Antibacterial activity of bactenecin 5 fragments and their interaction with phospholipid membranes

Rational Designed Hybrid Peptides Show up to a 6-Fold Increase in Antimicrobial Activity and Demonstrate Different Ultrastructural Changes as the Parental Peptides Measured by BioSAXS

Antimicrobial peptides (AMPs) are a promising class of compounds being developed against multi-drug resistant bacteria. Hybridization has been reported to increase antimicrobial activity. Here, two proline-rich peptides (consP1: VRKPPYLPRPRPRPL-CONH₂ and Bac5-v291: RWRRPIRRRPIRPPFWR-CONH₂) were combined with two arginine-isoleucine-rich peptides (optP1: KIILRIRWR-CONH₂ and optP7: KRRVRWIIW-CONH₂). Proline-rich antimicrobial peptides (PrAMPs) are known to inhibit the bacterial ribosome, shown also for Bac5-v291, whereas it is hypothesized a “dirty drug” model for the arginine-isoleucine-rich peptides. That hypothesis was underpinned by transmission electron microscopy and biological small-angle X-ray scattering (BioSAXS). The strength of BioSAXS is the power to detect ultrastructural changes in millions of cells in a short time (seconds) in a high-throughput manner. This information can be used to classify antimicrobial compounds into groups according to the ultrastructural changes they inflict on bacteria and how the bacteria react towards that assault. Based on previous studies, this correlates very well with different modes of action. Due to the novelty of this approach direct identification of the target of the antimicrobial compound is not yet fully established, more research is needed. More research is needed to address this limitation. The hybrid peptides showed a stronger antimicrobial activity compared to the proline-rich peptides, except when compared to Bac5-v291 against E. coli. The increase in activity compared to the arginine-isoleucine-rich peptides was up to 6-fold, however, it was not a general increase but was dependent on the combination of peptides and bacteria. BioSAXS experiments revealed that proline-rich peptides and arginine-isoleucine-rich peptides induce very different ultrastructural changes in E. coli, whereas a hybrid peptide (hyP7B5GK) shows changes, different to both parental peptides and the untreated control. These different ultrastructural changes indicated that the mode of action of the parental peptides might be different from each other as well as from the hybrid peptide hyP7B5GK. All peptides showed very low haemolytic activity, some of them showed a 100-fold or larger therapeutic window, demonstrating the potential for further drug development.

Natural and Synthetic Halogenated Amino Acids-Structural and Bioactive Features in Antimicrobial Peptides and Peptidomimetics

The 3D structure and surface characteristics of proteins and peptides are crucial for interactions with receptors or ligands and can be modified to some extent to modulate their biological roles and pharmacological activities. The introduction of halogen atoms on the side-chains of amino acids is a powerful tool for effecting this type of tuning, influencing both the physico-chemical and structural properties of the modified polypeptides, helping to first dissect and then rationally modify features that affect their mode of action. This review provides examples of the influence of different types of halogenation in amino acids that replace native residues in proteins and peptides. Examples of synthetic strategies for obtaining halogenated amino acids are also provided, focusing on some representative compounds and their biological effects. The role of halogenation in native and designed antimicrobial peptides (AMPs) and their mimetics is then discussed. These are in the spotlight for the development of new antimicrobial drugs to counter the rise of antibiotic-resistant pathogens. AMPs represent an interesting model to study the role that natural halogenation has on their mode of action and also to understand how artificially halogenated residues can be used to rationally modify and optimize AMPs for pharmaceutical purposes.

Caprine Bactenecins as Promising Tools for Developing New Antimicrobial and Antitumor Drugs

