NMR studies of lantibiotics. The structure of nisin in aqueous solution

Substrate Specificity of a Methyltransferase Involved in the Biosynthesis of the Lantibiotic Cacaoidin

Modification of the N- and C-termini of peptides enhances their stability against degradation by exopeptidases. The biosynthetic pathways of many peptidic natural products feature enzymatic modification of their termini, and these enzymes may represent a valuable pool of biocatalysts. The lantibiotic cacaoidin carries an N,N-dimethylated N-terminal amine group. Its biosynthetic gene cluster encodes the putative methyltransferase Cao4. In this work, we present reconstitution of the activity of the enzyme, which we termed CaoSC following standardized lanthipeptide nomenclature, using a heterologously produced peptide as the model substrate. In vitro methylation of diverse lanthipeptides revealed the substrate requirements of CaoSC. The enzyme accepts peptides of varying lengths and C-terminal sequences but requires dehydroalanine or dehydrobutyrine at the second position. CaoSC-mediated dimethylation of natural lantibiotics resulted in modestly enhanced antimicrobial activity of the lantibiotic haloduracin compared to that of the native compound. Improved activity and/or metabolic stability as a result of methylation illustrates the potential future application of CaoSC in the bioengineering of therapeutic peptides.

Synthesis, antimicrobial activity, and mechanistic studies of enterocin DD14, a leaderless two-peptide bacteriocin

Bacteriocins are promising alternatives to antibiotics in the food, veterinary and medical sectors, but their study and use is often hampered by the low yields and high costs associated with their purification from naturally occurring bacteria. Chemical synthesis has emerged as a means to overcome this limitation and design more active variants. In this study, microwave-assisted solid-phase peptide synthesis was used to produce the leaderless two-peptide bacteriocin enterocin DD14 (EntDD14), composed of EntDD14A (44 amino acids) and EntDD14B (43 amino acids). The resulting synthetic peptides, syn-EntDD14A and syn-EntDD14B, were tested against Gram-positive bacteria including Listeria, Staphylococcus and Enterococcus strains. Both peptides were found to be necessary for optimal, but not synergistic, antibacterial activity and to act through a pore-forming mechanism. Both peptides exhibited moderate cytotoxicity against eukaryotic cells.

A plant endophyte Staphylococcus hominis strain MBL_AB63 produces a novel lantibiotic, homicorcin and a position one variant

Here we report a jute endophyte Staphylococcus hominis strain MBL_AB63 isolated from jute seeds which showed promising antimicrobial activity against Staphylococcus aureus SG511 when screening for antimicrobial substances. The whole genome sequence of this strain, annotated using BAGEL4 and antiSMASH 5.0 to predict the gene clusters for antimicrobial substances identified a novel antimicrobial peptide cluster that belongs to the class I lantibiotic group. The predicted lantibiotic (homicorcin) was found to be 82% similar to a reported peptide epicidin 280 having a difference of seven amino acids at several positions of the core peptide. Two distinct peaks obtained at close retention times from a RP-HPLC purified fraction have comparable antimicrobial activities and LC-MS revealed the molecular mass of these peaks to be 3046.5 and 3043.2 Da. The presence of an oxidoreductase (homO) similar to that of epicidin 280- associated eciO or epilancin 15X- associated elxO in the homicorcin gene cluster is predicted to be responsible for the reduction of the first dehydrated residue dehydroalanine (Dha) to 2-hydroxypropionate that causes an increase of 3 Da mass of homicorcin 1. Trypsin digestion of the core peptide and its variant followed by ESI-MS analysis suggests the presence of three ring structures, one in the N-terminal and other two interlocking rings at the C-terminal region that remain undigested. Homicorcin exerts bactericidal activity against susceptible cells by disrupting the integrity of the cytoplasmic membrane through pore formation as observed under FE-SEM.

Studies of the highly potent lantibiotic peptide nisin Z in aqueous solutions of salts and biological buffer components

The lantibiotic nisin, usually used as a 2.5%w/w in NaCl and milk solids, has activity against a wide range of Gram-positive bacteria, especially food-borne pathogens, and has been used as a food preservative for decades without the development of significant resistance. It has been reported that the high purity (>95%) nisin Z form has activity against the Gram-negative speciesE. coli, which is significantly reduced in the presence of NaCl. This current study examined, by¹H NMR spectroscopy, the effects of NaCl, and a range of other salts, on the observed aqueous solution¹H NMR spectra of nisin Z in the pH 3-4 range, where nisin Z has its maximum stability. Nisin's mechanism of action involves binding to the polyoxygenated pyrophosphate moiety of lipid II, and in acidic solution the positively charged C-terminus region is reported to interact with the negative sulfate groups of SDS micelles, so the study was extended to include a number of polyoxygenated anions commonly used as buffers in many biological assays. In general, the biggest changes found were in the chemical shifts of protons in the hydrophobic N-terminus region, rather than the more polar C-terminus region. The effects seen on the addition of the salts (cations and anions) were not just an overall non-specific ionic strength effect, as different salts caused different effects, in an unpredictive manner. Similarly, the polyoxygenated anions behaved differently and not predictably, and neither the cations/anions, or polyoxygenated anions, constitute a Hofmeister or inverse Hofmeister series.

