Structure-activity analysis of brevinin 1E amide, an antimicrobial peptide from Rana esculenta

Individual substitution analogs of Mel(12-26), melittin's C-terminal 15-residue peptide: their antimicrobial and hemolytic actions

Residues 1-9 of M(12-26) (GLPALISWIKRKRQQ-NH2), the C-terminal 15-residue segment of melittin, were substituted individually to change the hydropathicities in these positions. Antimicrobial and hemolytic activities of these peptides were determined. The results showed increased antimicrobial activities with increased hydrophobicities at almost all the positions studied. The effects at positions 2, 5, 8 and 9 were significant while the effects at the other positions were small. These two groups of residues were located on the opposite faces of the alpha-helix. In other words, the hydrophobicities of the two faces were favorable, but one face (the more favorable face) contributed more to the antimicrobial activities than the other (the less favorable face). The hydrophobicity, not the amphipathicity, seems to be crucial for antimicrobial activity. In contrast, the hydrophobicity of one face was favorable but the other was unfavorable for the hemolytic activity, indicating that the amphipathicity may be important for hemolysis. Interestingly, the more favorable face for antimicrobial activity was located opposite to the favorable face for hemolytic activity, indicating the direction of the hydrophobic face for the antimicrobial activity and direction of the amphipathicity for the hemolytic activity were also important.

Human salivary MUC7 mucin peptides: effect of size, charge and cysteine residues on antifungal activity

We have previously shown that MUC7 (human salivary low-molecular-mass mucin) 20-mer: LAHQKPFIRKSYKCLHKRCR (residues 32-51 of the parent MUC7, with a net positive charge of 7) possesses a broad-spectrum antimicrobial activity [Bobek and Situ (2003) Antimicrob. Agents Chemother. 47, 645-652]. The aims of the present study were to determine the minimum peptide chain length and its location within the 20-mer region that retains potent antifungal activity against Candida albicans and Cryptococcus neoformans and to examine the effect of net charge of the peptide as well as the role of cysteine residues on the fungicidal activity. First, several C-terminal truncated MUC7 20-mer fragments (16-mer, 12-mer, 11-mer, 10-mer and 8-mer) and one N-terminal fragment (8-mer-N) were synthesized and tested. The results showed that MUC7 12-mer, located at the C-terminal region of MUC7 20-mer, having a net charge of +6 and exhibiting an amphipathic helical conformation, not only retained but exceeded the antifungal activity of that of 20-mer. Secondly, several variants of the 12-mer peptide containing a lower or no net positive charge, or no cysteine residues were synthesized and tested. A clear correlation between the net positive charge of the 12-mer, its potency and initial interaction of peptide with fungal cells was found by killing assays, fluorescence microscopy and fungal cell-membrane potential measurements. Furthermore, cysteine residues, which play a critical role in bacterial binding, were found to be not important for the fungicidal activity of these peptides. These results identified MUC7 12-mer as a potential candidate for development into a novel antifungal therapeutic agent.

Conformation, orientation, and adsorption kinetics of dermaseptin B2 onto synthetic supports at aqueous/solid interface

The antimicrobial activity of cationic amphipathic peptides is due mainly to the adsorption of peptides onto target membranes, which can be modulated by such physicochemical parameters as charge and hydrophobicity. We investigated the structure of dermaseptin B2 (Drs B2) at the aqueous/synthetic solid support interface and its adsorption kinetics using attenuated total reflection Fourier transform infrared spectroscopy and surface plasmon resonance. We determined the conformation and affinity of Drs B2 adsorbed onto negatively charged (silica or dextran) and hydrophobic supports. Synthetic supports of differing hydrophobicity were obtained by modifying silica or gold with omega-functionalized alkylsilanes (bromo, vinyl, phenyl, methyl) or alkylthiols. The peptide molecules adsorbed onto negatively charged supports mostly had a beta-type conformation. In contrast, a monolayer of Drs B2, mainly in the alpha-helical conformation, was adsorbed irreversibly onto the hydrophobic synthetic supports. The conformational changes during formation of the adsorbed monolayer were monitored by two-dimensional Fourier transform infrared spectroscopy correlation; they showed the influence of peptide-peptide interactions on alpha-helix folding on the most hydrophobic support. The orientation of the alpha-helical Drs B2 with respect to the hydrophobic support was determined by polarized attenuated total reflection; it was around 15 +/- 5 degrees. This orientation was confirmed and illustrated by a molecular dynamics study. These combined data demonstrate that specific chemical environments influence the structure of Drs B2, which could explain the many functions of antimicrobial peptides.