Proline-rich antimicrobial peptides (PR-AMPs) having a potent antimicrobial activity predominantly toward Gram-negative bacteria and negligible toxicity toward host cells, are attracting attention as new templates for developing antibiotic drugs. We have previously isolated and characterized several bactenecins that are promising in this respect, from the leukocytes of the domestic goat Capra hircus: ChBac5, miniChBac7.5N-α, and -β, as well as ChBac3.4. Unlike the others, ChBac3.4 shows a somewhat unusual pattern of activities for a mammalian PR-AMP: it is more active against bacterial membranes as well as tumor and, to the lesser extent, normal cells. Here we describe a SAR study of ChBac3.4 (RFRLPFRRPPIRIHPPPFYPPFRRFL-NH2) which elucidates its peculiarities and evaluates its potential as a lead for antimicrobial or anticancer drugs based on this peptide. A set of designed structural analogues of ChBac3.4 was explored for antibacterial activity toward drug-resistant clinical isolates and antitumor properties. The N-terminal region was found to be important for the antimicrobial action, but not responsible for the toxicity toward mammalian cells. A shortened variant with the best selectivity index toward bacteria demonstrated a pronounced synergy in combination with antibiotics against Gram-negative strains, albeit with a somewhat reduced ability to inhibit biofilm formation compared to native peptide. C-terminal amidation was examined for some analogues, which did not affect antimicrobial activity, but somewhat altered the cytotoxicity toward host cells. Interestingly, non-amidated peptides showed a slight delay in their impact on bacterial membrane integrity. Peptides with enhanced hydrophobicity showed increased toxicity, but in most cases their selectivity toward tumor cells also improved. While most analogues lacked hemolytic properties, a ChBac3.4 variant with two additional tryptophan residues demonstrated an appreciable activity toward human erythrocytes. The variant demonstrating the best tumor/nontumor cell selectivity was found to more actively initiate apoptosis in target cells, though its action was slower than that of the native ChBac3.4. Its antitumor effectiveness was successfully verified in vivo in a murine Ehrlich ascites carcinoma model. The obtained results demonstrate the potential of structural modification to manage caprine bactenecins’ selectivity and activity spectrum and confirm that they are promising prototypes for antimicrobial and anticancer drugs design.

Selective anticancer activity of synthetic peptides derived from the host defence peptide tritrpticin

Antimicrobial peptides (AMPs) constitute a diverse family of peptides with the ability to protect their host against microbial infections. In addition to their ability to kill microorganisms, several AMPs also exhibit selective cytotoxicity towards cancer cells and are collectively referred to as anticancer peptides (ACPs). Here a large library of AMPs, mainly derived from the porcine cathelicidin peptide, tritrpticin (VRRFPWWWPFLRR), were assessed for their anticancer activity against the Jurkat T cell leukemia line. These anticancer potencies were compared to the cytotoxicity of the peptides towards normal cells isolated from healthy donors, namely peripheral blood mononuclear cells (PBMCs) and red blood cells (RBCs; where hemolytic activity was assessed). Among the active tritrpticin derivatives, substitution of Arg by Lys enhanced the selectivity of the peptides towards Jurkat cells when compared to PBMCs. Additionally, the side chain length of the Lys residues was also optimized to further enhance the tritrpticin ACP selectivity at low concentrations. The mechanism of action of the peptides with high selectivity involved the permeabilization of the cytoplasmic membrane of Jurkat cells, without formation of apoptotic bodies. The incorporation of non-natural Lys-based cationic amino acids could provide a new strategy to improve the selectivity of other synthetic ACPs to enhance their potential for therapeutic use against leukemia cells.

Antibacterial Peptide BSN-37 Kills Extra- and Intra-Cellular Salmonella enterica Serovar Typhimurium by a Nonlytic Mode of Action