Isolation, Characterization and Structure Elucidation of a Novel Lantibiotic From Paenibacillus sp

We have isolated and characterized a novel antibacterial peptide, CMB001, following an extensive screening effort of bacterial species isolated from diverse environmental sources. The bacterium that produces CMB001 is characterized as a Gram (+) bacillus sharing approximately 98.9% 16S rRNA sequence homology with its closest match, Paenibacillus kyungheensis. The molecule has been purified to homogeneity from its cell-free supernatant by a three-step preparative chromatography process. Based on its primary structure, CMB001 shares 81% identity with subtilin and 62% with nisin. CMB001 is active mainly against Gram-positive bacteria and Mycobacteriaceae but it is also active against certain Gram-negative bacteria, including multi-drug resistant Acinetobacter baumannii. It retains full antibacterial activity at neutral pH and displays a low propensity to select for resistance among targeted bacteria. Based on NMR and mass spectrometry, CMB001 forms a unique 3D-structure comprising of a compact backbone with one α-helix and two pseudo-α-helical regions. Screening the structure against the Protein Data Bank (PDB) revealed a partial match with nisin-lipid II (1WCO), but none of the lantibiotics with known structures showed significant structural similarity. Due to its unique structure, resistance profile, relatively broad spectrum and stability under physiological conditions, CMB001 is a promising drug candidate for evaluation in animal models of bacterial infection.

A Chemical Biology Approach to Understanding Molecular Recognition of Lipid II by Nisin(1-12): Synthesis and NMR Ensemble Analysis of Nisin(1-12) and Analogues

Natural products that target lipid II, such as the lantibiotic nisin, are strategically important in the development of new antibacterial agents to combat the rise of antimicrobial resistance. Understanding the structural factors that govern the highly selective molecular recognition of lipid II by the N-terminal region of nisin, nisin(1-12), is a crucial step in exploiting the potential of such compounds. In order to elucidate the relationships between amino acid sequence and conformation of this bicyclic peptide fragment, we have used solid-phase peptide synthesis to prepare two novel analogues of nisin(1-12) in which the dehydro residues have been replaced. We have carried out an NMR ensemble analysis of one of these analogues and of the wild-type nisin(1-12) peptide in order to compare the conformations of these two bicyclic peptides. Our analysis has shown the effects of residue mutation on ring conformation. We have also demonstrated that the individual rings of nisin(1-12) are pre-organised to an extent for binding to the pyrophosphate group of lipid II, with a high degree of flexibility exhibited in the central amide bond joining the two rings.

Molecular Recognition of Lipid II by Lantibiotics: Synthesis and Conformational Studies of Analogues of Nisin and Mutacin Rings A and B

In response to the growing threat posed by antibiotic-resistant bacterial strains, extensive research is currently focused on developing antimicrobial agents that target lipid II, a vital precursor in the biosynthesis of bacterial cell walls. The lantibiotic nisin and related peptides display unique and highly selective binding to lipid II. A key feature of the nisin-lipid II interaction is the formation of a cage-like complex between the pyrophosphate moiety of lipid II and the two thioether-bridged rings, rings A and B, at the N-terminus of nisin. To understand the important structural factors underlying this highly selective molecular recognition, we have used solid-phase peptide synthesis to prepare individual ring A and B structures from nisin, the related lantibiotic mutacin, and synthetic analogues. Through NMR studies of these rings, we have demonstrated that ring A is preorganized to adopt the correct conformation for binding lipid II in solution and that individual amino acid substitutions in ring A have little effect on the conformation. We have also analyzed the turn structures adopted by these thioether-bridged peptides and show that they do not adopt the tight α-turn or β-turn structures typically found in proteins.

A novel bioengineered derivative of nisin displays enhanced antimicrobial activity against clinical Streptococcus agalactiae isolates

OBJECTIVES

Streptococcus agalactiae is the leading cause of neonatal disease worldwide, and infections caused by this opportunistic pathogen are becoming increasingly more prevalent in adults. With the global incidence of antimicrobial resistance continuing to rise, there is a recognised need for new therapeutic agents. Nisin is a potent antimicrobial peptide with demonstrated broad-spectrum activity against a range of clinically significant pathogens. This study aimed to examine the efficacy of nisin against a clinical population of S. agalactiae isolates and further to investigate the bioactivity of a novel bioengineered derivative of the peptide, designated nisin PV.

METHODS

A deferred antagonism assay was used to assess the bioactivity of wild-type nisin and nisin PV against 122 S. agalactiae isolates. Minimum inhibitory concentrations (MICs) were evaluated to determine the specific activity of both peptides. The genetic basis of nisin resistance among the isolate collection was investigated by PCR detection of the nsr gene.

RESULTS

In total, 91.0% (111/122) of the collection showed some level of susceptibility to nisin, whilst 9.0% (11/122) displayed complete resistance. Interestingly, the nisin derivative exhibited enhanced antimicrobial activity for 64.8% of the isolates. The frequency of the nsr gene conferring nisin resistance was 98.4% (120/122), suggesting that resistance may be linked to levels of expression of the protein or other regulatory elements.

CONCLUSION

This study indicates that there is potential for the use of nisin and its derivatives as therapeutic agents against S. agalactiae infections.

The expanding structural variety among bacteriocins from Gram-positive bacteria

Bacteria use various strategies to compete in an ecological niche, including the production of bacteriocins. Bacteriocins are ribosomally synthesized antibacterial peptides, and it has been postulated that the majority of Gram-positive bacteria produce one or more of these natural products. Bacteriocins can be used in food preservation and are also considered as potential alternatives to antibiotics. The majority of bacteriocins from Gram-positive bacteria had been traditionally divided into two major classes, namely lantibiotics, which are post-translationally modified bacteriocins, and unmodified bacteriocins. The last decade has seen an expanding number of ribosomally synthesized and post-translationally modified peptides (RiPPs) in Gram-positive bacteria that have antibacterial activity. These include linear azol(in)e-containing peptides, thiopeptides, bottromycins, glycocins, lasso peptides and lipolanthines. In addition, the three-dimensional (3D) structures of a number of modified and unmodified bacteriocins have been elucidated in recent years. This review gives an overview on the structural variety of bacteriocins from Gram-positive bacteria. It will focus on the chemical and 3D structures of these peptides, and their interactions with receptors and membranes, structure-function relationships and possible modes of action.