Role of amino acid residues within the disulfide loop of thanatin, a potent antibiotic peptide

Thanatin, a 21-residue peptide, is an inducible insect peptide with a broad range of activity against bacteria and fungi. It has a C-terminal disulfide loop, like the frog skin secretion antimicrobial peptides of the brevinin family. In this study, we tried to find the effect of a number of amino acids between the disulfide bond. Thanatin showed stronger antibacterial activity to Gram negative bacteria than other mutants, except Th1; whereas, the mutant peptides with deletion had higher activity to Gram positive bacteria than thanatin. An increase in the number of amino acid(s) using the alanine residue decreased the antibacterial activity in all of the bacteria. Th1 with deletion of threonine at position 15 (Thr(1)(2)) showed similar antibacterial activity against Gram-negative bacteria, but had higher activity against the Gram positive bacteria. In order to study the structure-function relationship, we measured liposome disruption by the peptides and CD spectra of the peptides. Th1 also showed the highest liposome leaking activity and alpha-helical propensity in the sodium dodecyl sulfate solution, compared with other peptides. Liposome disruption activity was closely correlated with the anti-Gram positive bacterial activity. All of the peptides showed no hemolytic activity. Th1 was considered to be useful as an antimicrobial peptide with broad spectrum without toxicity

Role of proline, cysteine and a disulphide bridge in the structure and activity of the anti-microbial peptide gaegurin 5

Gaegurin 5 (GGN5) is a cationic 24-residue anti-microbial peptide isolated from the skin of a Korean frog, Rana rugosa. It contains a central proline residue and an intra-residue disulphide bridge in its C-terminus, which are common to the anti-microbial peptides found in Ranidae. We determined the solution structure of GGN5 bound to SDS micelles for the first time and investigated the role of proline, cysteine and a disulphide bridge on the structure and activity of GGN5. GGN5 adopts an amphipathic alpha-helical structure spanning residues 3-20 kinked around Pro-14, which allows the hydrophobic residues to reside in the concave helical region, and a disulphide-bridged loop-like conformation in its C-terminus. By replacement of proline with alanine (PAGGN5), a straight and rigid helix was formed in the central region and was more stable than the kinked helix. Reduction of a disulphide bridge in the C-terminus (GGN5SH) maintained the loosely ordered loop-like conformation, while the replacement of two cysteines with serines (CSGGN5) caused the C-terminal conformation to be completely disordered. The magnitude of anti-microbial activity of the peptides was closely related to their helical stability in the order PAGGN5>GGN5>GGN5SH>CSGGN5, suggesting that the helical stability of the peptides is important for anti-microbial activity. On the other hand, the significant increase of haemolytic activity of PAGGN5 implies that a helical kink of GGN5 could be involved in the selectivity of target cells. The location of GGN5 and PAGGN5, analysed using paramagnetic probes, was mainly at the surface of SDS micelles, although the location of the N-terminal region was slightly different between them.

Structure-function relationship studies on the frog skin antimicrobial peptide tigerinin 1: design of analogs with improved activity and their action on clinical bacterial isolates

Structure-function relationships in antimicrobial peptides have been extensively investigated in order to obtain improved analogs. Most of these studies have targeted either alpha-helical peptides or beta-sheet peptides with multiple disulfide bridges. Tigerinins are short, nonhelical antimicrobial peptides with a single disulfide bridge. In this study, we have synthesized several analogs of tigerinin 1 with an aim to understand the structural basis of activity as well as improve its activity. The studies demonstrate that the loop structure of tigerinin 1 is essential for its optimal activity. However, linearization with increased cationic charges can compensate for loss of loop structure to some extent. Morphology of the cells after treatment with the active analogs shows extensive leakage of cytoplasmic contents. Tigerinin 1 and two of its analogs exhibit impressive activity against a variety of clinical bacterial isolates.