The increasing rates of resistance to traditional anti-Salmonella agents have made the treatment of invasive salmonellosis more problematic, which necessitates the search for new antimicrobial compounds. In this study, the action mode of BSN-37, a novel antibacterial peptide (AMP) from bovine spleen neutrophils, was investigated against Salmonella enterica serovar Typhimurium (S. Typhimurium). Minimum inhibitory concentrations (MICs) and time-kill kinetics of BSN-37 were determined. The cell membrane changes of S. Typhimurium CVCC541 (ST) treated with BSN-37 were investigated by testing the fluorescence intensity of membrane probes and the release of cytoplasmic β-galactosidase activity. Likewise, cell morphological and ultrastructural changes were also observed using scanning and transmission electron microscopes. Furthermore, the cytotoxicity of BSN-37 was detected by a CCK-8 kit and real-time cell assay. The proliferation inhibition of BSN-37 against intracellular S. Typhimurium was performed in Madin-Darby canine kidney (MDCK) cells. The results demonstrated that BSN-37 exhibited strong antibacterial activity against ST (MICs, 16.67 μg/ml), which was not remarkably affected by the serum salts at a physiological concentration. However, the presence of CaCl₂ led to an increase in MIC of BSN-37 by about 4-fold compared to that of ST. BSN-37 at the concentration of 100 μg/ml could completely kill ST after co-incubation for 6 h. Likewise, BSN-37 at different concentrations (50, 100, and 200 μg/ml) could increase the outer membrane permeability of ST but not impair its inner membrane integrity. Moreover, no broken and ruptured cells were found in the figures of scanning and transmission electron microscopes. These results demonstrate that BSN-37 exerts its antibacterial activity against S. Typhimurium by a non-lytic mode of action. Importantly, BSN-37 had no toxicity to the tested eukaryotic cells, even at a concentration of 800 μg/ml. BSN-37 could significantly inhibit the proliferation of intracellular S. Typhimurium.

Proline-Rich Peptides with Improved Antimicrobial Activity against E. coli, K. pneumoniae, and A. baumannii

Proline-rich antimicrobial peptides (PrAMPs) are promising agents to combat multi-drug resistant pathogens due to a high antimicrobial activity, yet low cytotoxicity. A library of derivatives of the PrAMP Bac5(1-17) was synthesized and screened to identify which residues are relevant for its activity. In this way, we discovered that two central motifs -PIRXP- cannot be modified, while residues at N- and C- termini tolerated some variations. We found five Bac5(1-17) derivatives bearing 1-5 substitutions, with an increased number of arginine and/or tryptophan residues, exhibiting improved antimicrobial activity and broader spectrum of activity while retaining low cytotoxicity toward eukaryotic cells. Transcription/translation and bacterial membrane permeabilization assays showed that these new derivatives still retained the ability to strongly inhibit bacterial protein synthesis, but also acquired permeabilizing activity to different degrees. These new Bac5(1-17) derivatives therefore show a dual mode of action which could hinder the selection of bacterial resistance against these molecules.

In vitro and in vivo properties of the bovine antimicrobial peptide, Bactenecin 5

Antimicrobial peptides (AMP), part of the innate immune system, are well studied for their ability to kill pathogenic microorganisms. However, many also possess important immunomodulatory effects, and this area has potential for the development of novel therapies to supplement traditional methods such as the use of antibiotics. Here, we characterise the microbicidal and immunomodulatory potential of the proline-rich bovine AMP, Bactenecin 5 (Bac5). We demonstrate broad antimicrobial activity, including against some mycobacterial species, which are important pathogens of fish, cattle and humans. Bac5 is able to activate macrophage-like THP-1 cells and can synergistically trigger the upregulation of tnf-α when co-stimulated with M. marinum. Furthermore, Bac5 sensitises A549 epithelial cells to stimulation with TNF-α. For the first time, we characterise the activity of Bac5 in vivo, and show it to be a potent chemokine for macrophages in the zebrafish (Danio rerio) embryo model of infection. Bac5 also supports the early recruitment of neutrophils in the presence of M. marinum. In the absence of host adaptive immunity, exogenous injected Bac5 is able to slow, although not prevent, infection of zebrafish with M. marinum.

Improving the Activity of Trp-Rich Antimicrobial Peptides by Arg/Lys Substitutions and Changing the Length of Cationic Residues

Antimicrobial peptides (AMPs) constitute a promising alternative for the development of new antibiotics that could potentially counteract the growing number of antibiotic-resistant bacteria. However, the AMP structure⁻function relationships remain unclear and detailed studies are still necessary. The positively charged amino acid residues (Arg and Lys) play a crucial role in the activity of most AMPs due to the promotion of electrostatic interactions between the peptides and bacterial membranes. In this work we have analyzed the antimicrobial and structural properties of several Trp-rich AMPs containing exclusively either Arg or Lys as the positively charged residues. Their antimicrobial activity and mechanism of action were investigated, showing that Lys residues give rise to a reduced antimicrobial potency for most peptides, which was correlated, in turn, with a decrease in their ability to permeabilize the cytoplasmic membrane of Escherichia coli. Additionally, the presence of Arg and Lys renders the peptides susceptible to degradation by proteases, such as trypsin, limiting their therapeutic use. Therefore, modifications of the side chain length of Arg and Lys were investigated in an attempt to improve the protease resistance of AMPs. This approach resulted in enhanced stability to trypsin digestion, and in several cases, shorter sidechains conserved or even improved the antimicrobial activity. All together, these results suggest that Arg-to-Lys substitutions, coupled with side chain length modifications, can be extremely useful for improving the activity and stability of AMPs.