Stoichiometry and structure of a lantibiotic maturation complex

Lantibiotics are ribosomally synthesized antimicrobial peptides secreted by mainly Gram-positive bacteria. Class 1 lantibiotics mature via two modification steps introduced by a modification LanBC complex. For the lantibiotic nisin, the dehydratase NisB catalyzes the dehydration of serine and threonine residues in the so-called core peptide. Second, five (methyl)-lanthionine rings are introduced in a regio- and stereospecific manner by the cyclase NisC. Here, we characterized the assembly of the NisBC complex in vitro, which is only formed in the presence of the substrate. The complex is composed of a NisB dimer, a monomer of NisC and one prenisin molecule. Interestingly, the presence of the last lanthionine ring prevented complex formation. This stoichiometry was verified by small-angle X-ray scattering measurements, which revealed the first structural glimpse of a LanBC complex in solution.

Revisión: Espectro antimicrobiano, estructura, propiedades y mode de acción de la nisina, una bacteriocina producida por Lactococcus lactis/Review: Antimicrobial spectrum, structure, properties and mode of action of nisin, a bacteriocin produced by Lactococcus lactis

Nisin is a 34 amino acid antibacterial peptide produced by certain strains of Lactococcus lactis. This bacteriocin has found wide application as a food preservative owing to its non-toxic nature, its heat stability at acidic pH, its inactivation by proteolytic enzymes in the digestive tract and, especially, to its antimicrobial activity against a broad range of Gram-positive organisms, including food pathogens of concern in food industry such as Clostridium botulinum and Listeria monocytogenes. However, the use of nisin has the limitation that its solubility and stability decrease progressively as the environ mental pH increases. The two natural variants of nisin, named nisin A and nisin Z, are ribosomally synthesized as 57 amino acid precursor peptides which are subjected to further modifications. The mature peptide displays several unusual features, such as the presence of dehydrated amino acids and lanthionine rings. Insertion of the peptide into the cytoplasmic membrane of susceptible cells leads to the formation of pores, dissipating the membrane potential and pH gradients.

Docking and molecular dynamics simulations of the ternary complex nisin2:lipid II

Lanthionine antibiotics are an important class of naturally-occurring antimicrobial peptides. The best-known, nisin, is a commercial food preservative. However, structural and mechanistic details on nisin-lipid II membrane complexes are currently lacking. Recently, we have developed empirical force-field parameters to model lantibiotics. Docking and molecular dynamics (MD) simulations have been used to study the nisin₂:lipid II complex in bacterial membranes, which has been put forward as the building block of nisin/lipid II binary membrane pores. An Ile1Trp mutation of the N-terminus of nisin has been modelled and docked onto lipid II models; the computed binding affinity increased compared to wild-type. Wild-type nisin was also docked onto three different lipid II structures and a stable 2:1 nisin:lipid II complex formed. This complex was inserted into a membrane. Six independent MD simulations revealed key interactions in the complex, specifically the N-terminal engagement of nisin with lipid II at the pyrophosphate and C-terminus of the pentapeptide chain. Nisin² inserts into the membrane and we propose this as the first step in pore formation, mediated by the nisin N-terminus–lipid II pentapeptide hydrogen bond. The lipid II undecaprenyl chain adopted different conformations in the presence of nisin, which may also have implications for pore formation.

Potentiating the Activity of Nisin against Escherichia coli

Lantibiotics are antimicrobial (methyl)lanthionine-containing peptides produced by various Gram-positive bacteria. The model lantibiotic, nisin, binds lipid II in the cell membrane. Additionally, after binding it can insert into the membrane creating a pore. Nisin can efficiently inhibit the growth of Gram-positive bacteria and resistance is rarely observed. However, the activity of lantibiotics is at least 100-fold lower against certain Gram-negative bacteria. This is caused by the fact that Gram-negative bacteria have an outer membrane that hinders the peptides to reach lipid II, which is located in the inner membrane. Improving the activity of lantibiotics against Gram-negative bacteria could be achieved if the outer membrane traversing efficiency is increased. Here, several anti-Gram-negative peptides (e.g., apidaecin 1b, oncocin), or parts thereof, were fused to the C-terminus of either a truncated version of nisin containing the first three/five rings or full length nisin. The activities of these fusion peptides were tested against Gram-negative pathogens. Our results showed that when an eight amino acids (PRPPHPRL) tail from apidaecin 1b was attached to nisin, the activity of nisin against Escherichia coli CECT101 was increased more than two times. This research presents a new and promising method to increase the anti-Gram-negative activity of lantibiotics.

Type AII lantibiotic bovicin HJ50 with a rare disulfide bond: structure, structure-activity relationships and mode of action

Lantibiotics are ribosomally synthesized antimicrobial peptides containing unusual amino acids. As promising alternatives to conventional antibiotics, they have a high potential for alleviating the problem of emergent antibiotic resistance, with possible applications in many industries that have antibacterial demand. Bovicin HJ50 is a type AII lantibiotic, the largest group of lantibiotics, comprising a linear N-terminal region and a globular C-terminal region. Interestingly, bovicin H50 has a disulfide bond that is rare in this group. Owing to limited information about the spatial structures of type AII lantibiotics, the functional regions of this type and the role of the disulfide bond are still unknown. In the present study, we resolved the solution structure of bovicin HJ50 using NMR spectroscopy. This is the first spatial structure of a type AII lantibiotic. Bovicin HJ50 exhibited high flexibility in aqueous solution, whereas varied rigidities were observed in the different rings with the conserved ring A being the most rigid. The charged residues Lys¹¹, Asp¹² and Lys³⁰, as well as the essential disulfide bond were critical for antimicrobial activity. Importantly, bovicin HJ50 showed not only peptidoglycan precursor lipid II-binding ability, but also pore-forming activity, which is significantly different from other bacteriostatic type AII lantibiotics, suggesting a novel antimicrobial mechanism.