Structure-function studies on the amphibian peptide brevinin 1E: translocating the cationic segment from the C-terminal end to a central position favors selective antibacterial activity

Brevinin 1E, which has the sequence FLPLLAGLAANFLPKIFCKITRKC, is an antimicrobial peptide isolated from the skin secretions of the European frog Rana esculenta. Both the linear and the disulfide-bridged forms have relatively broad-spectrum antibacterial as well as hemolytic activities. The antibacterial and hemolytic activities and biophysical properties of synthetic peptides corresponding to brevinin 1E and its analog in which the segment CKITRKC has been transposed to a central location resulting in the sequence FLPLLAGLCKITRKCAANFLPKIF have been investigated. Our studies indicate that the analog peptide has antibacterial activity comparable with brevinin 1E, but with considerably reduced hemolytic activity. The linear variant of the analog has no hemolytic activity, unlike the linear form of brevinin 1E. The biological activities can be explained on the basis of relative affinities for anionic and zwitterionic lipids. A cluster of cationic amino acids flanked on one side by a hydrophobic stretch of amino acids and another side composed of apolar amino acids appears to favor preferential antibacterial activity.

Structural study of novel antimicrobial peptides, nigrocins, isolated from Rana nigromaculata

Novel cationic antimicrobial peptides, named nigrocin 1 and 2, were isolated from the skin of Rana nigromaculata and their amino acid sequences were determined. These peptides manifested a broad spectrum of antimicrobial activity against various microorganisms with different specificity. By primary structural analysis, it was revealed that nigrocin 1 has high sequence homology with brevinin 2 but nigrocin 2 has low sequence homology with any other known antimicrobial peptides. To investigate the structure-activity relationship of nigrocin 2, which has a unique primary structure, circular dichroism (CD) and homonuclear nuclear magnetic resonance spectroscopy (NMR) studies were performed. CD investigation revealed that nigrocin 2 adopts mainly an alpha-helical structure in trifluoroethanol (TFE)/H(2)O solution, sodium dodecyl sulfate (SDS) micelles, and dodecylphosphocholine micelles. The solution structures of nigrocin 2 in TFE/H(2)O (1:1, v/v) solution and in SDS micelles were determined by homonuclear NMR. Nigrocin 2 consists of a typical amphipathic alpha-helix spanning residues 3-18 in both 50% TFE solution and SDS micelles. From the structural comparison of nigrocin 2 with other known antimicrobial peptides, nigrocin 2 could be classified into the family of antimicrobial peptides containing a single linear amphipathic alpha-helix that potentially disrupts membrane integrity, which would result in cell death.

Tigerinins: novel antimicrobial peptides from the Indian frog Rana tigerina

Four broad-spectrum, 11 and 12 residue, novel antimicrobial peptides have been isolated from the adrenaline-stimulated skin secretions of the Indian frog Rana tigerina. Sequences of these peptides have been determined by automated Edman degradation, by mass spectral analysis and confirmed by chemical synthesis. These peptides, which we have named as tigerinins, are characterized by an intramolecular disulfide bridge between two cysteine residues forming a nonapeptide ring. This feature is not found in other amphibian peptides. Conformational analysis indicate that the peptides tend to form beta-turn structures. The peptides are cationic and exert their activity by permeabilizing bacterial membranes. Tigerinins represent the smallest, nonhelical, cationic antimicrobial peptides from amphibians.