Probing the disparate effects of arginine and lysine residues on antimicrobial peptide/bilayer association

Antimicrobial peptides (AMPs) are key components of the innate immune response and represent promising templates for the development of broad-spectrum alternatives to conventional antibiotics. Most AMPs are short, cationic peptides that interact more strongly with negatively charged prokaryotic membranes than net neutral eukaryotic ones. Both AMPs and synthetic analogues with arginine-like side chains are more active against bacteria than those with lysine-like amine groups, though the atomistic mechanism for this increase in potency remains unclear. To examine this, we conducted comparative molecular dynamics simulations of a model negatively-charged membrane system interacting with two mutants of the AMP KR-12: one with lysine residues mutated to arginines (R-KR12) and one with arginine residues mutated to lysine (K-KR12). Simulations show that both partition analogously to the bilayer and display similar preferences for hydrogen bonding with the anionic POPGs. However, R-KR12 binds stronger to the bilayer than K-KR12 and forms significantly more hydrogen bonds, leading to considerably longer interaction times. Additional simulations with methylated R-KR12 and charge-modified K-KR12 mutants show that the extensive interaction seen in the R-KR12 system is partly due to arginine's strong atomic charge distribution, rather than being purely an effect of the greater number of hydrogen bond donors. Finally, free energy simulations reveal that both peptides are disordered in solution but form an amphipathic α-helix when inserted into the bilayer headgroup region. Overall, these results highlight the role of charge and hydrogen bond strength in peptide bilayer insertion, and offer potential insights for designing more potent analogues in the future.

Structure of the mammalian antimicrobial peptide Bac7(1-16) bound within the exit tunnel of a bacterial ribosome

Proline-rich antimicrobial peptides (PrAMPs) produced as part of the innate immune response of animals, insects and plants represent a vast, untapped resource for the treatment of multidrug-resistant bacterial infections. PrAMPs such as oncocin or bactenecin-7 (Bac7) interact with the bacterial ribosome to inhibit translation, but their supposed specificity as inhibitors of bacterial rather than mammalian protein synthesis remains unclear, despite being key to developing drugs with low toxicity. Here, we present crystal structures of the Thermus thermophilus 70S ribosome in complex with the first 16 residues of mammalian Bac7, as well as the insect-derived PrAMPs metalnikowin I and pyrrhocoricin. The structures reveal that the mammalian Bac7 interacts with a similar region of the ribosome as insect-derived PrAMPs. Consistently, Bac7 and the oncocin derivative Onc112 compete effectively with antibiotics, such as erythromycin, which target the ribosomal exit tunnel. Moreover, we demonstrate that Bac7 allows initiation complex formation but prevents entry into the elongation phase of translation, and show that it inhibits translation on both mammalian and bacterial ribosomes, explaining why this peptide needs to be stored as an inactive pro-peptide. These findings highlight the need to consider the specificity of PrAMP derivatives for the bacterial ribosome in future drug development efforts.