Genipin cross-linked nanocomposite films for the immobilization of antimicrobial agent

Cellulose nanocrystal (CNC) reinforced chitosan based antimicrobial films were prepared by immobilizing nisin on the surface of the films. Nanocomposite films containing 18.65 μg/cm(2) of nisin reduced the count of L. monocytogenes by 6.73 log CFU/g, compared to the control meat samples (8.54 log CFU/g) during storage at 4 °C in a Ready-To-Eat (RTE) meat system. Film formulations containing 9.33 μg/cm(2) of nisin increased the lag phase of L. monocytogenes on meat by more than 21 days, whereas formulations with 18.65 μg/cm(2) completely inhibited the growth of L. monocytogenes during storage. Genipin was used to cross-link and protect the activity of nisin during storage. Nanocomposite films cross-linked with 0.05% w/v genipin exhibited the highest bioactivity (10.89 μg/cm(2)) during the storage experiment, as compared to that of the un-cross-linked films (7.23 μg/cm(2)). Genipin cross-linked films were able to reduce the growth rate of L. monocytogenes on ham samples by 21% as compared to the un-cross-linked films. Spectroscopic analysis confirmed the formation of genipin-nisin-chitosan heterocyclic cross-linked network. Genipin cross-linked films also improved the swelling, water solubility, and mechanical properties of the nanocomposite films.

Lantibiotic immunity: inhibition of nisin mediated pore formation by NisI

Nisin, a 3.4 kDa antimicrobial peptide produced by some Lactococcus lactis strains is the most prominent member of the lantibiotic family. Nisin can inhibit cell growth and penetrates the target Gram-positive bacterial membrane by binding to Lipid II, an essential cell wall synthesis precursor. The assembled nisin-Lipid II complex forms pores in the target membrane. To gain immunity against its own-produced nisin, Lactococcus lactis is expressing two immunity protein systems, NisI and NisFEG. Here, we show that the NisI expressing strain displays an IC₅₀ of 73±10 nM, an 8–10-fold increase when compared to the non-expressing sensitive strain. When the nisin concentration is raised above 70 nM, the cells expressing full-length NisI stop growing rather than being killed. NisI is inhibiting nisin mediated pore formation, even at nisin concentrations up to 1 µM. This effect is induced by the C-terminus of NisI that protects Lipid II. Its deletion showed pore formation again. The expression of NisI in combination with externally added nisin mediates an elongation of the chain length of the Lactococcus lactis cocci. While the sensitive strain cell-chains consist mainly of two cells, the NisI expressing cells display a length of up to 20 cells. Both results shed light on the immunity of lantibiotic producer strains, and their survival in high levels of their own lantibiotic in the habitat.

Structural characterization of four prochlorosins: a novel class of lantipeptides produced by planktonic marine cyanobacteria

Prochlorosins make up a class of secondary metabolites produced by strains of Prochlorococcus, single-cell, planktonic marine cyanobacteria. These polycyclic peptides contain lanthionine and methyllanthionine residues that result in thioether cross-links. In Prochlorococcus MIT9313, a single enzyme, ProcM, catalyzes the posttranslational modification of 29 linear peptide substrates to generate a library of highly diverse cyclic peptides. To investigate the catalytic promiscuity of ProcM, we chose four prochlorosins previously demonstrated to be produced by the organism for detailed structural characterization. Nuclear magnetic resonance studies allowed unambiguous assignment of the ring topologies, demonstrating a high degree of topological diversity. The stereochemistry of the lanthionine and methyllanthionine residues was determined by gas chromatography and mass spectrometry for seven prochlorosins. All methyllanthionines had the (2S,3S,6R) configuration, and the lanthionines had the (2S,6R) configuration, irrespective of the direction of cyclization, ring size, or ring topology. These findings indicate that most, if not all, of the rings in prochlorosins are formed enzymatically by ProcM lanthionine synthetase and not by a nonenzymatic process as previously suggested.

Solution structure by nuclear magnetic resonance of the two lantibiotics 97518 and NAI-107

Lantibiotics 97518 and NAI-107, produced by the related genera Planomonospora and Microbispora respectively, are members of a family of nisin-related compounds. They represent promising compounds to treat infections caused by multiresistant Gram-positive pathogens. Despite their similar structure and a similar antibacterial spectrum, the two lantibiotics exhibit significant differences in their potency. To gain an insight into the structure-activity relationships, their conformational properties in solution are determined by NMR. After carrying out an NOE analysis of 2D (1)H NMR spectra, high-resolution 3D structures are determined using molecular dynamics simulations.

Complete covalent structure of nisin Q, new natural nisin variant, containing post-translationally modified amino acids

The third member of the nisin variant, nisin Q, produced by Lactococcus lactis 61-14, is a ribosomally-synthesized antimicrobial peptide, the so-called lantibiotic containing post-translationally modified amino acids such as lanthionine and dehydroalanine. Here, we determined the complete covalent structure of nisin Q, consisting of 34 amino acids, by two-dimensional (1)H nuclear magnetic resonance (NMR) spectroscopy. Sequential assignment of nisin Q containing the unusual amino acids was performed by total correlation spectroscopy (TOCSY) and nuclear Overhauser enhancement spectroscopy (NOESY). The observed long range nuclear Overhauser effect (NOE) in nisin Q indicated assignment of all five sets of lanthionines that intramolecularly bridge residues 3-7, 8-11, 13-19, 23-26, and 25-28. Consequently, the covalent structure of nisin Q was determined to hold the same thioether linkage formation as the other two nisins, but to harbor the four amino acid substitutions, in contrast with nisin A.

