differ in their response to chain unsaturation. For example, compared to PC bilayers, the inclusion of a first double bond in PG lipids results in a smaller incremental change to the area per lipid and bilayer thickness. However, the extrapolated lipid area of saturated PG lipids to infinite chain length is found to be similar to that of PCs, an indication of the glycerol-carbonyl backbone's pivotal role in influencing the lipid-water interface.

Phase transitions and spatially ordered counterion association in ionic-lipid membranes: theory versus experiment

Aqueous dispersions of phosphatidylglycerol (PG) lipids may present an anomalous chain-melting transition at low ionic strengths, as seen by different experimental techniques such as calorimetry or light scattering. The anomaly disappears at high ionic strengths or for longer acyl-chain lengths. In this article, we use a statistical model for the bilayer that distinguishes both lipid chain and headgroup states in order to compare model and experimental thermotropic and electrical properties. The effective van der Waals interactions among hydrophobic chains compete with the electrostatic repulsions between polar headgroups, which may be ionized (counterion dissociated) or electrically neutral (associated with counterions). Electric degrees of freedom introduce new thermotropic charge-ordered phases in which headgroup charges may be spatially ordered, depending on the electrolyte ionic strength, introducing a new rationale for experimental data on PGs. The thermal phases presented by the model for different chain lengths, at fixed ionic strength, compare well with an experimental phase diagram constructed on the basis of differential scanning calorimetry profiles. In the case of dispersions of DMPG (dimyristoyl phosphatidylglycerol) with added monovalent salt, the model properties reproduce the main features displayed by data from differential scanning calorimetry as well as the characteristic profile for the degree of ionization of the bilayer surface across the anomalous transition region, obtained from the theoretical interpretation of electrokinetic (conductivity and electrophoretic mobility) measurements.

Interactions of biocidal guanidine hydrochloride polymer analogs with model membranes: a comparative biophysical study

Four synthesized biocidal guanidine hydrochloride polymers with different alkyl chain length, including polyhexamethylene guanidine hydrochloride and its three new analogs, were used to investigate their interactions with phospholipids vesicles mimicking bacterial membrane. Characterization was conducted by using fluorescence dye leakage, isothermal titration calorimetry, and differential scanning calorimetry. The results showed that the gradually lengthened alkyl chain of the polymer increased the biocidal activity, accompanied with the increased dye leakage rate and the increased binding constant and energy change value of polymer-membrane interaction. The polymer-membrane interaction induced the change of pretransition and main phase transition (decreased temperature and increased width) of phospholipids vesicles, suggesting the conformational change in the phospholipids headgroups and disordering in the hydrophobic regions of lipid membranes. The above information revealed that the membrane disruption actions of guanidine hydrochloride polymers are the results of the polymer's strong binding to the phospholipids membrane and the subsequent perturbations of the polar headgroups and hydrophobic core region of the phospholipids membrane. The alkyl chain structure significantly affects the binding constant and energy change value of the polymer-membrane interactions and the perturbation extent of the phospholipids membrane, which lead to the different biocidal activity of the polymer analogs. This work provides important information about the membrane disruption action mechanism of biocidal guanidine hydrochloride polymers.