Guanidino groups greatly enhance the action of antimicrobial peptidomimetics against bacterial cytoplasmic membranes

Antimicrobial peptides or their synthetic mimics are a promising class of potential new antibiotics. Herein we assess the effect of the type of cationic side chain (i.e., guanidino vs. amino groups) on the membrane perturbing mechanism of antimicrobial α-peptide-β-peptoid chimeras. Langmuir monolayers composed of 1,2-dipalmitoyl-sn-glycero-3-phosphatidylglycerol (DPPG) were used to model cytoplasmic membranes of both Gram-positive and Gram-negative bacteria, while lipopolysaccharide Kdo2-lipid A monolayers were mimicking the outer membrane of Gram-negative species. We report the results of the measurements using an array of techniques, including high-resolution synchrotron surface X-ray scattering, epifluorescence microscopy, and in vitro antimicrobial activity to study the molecular mechanisms of peptidomimetic interaction with bacterial membranes. We found guanidino group-containing chimeras to exhibit greater disruptive activity on DPPG monolayers than the amino group-containing analogues. However, this effect was not observed for lipopolysaccharide monolayers where the difference was negligible. Furthermore, the addition of the nitrobenzoxadiazole fluorophore did not reduce the insertion activity of these antimicrobials into both model membrane systems examined, which may be useful for future cellular localization studies.

Structure-activity relationships of the antimicrobial peptide arasin 1 - and mode of action studies of the N-terminal, proline-rich region

Arasin 1 is a 37 amino acid long proline-rich antimicrobial peptide isolated from the spider crab, Hyas araneus. In this work the active region of arasin 1 was identified through structure-activity studies using different peptide fragments derived from the arasin 1 sequence. The pharmacophore was found to be located in the proline/arginine-rich NH(2) terminus of the peptide and the fragment arasin 1(1-23) was almost equally active to the full length peptide. Arasin 1 and its active fragment arasin 1(1-23) were shown to be non-toxic to human red blood cells and arasin 1(1-23) was able to bind chitin, a component of fungal cell walls and the crustacean shell. The mode of action of the fully active N-terminal arasin 1(1-23) was explored through killing kinetic and membrane permeabilization studies. At the minimal inhibitory concentration (MIC), arasin 1(1-23) was not bactericidal and had no membrane disruptive effect. In contrast, at concentrations of 5×MIC and above it was bactericidal and interfered with membrane integrity. We conclude that arasin 1(1-23) has a different mode of action than lytic peptides, like cecropin P1. Thus, we suggest a dual mode of action for arasin 1(1-23) involving membrane disruption at peptide concentrations above MIC, and an alternative mechanism of action, possibly involving intracellular targets, at MIC.

Structural and functional studies on a proline-rich peptide isolated from swine saliva endowed with antifungal activity towards Cryptococcus neoformans

A proline-rich peptide of 2733Da, isolated from pig parotid granule preparations was tested against different pathogenic fungi. It showed interesting antifungal activity towards a clinical isolate of Cryptococcus neoformans, with an EC(50) of 2.2μM. Neither cytotoxic nor haemolytic effects were observed towards mammalian cells. Circular dichroism and infrared spectroscopic studies showed that the peptide adopted a combination of polyproline type-II, β-turn and unordered conformations at physiological temperatures. Temperature dependent experiments evidenced a tendency to adopt a polyproline-II helix conformation. From experiments with lipid vesicles, Neutral Red Uptake (NRU), haemolytic assays, and confocal microscopy studies, it could be hypothesized that the peptide may exert its antifungal effect by interacting with an intracellular target rather than through membrane damage.

Comparative activity and mechanism of action of three types of bovine antimicrobial peptides against pathogenic Prototheca spp

The yeast-like algae of the genus Prototheca are ubiquitous saprophytes causing infections in immunocompromised patients and granulomatous mastitis in cattle. Few available therapies and the rapid spread of resistant strains worldwide support the need for novel drugs against protothecosis. Host defence antimicrobial peptides inactivate a wide array of pathogens and are a rich source of leads, with the advantage of being largely unaffected by microbial resistance mechanisms. Three structurally diverse bovine peptides [BMAP-28, Bac5 and lingual antimicrobial peptide (LAP)] have thus been tested for their capacity to inactivate Prototheca spp. In minimum inhibitory concentration (MIC) assays, they were all effective in the micromolar range against clinical mastitis isolates as well as a Prototheca wickerhamii reference strain. BMAP-28 sterilized Prototheca cultures within 30-60 min at its MIC, induced cell permeabilization with near 100% release of cellular adenosine triphosphate and resulted in extensive surface blebbing and release of intracellular material as observed by scanning electron microscopy. Bac5 and LAP inactivated Prototheca following 3-6 h incubation at fourfold their MIC and did not result in detectable surface damage despite 70-90% killing, suggesting they act via non-lytic mechanisms. In circular dichroism studies, the conformation of BMAP-28, but not that of Bac5 or LAP, was affected by interaction with liposomes mimicking algal membranes. Our results indicate that BMAP-28, Bac5 and LAP kill Prototheca with distinct potencies, killing kinetics, and modes of action and may be appropriate for protothecal mastitis treatment. In addition, the ability of Bac5 and LAP to act via non-lytic mechanisms may be exploited for the development of target-selective drugs.