Elucidation of the antimicrobial mechanism of mutacin 1140

Mutacin 1140 and nisin A are peptide antibiotics that belong to the lantibiotic family. N-Terminal rings A and B of nisin A and mutacin 1140 (lipid II-binding domain) share many structural and sequence similarities. Nisin A binds lipid II and thus disrupts cell wall synthesis and also forms transmembrane pores. Very little is known about mutacin 1140 in this regard. We performed fluorescence-based studies using a bacteria-mimetic membrane system. The results indicated that lipid II monomers are arranged differently in the mutacin 1140 complex than in the nisin A complex. These differences in complex formation may be attributed to the fact that nisin A uses lipid II to form a distinct pore complex, while mutacin 1140 does not form pores in this membrane system. Further experiments demonstrated that the mutacin 1140-lipid II and nisin A-lipid II complexes are very stable and capable of withstanding competition from each other. Transmembrane electrical potential experiments using a Streptococcus rattus strain, which is sensitive to mutacin 1140, demonstrated that mutacin 1140 does not form pores in this strain even at a concentration 8 times higher than the minimum inhibitory concentration (MIC). Circular complexes of mutacin 1140 and nisin A were observed by electron microscopy, providing direct evidence for a lateral assembly mechanism for these antibiotics. Mutacin 1140 did exhibit a membrane disruptive function in another commonly used artificial bacterial membrane system, and its disruptive activity was enhanced by increasing amounts of anionic phospholipids.

Gene-encoded antibiotics made in bacteria

Production of antimicrobial peptides and proteins is very common among bacteria and a variety of such substances has been described. In general Gram-negative bacteria produce protein bacteriocins (e.g. colicins) with narrow action spectra based on receptor-mediated activity. They produce comparatively few peptides, such as the post-translationally modified microcin B17. In contrast Gram-positive bacteria tend to produce peptide bacteriocins smaller than 10 kDa and of wider activity spectra. These show particular potential for application. They can be divided into unmodified peptides (e.g. lactococcins, lactacins, pediocins) and lanthionine-containing peptides (lantibiotics, e.g. nisin, epidermin, Pep5). The unmodified peptides are mostly hydrophobic or amphiphilic and act by disturbing the function of the cytoplasmic membrane. They are synthesized as prepeptides with a characteristic N-terminal leader peptide. In some cases genes for immunity peptides were found in close proximity to structural genes; furthermore, two-component response regulators seem to be involved in the regulation of their synthesis. The biosynthetic genes for lantibiotics are also organized in operons. Lantibiotic gene clusters include genes encoding the unique enzymes which dehydrate serine and threonine and form the characteristic thioether-bridged lanthionines. Three types of lantibiotics are currently distinguished on the basis of structural features and functional aspects: type A, which include elongated, amphiphilic, pore-forming peptides (e.g. nisin); type B, which are of globular shape and inhibit phospholipases (e.g. duramycins); and type C (e.g. actagardine) with intermediate features which act by inhibiting bacterial cell wall biosynthesis.

Step-wise and pre-organization induced synthesis of a crossed alkene-bridged nisin Z DE-ring mimic by ring-closing metathesis

This paper describes two approaches for the synthesis of a crossed alkene-bridged mimic of the thioether ring system of the nisin Z DE-fragment. The first approach comprised the stepwise total synthesis featuring a cross metathesis and a macrolactamization on a solid support followed by a ring-closing metathesis in solution. Via this route the title compound was obtained in an overall yield of 7% (85% on average for 16 reaction steps). In the second approach, the linear precursor peptide was subjected to ring-closing metathesis and the bicyclic peptide with the correct side chain connectivity pattern was obtained in yields up to 95%. The preferred formation of the bicyclic crossed alkene-bridged mimic of the DE-ring suggests a favorable pre-organization of the linear precursor peptide.

Inhibition of Bacillus anthracis and potential surrogate bacilli growth from spore inocula by nisin and other antimicrobial peptides

The ability of nisin, synthetic temporin analogs, magainins, defensins, and cecropins to inhibit Bacillus anthracis, Bacillus cereus, Bacillus thuringiensis, Bacillus mycoides, and Bacillus subtilis growth from spore inocula was determined using well diffusion assays. Nisin, magainin II amide, and defensins were inhibitory in screening against B. anthracis Sterne or B. cereus ATCC 7004, but only nisin inhibited virulent B. anthracis strains. The MICs of nisin against the 10 Bacillus strains examined were 0.70 to 13.51 microg/ml. Synthetic temporin analogs also inhibited B. anthracis but were not as potent as nisin. None of the strains examined were appropriate B. anthracis surrogates for testing sensitivity to antimicrobial peptides.

Molecular and genetic characterization of a novel nisin variant produced by Streptococcus uberis

Streptococcus uberis is one of the principal causative agents of bovine mastitis. In this study, we report that S. uberis strain 42 produces a lantibiotic, nisin U, which is 78% identical (82% similar) to nisin A from Lactococcus lactis. The 15.6-kb nisin U locus comprises 11 open reading frames, similar in putative functionality but differing in arrangement from that of the nisin A biosynthetic cluster. The nisin U producer strain exhibits specific resistance (immunity) to nisin U and cross-resistance to nisin A, a finding consistent with the 55% sequence similarity of their respective immunity peptides. Homologues of the nisin U structural gene were identified in several additional S. uberis strains, and in each case cross-protective immunity was expressed to nisin A and to the other producers of nisin U and its variants. To our knowledge, this is the first report both of characterization of a bacteriocin by S. uberis, as well as of a member of the nisin family of peptides in a species other than L. lactis.