Induction of synthesis of an antimicrobial peptide in the skin of the freeze-tolerant frog, Rana sylvatica, in response to environmental stimuli

An extract of skin taken from specimens of the freeze-tolerant wood frog, Rana sylvatica, that were collected from cold (<7 degrees C) ponds and maintained at 5 degrees C lacked detectable antimicrobial activity. In contrast, an extract of skin taken from specimens maintained at 30 degrees C for 3 weeks under laboratory conditions contained a high concentration (approximately 4 nmol/g) of a single antimicrobial peptide of the brevinin-1 family (FLPVVAGLAAKVLPSIICAVTKKC). The peptide inhibited growth of Escherichia coli (minimum inhibitory concentration 45 microM) and Staphylococcus aureus (minimum inhibitory concentration 7 microM). The data suggest that synthesis of the peptide is induced when the animal is in an environment that promotes the growth of microorganisms consistent with a role in the animal's defense strategy.

Antibacterial peptides isolated from insects

Insects are amazingly resistant to bacterial infections. To combat pathogens, insects rely on cellular and humoral mechanisms, innate immunity being dominant in the latter category. Upon detection of bacteria, a complex genetic cascade is activated, which ultimately results in the synthesis of a battery of antibacterial peptides and their release into the haemolymph. The peptides are usually basic in character and are composed of 20-40 amino acid residues, although some smaller proteins are also included in the antimicrobial repertoire. While the proline-rich peptides and the glycine-rich peptides are predominantly active against Gram-negative strains, the defensins selectively kill Gram-positive bacteria and the cecropins are active against both types. The insect antibacterial peptides are very potent: their IC50 (50% of the bacterial growth inhibition) hovers in the submicromolar or low micromolar range. The majority of the peptides act through disintegrating the bacterial membrane or interfering with membrane assembly, with the exception of drosocin, apidaecin and pyrrhocoricin which appear to deactivate a bacterial protein in a stereospecific manner. In accordance with their biological function, the membrane-active peptides form ordered structures, e.g. alpha-helices or beta-pleated sheets and often cast permeable ion-pores. Their cytotoxic properties were exploited in in vivo studies targeting tumour progression. Although the native peptides degrade quickly in biological fluids other than insect haemolymph, structural modifications render the peptides resistant against proteases without sacrificing biological activity. Indeed, a pyrrhocoricin analogue shows lack of toxicity in vitro and in vivo and protects mice against experimental Escherichia coli infection. Careful selection of lead molecules based on the insect antibacterial peptides may extend their utility and produce viable alternatives to the conventional antimicrobial compounds for mammalian therapy.

Solution structure of the antimicrobial peptide gaegurin 4 by H and 15N nuclear magnetic resonance spectroscopy

Gaegurin 4 (GGN4) is a 37-residue antimicrobial peptide isolated from the skin of a Korean frog, Rana rugosa. This peptide shows a broad range of activity against prokaryotic cells but shows very little hemolytic activity against human red blood cells. The solution structure of GGN4 was studied by using circular dichroism (CD) and NMR spectroscopy. CD investigations revealed that GGN4 adopts mainly an alpha-helical conformation in trifluoroethanol/water solution, in dodecylphosphocholine and in SDS micelles, but adopts random structure in aqueous solution. By using both homonuclear and heteronuclear NMR experiments, complete 1H and 15N resonance assignments were obtained for GGN4 in 50% trifluoroethanol/water solution. The calculated structures of GGN4 consist of two amphipathic alpha-helices extending from residues 2-10 and from residues 16-32. These two helices are connected by a flexible loop spanning between the residues 11 and 15. By using enzyme digestion and matrix-assisted laser desorption/ionization mass spectroscopy, we confirmed that GGN4 contains a disulfide bridge formed between the residues Cys31 and Cys37 in its C-terminus. The effect of disulfide bridge on the structure and the activity of GGN4 was investigated. The reduced form of GGN4 revealed a similar activity and conformation to native GGN4, suggesting that the disulfide bridge does not strongly affect the conformation and the antimicrobial activity of GGN4.

Design and synthesis of novel antimicrobial pseudopeptides with selective membrane-perturbation activity

By incorporating carbamate bond(s) into a cytolytic peptide, novel pseudopeptides with potent antibacterial activity and low hemolytic activity were synthesized. Circular dichroism spectra suggested that the incorporation of carbamate bond(s) decrease the alpha helical conformation of the peptide in lipid membrane circumstances, which must be regarded as a major factor for the separation of antibacterial activity from cytotoxic activity for mammalian cell. Experiments in which dye was released from vesicles indicated that the potent antibacterial activity and low hemolytic activity of the pseudopeptides must be due to their great lipid membrane selectivity. The present result suggest that backbone modifications can be a great tool for developing pseudopeptides with improved biological activity and bioavailability from cytolytic peptides.