Proline-rich antimicrobial peptides: converging to a non-lytic mechanism of action

Proline-rich antimicrobial peptides are a group of cationic host defense peptides of vertebrates and invertebrates characterized by a high content of proline residues, often associated with arginine residues in repeated motifs. Those isolated from some mammalian and insect species, although not evolutionarily related, use a similar mechanism to selectively kill Gram-negative bacteria, with a low toxicity to animals. Unlike other types of antimicrobial peptides, their mode of action does not involve the lysis of bacterial membranes but entails penetration into susceptible cells, where they then act intracellularly. Some aspects of the transport system and cytoplasmic targets have been elucidated. These features make them attractive both as anti-infective lead compounds and as a new class of potential cell-penetrating peptides capable of internalising membrane-impermeant drugs into both bacterial and eukaryotic cells.

Structural and functional characterization of the porcine proline–rich antifungal peptide SP-B isolated from salivary gland granules

A 1905-Da cationic proline-rich peptide, named SP-B, was recently isolated by our group as the main component of salivary gland granules, and its primary sequence fully characterized by means of automated Edman sequencing and LC-MS/MS tools. In the present study SP-B is shown to possess antifungal activity when challenged with strains of Cryptococcus neoformans, Candida albicans and Aspergillus fumigatus, while only negligible antibacterial activity was detected. Furthermore, SP-B was found to be non-cytotoxic when tested on fibroblast cell lines. To obtain information regarding its structure affinity, capillary electrophoresis (CE), circular dichroism (CD) and attenuated total reflection (ATR)-FT/IR experiments were performed. CE revealed a pH dependence of the hydrodynamic radial dimensions both in aqueous and 2,2,2-trifluoroethanol solutions. CD and ATR-FT/IR measurements confirmed the structure-pH relationship, revealing a secondary structure composed of mixed proportions of polyproline-II, unordered and turn motifs, the last being more evident in the zwitterionic form of the peptide. From these findings SP-B peptide could be classified as a new member of the proline-rich antimicrobial peptide family.

Toward Understanding the Cationicity of Defensins

Human defensins are a family of small antimicrobial proteins found predominantly in leukocytes and epithelial cells that play important roles in the innate and adaptive immune defense against microbial infection. The most distinct molecular feature of defensins is cationicity, manifested by abundant Arg and/or Lys residues in their sequences. Sequence analysis indicates that Arg is strongly selected over Lys in alpha-defensins but not in beta-defensins. To understand this Arg/Lys disparity in defensins, we chemically synthesized human alpha-defensin 1 (HNP1) and several HNP1 analogs where three Arg residues were replaced by each of the following six alpha-amino acids: Lys, ornithine (Orn), diaminobutyric acid (Dab), diaminopropionic acid (Dap), N,N-dimethyl-Lys ((diMe)Lys), and homo-Arg ((homo)Arg). In addition, we prepared human beta-defensin 1 (hBD1) and (Lys-->Arg)hBD1 in which all four Lys residues were substituted for Arg. Bactericidal activity assays revealed the following. 1) Arg-containing HNP1 and (Lys-->Arg)hBD1 are functionally better than Lys-HNP1 and hBD1, respectively; the difference between Arg and Lys is more evident in the alpha-defensin than in the beta-defensin and is more evident at low salt concentrations than at high salt concentrations. 2) For HNP1, the Arg/Lys disparity is much more pronounced with Staphylococcus aureus than with Escherichia coli, and the Arg-rich HNP1 kills bacteria faster than its Lys-rich analog. 3) Arg and Lys appear to have optimal chain lengths for bacterial killing as shortening Lys or lengthening Arg in HNP1 invariably becomes functionally deleterious. Our findings provide insights into the Arg/Lys disparity in defensins, and shed light on the cationicity of defensins with respect to their antimicrobial activity and specificity.