Synthesis of a Cyclic Peptide Containing Norlanthionine: Effect of the Thioether Bridge on Peptide Conformation

Two diastereomeric analogues of ring C of nisin incorporating a novel norlanthionine residue have been synthesized via a triply orthogonal protecting group strategy. A full structural study was carried out by NMR, which elucidated the conformational properties of the two peptides and enabled the identity of each diastereoisomer to be proposed.

Identification of the lantibiotic nisin Q, a new natural nisin variant produced by Lactococcus lactis 61-14 isolated from a river in Japan

Lactococcus lactis 61-14 isolated from river water produced a bacteriocin active against a wide range of Gram-positive bacteria. N-terminal amino acid sequencing, mass spectral analysis of the purified bacteriocin, and genetic analysis using nisin-specific primers showed that the bacteriocin was a new natural nisin variant, termed nisin Q. Nisin Q and nisin A differ in four amino acids in the mature peptide and two in the leader sequence.

Peptide conformational changes induced by tryptophan-phosphocholine interactions in a micelle

Sodium dodecylsulfate (SDS) and dodecylphosphocholine (DPC) micelles are often used to mimic the membrane- or receptor-bound states of peptides in NMR studies. From the present examination of a 26-residue analog of exendin-4 (TrEX4) by NMR and CD in water, aqueous 30% trifluoroethanol (TFE), and bound to both SDS and DPC micelles, it is clear that these two lipid micelles can yield very different peptide structures. The Trp-cage fold (also observed in 30% TFE) is present when TrEX4 is bound to SDS micelles; however, tertiary structure is absent in the presence of DPC micelles. The loss of tertiary structure is attributed to an energetically favorable interaction (estimated as 2-3 kcal/mol) of the tryptophan side chain with the phosphocholine head groups. These dramatic structural differences suggest that care must be taken when using either SDS or DPC to mimic the membrane- or receptor-bound states.

Two-peptide bacteriocins produced by lactic acid bacteria

Bacteriocins from lactic acid bacteria are ribosomally produced peptides (usually 30-60 amino acids) that display potent antimicrobial activity against certain other Gram-positive organisms. They function by disruption of the membrane of their targets, mediated in at least some cases by interaction of the peptide with a chiral receptor molecule (e.g., lipid II or sugar PTS proteins). Some bacteriocins are unmodified (except for disulfide bridges), whereas others (i.e. lantibiotics) possess extensive post-translational modifications which include multiple monosulfide (lanthionine) bridges and dehydro amino acids as well as possible keto amide residues at the N-terminus. Most known bacteriocins are biologically active as single peptides. However, there is a growing class of two peptide systems, both unmodified and lantibiotic, which are fully active only when both partners are present (usually 1:1). In some cases, neither peptide has activity by itself, whereas in others, the activity of one is enhanced by the other. This review discusses the classification, structure, production, regulation, biological activity, and potential applications of such two-peptide bacteriocins.

Engineering a disulfide bond and free thiols in the lantibiotic nisin Z

The antimicrobial peptide nisin contains the uncommon amino acid residues lanthionine and methyl-lanthionine, which are post-translationally formed from Ser, Thr and Cys residues. To investigate the importance of these uncommon residues for nisin activity, a mutant was designed in which Thr13 was replaced by a Cys residue, which prevents the formation of the thioether bond of ring C. Instead, Cys13 couples with Cys19 via an intramolecular disulfide bridge, a bond that is very unusual in lantibiotics. NMR analysis of this mutant showed a structure very similar to that of wild-type nisin, except for the configuration of ring C. The modification was accompanied by a dramatic reduction in antimicrobial activity to less than 1% of wild-type activity, indicating that the lanthionine of ring C is very important for this activity. The nisin Z mutants S5C and M17C were also isolated and characterized; they are the first lantibiotics known that contain an additional Cys residue that is not involved in bridge formation but is present as a free thiol. Secretion of these peptides by the lactococcal producer cells, as well as their antimicrobial activity, was found to be strongly dependent on a reducing environment. Their ability to permeabilize lipid vesicles was not thiol-dependent. Labeling of M17C nisin Z with iodoacetamide abolished the thiol-dependence of the peptide. These results show that the presence of a reactive Cys residue in nisin has a strong effect on the antimicrobial properties of the peptide, which is probably the result of interaction of these residues with thiol groups on the outside of bacterial cells.

Solution structure of plantaricin C, a novel lantibiotic

Plantaricin C, a bacteriocin produced by a Lactobacillus plantarum strain of dairy origin, is a lantibiotic. One dehydroalanine, one lanthionine and three beta-methyl-lanthionine residues were found in its 27 amino acid sequence. The plantaricin C structure has two parts: the first comprises the six NH2-terminal residues, four of which are lysines, which confer a strong positive charge to this stretch. The amino acids in positions 7 and 27 form the lanthionine bridge, giving a globular conformation to the rest of the molecule. The beta-methyl-lanthionine bridges are established between residues 12-15, 13-18 and 23-26. This central region has a charge distribution compatible with an amphipathic alpha-helix, through which plantaricin C would become inserted into the membrane matrix of sensitive organisms, provoking the opening of pores and leakage of the cytoplasmic content.