Structural features of helical antimicrobial peptides: their potential to modulate activity on model membranes and biological cells

Antibacterial, membrane-lytic peptides belong to the innate immune system and host defense mechanism of a multitude of animals and plants. The largest group of peptide antibiotics comprises peptides which fold into an amphipathic alpha-helical conformation when interacting with the target. The activity of these peptides is thought to be determined by global structural parameters rather than by the specific amino acid sequence. This review is concerned with the influence of structural parameters, such as peptide helicity, hydrophobicity, hydrophobic moment, peptide charge and the size of the hydrophobic/hydrophilic domain, on membrane activity and selectivity. The potential of these parameters to increase the antibacterial activity and to improve the prokaryotic selectivity of natural and model peptides is assessed. Furthermore, biophysical studies are summarized which elucidated the molecular basis for activity and selectivity modulations on the level of model membranes. Finally, the knowledge about the role of peptide structural parameters is applied to understand the different activity spectra of natural membrane-lytic peptides.

Interaction of antimicrobial peptides with biological and model membranes: structural and charge requirements for activity

Species right across the evolutionary scale from insects to mammals use peptides as part of their host-defense system to counter microbial infection. The primary structures of a large number of these host-defense peptides have been determined. While there is no primary structure homology, the peptides are characterized by a preponderance of cationic and hydrophobic amino acids. The secondary structures of many of the host-defense peptides have been determined by a variety of techniques. The acyclic peptides tend to adopt helical conformation, especially in media of low dielectric constant, whereas peptides with more than one disulfide bridge adopt beta-structures. Detailed investigations have indicated that a majority of these host-defense peptides exert their action by permeabilizing microbial membranes. In this review, we discuss structural and charge requirements for the interaction of endogenous antimicrobial peptides and short peptides that have been derived from them, with membranes.

The dermaseptin precursors: a protein family with a common preproregion and a variable C-terminal antimicrobial domain

Preprodermaseptins are a group of antimicrobial peptide precursors found in the skin of a variety of frog species. Precursors of this family have very similar N-terminal preprosequences followed by markedly different C-terminal domains that correspond to mature antimicrobial peptides. Some of these peptides are 24-34 amino acids long and form well-behaved amphipathic alpha-helices, others are disulfide-linked peptides of 20-46 residues, still others, highly hydrophobic, are the smallest antimicrobial peptides known so far being only 10-13 residues in length. All these peptides are broad-spectrum microbicides that kill many bacteria, protozoa, yeasts and fungi by destroying or permeating the microbial membrane. In frogs belonging to the genus Phyllomedusinae, preprodermaseptins encoded peptides also include dermorphins and deltorphins, D-amino acid-containing heptapeptides which are very potent and specific agonists of the mu- or delta-opioid receptors. The remarkable similarity between preproregions of precursors that give rise to peptides with very different primary structures, conformations and activities suggests that the corresponding genes originate from a common ancestor. The high conservation of the precursor prepropart indicates that this region must have an important function.

Antimicrobial activity and conformation of gaegurin-6 amide and its analogs

A function of the intra-disulfide bridge located at the C-terminal of Rana peptides has not been extensively studied. To investigate the function of the disulfide bridge related to the activity and the structure, we chose Gaegurin-6, isolated from Rana rugosa as a model peptide and synthesized linear analogs. The reduction of the disulfide bridge resulted in the complete loss of antimicrobial activity while replacements of cysteines by serines retained antimicrobial activity. Circular dichroism spectra from a titration of the peptides in sodium dodecyl sulfate indicated that the disulfide bridge of Gaegurin-6 might stabilize the induction of an alpha helical structure in lipid membranes and the alpha helical forming propensity of the peptides correlated with antimicrobial activity.