Semisynthesis of a glycosylated Im7 analogue for protein folding studies

To establish a system to address questions concerning the influence of glycosylation on protein folding pathways, we have developed a semisynthetic route toward the immunity protein Im7. This fourhelix protein has been used extensively as model protein for folding studies. Native chemical ligation (NCL) affords an N-linked chitobiose glycoprotein analogue of Im7 with an Ala29Cys mutation. The semisynthetic approach relies on the solid-phase peptide synthesis (SPPS) of N-terminal thioesters (including helix I), in glycosylated or unglycosylated form, in combination with the expression of the C-terminal fragment of Im7 (containing helices II-IV). Detailed kinetic and thermodynamic analysis of the protein folding behavior reveals that semisynthetic Im7 analogues are well suited for protein folding studies and that the folding mechanism of the glycoprotein of this Im7 variant is not significantly altered over the unglycosylated analogue.

Vaginal microbicides: a novel approach to preventing sexual transmission of HIV

The AIDS epidemic continues its unrelentless expansion. According to the Joint United Nations Programme on HIV/AIDS, there are more than 40 million people living with HIV, and more than 15,000 new infections occur every day. One approach to curbing HIV is the development of topical microbicidal agents or microbicides. These are compounds designed to protect the body's mucosal surfaces from infection by sexually transmitted disease-causing pathogens, including HIV. Several candidates are in preclinical stages; however, only a handful have been tested in humans for safety, and even fewer are ready for clinical efficacy trials. In this update, we describe microbicide research and development, including preclinical screening algorithms, ideal properties, compounds in the pipeline, and future prospects. This review is based on a previous work, which has been updated to contain new information, especially regarding microbicide candidates in preclinical and clinical stages of development.

Antimicrobial activity of novel dendrimeric peptides obtained by phage display selection and rational modification

A large 10-mer phage peptide library was panned against whole Escherichia coli cells, and an antimicrobial peptide (QEKIRVRLSA) was selected. The peptide was synthesized in monomeric and dendrimeric tetrabranched form (multiple antigen peptide [MAP]), which generally allows a dramatic increase of peptide stability to peptidases and proteases. The antibacterial activity of the dendrimeric peptide against E. coli was much higher than that of the monomeric form. Modification of the original sequence, by residue substitution or sequence shortening, produced three different MAPs, M4 (QAKIRVRLSA), M5 (KIRVRLSA), and M6 (QKKIRVRLSA) with enhanced stability to natural degradation and antimicrobial activity against a large panel of gram-negative bacteria. The MICs of the most potent peptide, M6, were as low as 4 to 8 microg/ml against recent clinical isolates of multidrug-resistant Pseudomonas aeruginosa and members of the Enterobacteriaceae. The same dendrimeric peptides showed high stability to blood proteases, low hemolytic activity, and low cytotoxic effects on eukaryotic cells, making them promising candidates for the development of new antibacterial drugs.

Antimicrobial activity of Bac7 fragments against drug-resistant clinical isolates

Ten peptides from 13 to 35 residues in length and covering the whole sequence of the Pro-rich peptide Bac7 were synthesized to identify the domain responsible for its antimicrobial activity. At least 16 residues of the highly cationic N-terminal sequence were required to maintain the activity against Gram-negative bacteria. The fragments Bac7(1-35) and, to a lesser extent, Bac7(1-16) proved active against a panel of antibiotic-resistant clinical isolates of Gram-negative bacteria, with the notable exception of Burkholderia cepacia. In addition, when tested against fungi, the longer fragment was also active against collection strains and clinical isolates of Cryptococcus neoformans, but not towards clinical isolates of Candida albicans.