Computational study of nisin interaction with model membrane

Nisin is a 34-residue lantibiotic widely used as food preservative. Its mode of action on the bacterial cytoplasmic membrane is unclear. It should form ion channels but a molecular description of the interaction between nisin and phospholipids is lacking. The interactions between nisin and a membrane and the influence of phospholipids are here analysed by molecular modelling. The NMR structures of nisin in a micellar environment were previously determined (Van den Hooven et al., Eur. J. Biochem. 235 (1996) 382-393) Those structures were used to start with. They were refined by running a Monte Carlo procedure at a model lipid/water interface. It was shown that nisin is adsorbing onto the interface, with its N-terminal moiety more deeply inserted in lipids than the C-end, indicating distinct hydrophobic properties of the N- and C-domains. Therefore, we suggest that the N-terminal part is implied in the insertion of nisin in lipids, while the C-terminal moiety could be involved in the initial interaction with the membrane surface. Modelling the interaction of nisin with different neutral or anionic phospholipids shows that it disturbs the lipid organisation. The disturbance is maximal with phosphatidylglycerol. In this system, nisin curves the surface of phosphatidylglycerol layer round suggesting it could induce micelle formation. This could be a preliminary step to pore formation. It suggests that phosphatidylglycerol could have a direct action on nisin insertion and on ion channel formation. Appearance of a curvature also agrees with the 'wedge model' proposed in the literature for the nisin pore formation.

Lantibiotics: biosynthesis and biological activities of uniquely modified peptides from gram-positive bacteria

A plethora of novel gene-encoded antimicrobial peptides from animals, plants and bacteria has been described during the last decade. Many of the bacterial peptides possess modified building blocks such as thioethers and thiazoles or unsaturated and stereoinverted amino acids, which are unique among ribosomally made peptides. Genetic and biochemical studies of many of these peptides, mostly the so-called lantibiotics, have revealed the degree to which cells are capable of transforming peptides by posttranslational modification. The biosynthesis follows a general scheme: Precursor peptides are first modified and then proteolytically activated; the latter may occur prior to, concomitantly with or after export from the cell. The genes for the biosynthetic machinery are organized in clusters and include information for the antibiotic prepeptide, the modification enzymes and accessory functions such as dedicated proteases and ABC transporters as well as immunity factors and regulatory proteins. These fundamental aspects are discussed along with the biotechnological potential of the peptides and of the biosynthesis enzymes, which could be used for construction of novel, peptide-based biomedical effector molecules.

The lantibiotic nisin induces transmembrane movement of a fluorescent phospholipid

Nisin is a pore-forming antimicrobial peptide. The capacity of nisin to induce transmembrane movement of a fluorescent phospholipid in lipid vesicles was investigated. Unilamellar phospholipid vesicles that contained a fluorescent phospholipid (1-acyl-2-(6-[(7-nitro-2-1, 3-benzoxadiazol-4-yl)amino]caproyl)-sn-glycero-3-phosphocholine) in the inner leaflet of the bilayer were used. Nisin-induced movement of the fluorescent phospholipid from the inner leaflet to the outer leaflet of the membrane reached stable levels, which were dependent on the concentration of nisin added. The rate constant k of this nisin-induced transmembrane movement increased with the nisin concentration but was not dependent on temperature within the range of 5 to 30 degrees C. In contrast, the rate constant of movement of fluorescent phospholipid from vesicle to vesicle strongly depended on temperature. The data indicate that nisin transiently disturbs the phospholipid organization of the target membrane.

Structural variations in nisin associated with different membrane mimicking and pH environments

Nisin is a membrane active antimicrobial peptide containing unusual dehydrated amino acid residues. The secondary structure of nisin in aqueous solution, membrane mimicking solvents and at various pH values was investigated using circular dichroism. In aqueous solution nisin is largely randomly coiled. In liposomes and at pH 6 and above, however, the presence of a maximum at 195 nm and a minimum at 190 nm was notable and indicative of beta-turn formation in these environments. This change in structure was speculated to result in an increasing unavailability of the site for initial reaction of peptide and membrane at higher pH.

Constitution and solution conformation of the antibiotic mersacidin determined by NMR and molecular dynamics

The solution structure of the tetracyclic lantibiotic mersacidin in methanol (CD3OH) has been determined by NMR followed by distance bound driven dynamics and subsequent restrained molecular dynamics simulations combined with an iterative relaxation matrix approach and alternatively by a simulated annealing protocol. The molecular dynamics simulations were performed with the AMBER program system and with the INSIGHT program package. The distance bound driven dynamics calculation was conducted using a modified version of the DISGEO II program. The interproton distance restraints were derived from jump symmetrized rotating-frame Overhauser enhancement and exchange (JS-ROESY) spectra, which yield optimum sensitivity for medium-sized molecules like mersacidin. The connectivities via the sulfide bridges were unambiguously confirmed by heteronuclear NMR techniques (heteronuclear single quantum coherence and heteronuclear multiple bond correlation methods). Due to the tetracyclic structure, mersacidin exhibits a rather rigid globular shape, which neither belongs to the duramycin nor to the nisin structure type lantibiotics. The resulting structures for the simulated annealing protocol of restrained and subsequent free molecular dynamics were compared and found to be very similar.

Structural characterization of free and membrane-bound nisin by infrared spectroscopy

This study reports two new trends about nisin affinity for lipid membranes. First, there is a very strong dependence of nisin binding on the membrane surface charge. As illustrated in this work, the binding of nisin is much greater for phosphatidylglycerol (PG) than for phosphatidylcholine (PC) membranes. This can be rationalized by electrostatic attraction between the positively charged peptide and the negatively charged PG. Second, the affinity of nisin shows a very weak dependence on the lipid phase, the binding to fluid or gel phase membranes being nearly equivalent. Therefore, our results suggest that nisin behaves as an extrinsic peptide. This work also presents the first piece of information relative to the structure of membrane-bound nisin. The Amide I band of the peptide is different for free nisin in water and for membrane-bound nisin. By analyzing this region using self-deconvolution and band fitting, and by comparing with results obtained from nisin dissolved in various H2O/trifluoroethanol mixtures, it can be inferred that the binding of nisin to phospholipid membranes leads to an increased proportion of beta-turns.