On-Resin Native Chemical Ligation for Cyclic Peptide Synthesis1,2

A novel cysteine derivative, N(alpha)-trityl-S-(9H-xanthen-9-yl)-l-cysteine [Trt-Cys(Xan)-OH] has been introduced for peptide synthesis, specifically for application to a new strategy for the preparation of cyclic peptides. The following steps were carried out to synthesize the cyclic model peptide cyclo(Cys-Thr-Abu-Gly-Gly-Ala-Arg-Pro-Asp-Phe): (i). side-chain anchoring of Fmoc-Asp-OAl via its free beta-carboxyl as a p-alkoxybenzyl ester to a solid support; (ii). stepwise chain elongation of the peptide by standard Fmoc/tBu solid-phase chemistry; (iii). removal of the N-terminal Fmoc group; (iv). coupling of Trt-Cys(Xan)-OH; (v). selective Pd(0)-promoted cleavage of the C-terminal allyl ester; (vi). coupling of the C-terminal residue, i.e., H-Phe-SBzl, preactivated as a thioester; (vii). selective removal of the N(alpha)-Trt and S-Xan protecting groups under very mild acid conditions; (viii). on-resin cyclization by native chemical ligation in an aqueous milieu; and (ix). final acidolytic cleavage of the cyclic peptide from the resin. The strategy was evaluated for three supports: poly[N,N-dimethacrylamide-co-poly(ethylene glycol)] (PEGA), cross-linked ethoxylate acrylate resin (CLEAR), and poly(ethylene glycol)-polystyrene (PEG-PS) graft resin supports. For PEGA and CLEAR, the desired cyclic product was obtained in 76-86% overall yield with initial purities of approximately 70%, whereas for PEG-PS (which does not swell nearly as well in water), results were inferior. Solid-phase native chemical ligation/cyclization methodology appears to have advantages of convenience and specificity, which make it promising for further generalization.

Fmoc solid-phase synthesis of peptide thioesters by masking as trithioortho esters

[reaction: see text] Total chemical synthesis of proteins by chemoselective ligation relies on C-terminal peptide thioesters as building blocks. Their preparation by standard Fmoc solid-phase peptide synthesis is made difficult by the lability of thioesters to aminolysis by the secondary amines used for removal of the Fmoc group. Here we present a novel backbone amide linker (BAL) strategy for their synthesis in which the thioester functionality is masked as a trithioortho ester throughout the synthesis.

Selective cytotoxicity following Arg-to-Lys substitution in tritrpticin adopting a unique amphipathic turn structure

In antimicrobial peptides, the cationic property due to basic amino acids has been widely recognized as an important factor to promote electrostatic interaction with negatively charged phospholipids. However, little is known about the differences between two basic residues, Arg and Lys, in membrane binding affinity. Tritrpticin is an Arg- or Trp-rich antimicrobial peptide with a broad spectrum of antibacterial and antifungal activity. To investigate the structural and functional differences between Arg and Lys residues, here we designed and synthesized Arg-containing peptides, tritrpticin and SYM11, and their counterpart Lys-substituted peptides, TRK and SYM11KK, respectively. Although there were no remarkable conformational differences between Arg-containing and Lys-substituted peptides, TRK and SYM11KK exhibited almost two-fold enhanced antibacterial activity but significantly reduced hemolytic activity as compared to tritrpticin and SYM11, respectively. Furthermore, Arg-containing peptides showed strong binding affinity to both zwitterionic and anionic liposomes, whereas Lys-substituted peptides interacted weakly with zwitterionic liposomes but strongly with anionic liposomes. These results suggest that the primary amine of Lys interacts less electrostatically with zwitterionic phospholipids than the guanidinium group of Arg. Our results obtained in this study may be helpful in the design of drugs that target negatively charged phospholipids.

An Update on Vaginal Microbicides

In response to an increasing prevalence of sexually transmitted diseases, especially AIDS, efforts to prevent further infections have been heightened. One of those approaches has been the development of topical microbicidal agents or microbicides. These are compounds designed to protect the body's mucosal surfaces from infection by sexually transmitted disease-causing pathogens. Numerous candidates are currently in preclinical stages; however, only a handful have been tested for safety, and even fewer are ready for clinical efficacy trials. In this update, we describe some of the specific features of microbicide research and development, including preclinical screening algorithms, candidate's ideal properties, examples of compounds presently in the pipeline, and future prospects.