Role of transmembrane pH gradient and membrane binding in nisin pore formation

Nisin is a cationic antimicrobial peptide that belongs to the group of lantibiotics. It is thought to form oligomeric pores in the target membrane by a mechanism that requires the transmembrane electrical potential delta psi and that involves local pertubation of the lipid bilayer structure. Here we show that nisin does not form exclusively voltage-dependent pores: even in the absence of a delta psi, nisin is able to dissipate the transmembrane pH gradient (delta pH) in sensitive Lactococcus lactis cells and proteoliposomes. The rate of dissipation increases with the magnitude of the delta pH. Nisin forms pores only when the delta pH is inside alkaline. The efficiency of delta psi-induced pore formation is strongly affected by the external pH, whereas delta pH-induced pore formation is rather insensitive to the external pH. Nisin(1-12), an amino-terminal fragment of nisin, and (des-deltaAla5)-(nisin(1-32) amide have a strongly reduced capacity to dissipate the delta psi and delta pH in cytochrome c oxidase proteoliposomes and L. lactis cells. Both variants bind with reduced efficiency to liposomes containing negatively charged phospholipids, suggesting that both ring A and rings C to E play a role in membrane binding. Nisin(1-12) competes with nisin for membrane binding and antagonizes pore formation. These findings are consistent with the wedge model of nisin-induced pore formation.

Structure and biological activity of chemically modified nisin A species

Nisin, a 34-residue peptide bacteriocin, contains the less common amino acids lanthionine, beta-methyl-lanthionine, dehydroalanine (Dha), and dehydrobutyrine (Dhb). Several chemically modified nisin A species were purified by reverse-phase HPLC and characterized by two-dimensional NMR and electrospray mass spectrometry. Five constituents, [2-hydroxy-Ala5]nisin, [Ile4-amide,pyruvyl-Leu6]des-Dha5-nisin, [Met(O)21]nisin, [Ser33]nisin, and nisin-(1-32)-peptide amide, were found in a commercial nisin sample. A further species, [2-hydroxy-Ala5]nisin-(1-32)-peptide amide, was obtained by freeze drying an acidic nisin solution. These compounds are formed by chemical modification of nisin: the addition of a water molecule to the dehydroalanine residues, which can lead to the cleavage of the polypeptide chain, or the oxidation of methionine residues. The 2-hydroxyalanine-containing products have a limited stability; they are spontaneously converted into the corresponding des-dehydroalanine derivatives. The growth-inhibiting activity of the modified nisins towards different bacteria was determined. The 2-hydroxyalanine-containing species and the des-dehydroalanine derivative show a strong reduction in biological activity as compared to native nisin. [Met(O)21]nisin and [Ser33]nisin show moderate or no reduction in biological activity.

Structure-activity relationships in the peptide antibiotic nisin: role of dehydroalanine 5

A mutant of the peptide antibiotic nisin in which the dehydroalanine residue at position 5 has been replaced by an alanine has been produced and structurally characterized. It is shown to have activity very similar to that of wild-type nisin in inhibiting growth of Lactococcus lactis and Micrococcus luteus but is very much less active than nisin as an inhibitor of the outgrowth of spores of Bacillus subtilis. These observations, which parallel those of W. Liu and J. N. Hansen (Appl. Environ. Microbiol. 59:648-651, 1993) on the corresponding mutant of the related antibiotic subtilin, are discussed in terms of the mechanism(s) of action of these antibiotics.

Structure-activity relationships in the peptide antibiotic nisin: antibacterial activity of fragments of nisin

The post-translationally modified peptide antibiotic nisin has been cleaved by a number of proteases and the fragments produced purified, characterised chemically, and assayed for activity in inhibiting the growth of Lactococcus lactis MG1614 and Micrococcus luteus NCDO8166. These results provide information on the importance of different parts of the nisin molecule for its growth-inhibition activity. Removal of the C-terminal five residues leads to approximately a 10-fold decrease in potency, while removal of a further nine residues, encompassing two of the lanthionine rings, leads to a 100-fold decrease. There are some differences between analogous fragments of nisin and subtilin, suggesting possible subtle differences in mode of action. Cleavage within, or removal of, lanthionine ring C essentially abolishes the activity of nisin. The fragment nisin1-12 is inactive itself, and specifically antagonises the growth-inhibitory action of nisin. These results are discussed in terms of current models for the mechanism of action of nisin.

Interaction of the lantibiotic nisin with membranes revealed by fluorescence quenching of an introduced tryptophan

Nisin is a lantibiotic produced by strains of Lactococcus lactis subsp. lactis. The target for nisin action is the cytoplasmic membrane of gram-positive bacteria. To aid understanding of its mode of action, the interaction of nisin with vesicles of differing phospholipid composition were investigated by fluorescence techniques, using a variant of nisin in which the isoleucine at position 30 was replaced by a tryptophan residue. Activity of the site-directed variant containing tryptophan was established to be similar to that of the wild-type peptide. Fluorescence experiments showed a blue shift of the emission wavelength maximum in the presence of lipid vesicles, indicating that the tryptophan residue enters a more hydrophobic environment. Quenching experiments with aqueous and membrane-restricted quenchers (iodide and spin-labelled lipids, respectively) both confirmed a non-aqueous environment for the Trp30 residue, and implied that the residue resides between 0.36 nm and 0.52 nm from the centre of the membrane, depending on the lipid identity. The results clearly demonstrate that nisin interacts strongly with the hydrophobic phase of lipid vesicles. This interaction is stronger in the presence of negatively charged lipids suggesting their importance in the functional interaction of nisin with membranes.