Construction and synthesis of lactoferricin derivatives with enhanced antibacterial activity

Recent Advances on Antimicrobial Peptides from Milk: Molecular Properties, Mechanisms, and Applications

Traditional antibiotics are facing a tremendous challenge due to increased antimicrobial resistance; hence, there is an urgent need to find novel antibiotic alternatives. Milk protein-derived antimicrobial peptides (AMPs) are currently attracting substantial attention considering that they showcase an extensive spectrum of antimicrobial activities, with slower development of antimicrobial resistance and safety of raw materials. This review summarizes the molecular properties, and activity mechanisms and highlights the applications and limitations of AMPs derived from milk proteins comprehensively. Also the analytical technologies, especially bioinformatics methodologies, applied in the process of screening, identification, and mechanism illustration of AMPs were underlined. This review will give some ideas for further research and broadening of the applications of milk protein-derived AMPs in the food field.

Synergy by Perturbing the Gram-Negative Outer Membrane: Opening the Door for Gram-Positive Specific Antibiotics

New approaches to target antibacterial agents toward Gram-negative bacteria are key, given the rise of antibiotic resistance. Since the discovery of polymyxin B nonapeptide as a potent Gram-negative outer membrane (OM)-permeabilizing synergist in the early 1980s, a vast amount of literature on such synergists has been published. This Review addresses a range of peptide-based and small organic compounds that disrupt the OM to elicit a synergistic effect with antibiotics that are otherwise inactive toward Gram-negative bacteria, with synergy defined as a fractional inhibitory concentration index (FICI) of <0.5. Another requirement for the inclusion of the synergists here covered is their potentiation of a specific set of clinically used antibiotics: erythromycin, rifampicin, novobiocin, or vancomycin. In addition, we have focused on those synergists with reported activity against Gram-negative members of the ESKAPE family of pathogens namely, Escherichia coli, Pseudomonas aeruginosa, Klebsiella pneumoniae, and/or Acinetobacter baumannii. In cases where the FICI values were not directly reported in the primary literature but could be calculated from the published data, we have done so, allowing for more direct comparison of potency with other synergists. We also address the hemolytic activity of the various OM-disrupting synergists reported in the literature, an effect that is often downplayed but is of key importance in assessing the selectivity of such compounds for Gram-negative bacteria.

LFchimera: a synthetic mimic of the two antimicrobial domains of bovine lactoferrin

Saliva is essential for the maintenance of oral health. When salivary flow is impaired, the risk of various oral diseases such as caries and candidiasis increases drastically. Under healthy conditions, saliva provides effective protection against microbial colonization by the collaborative action of numerous host-defense molecules. This review describes how saliva has been the guideline for the design and characterization of a heterodimeric antimicrobial construct called LFchimera. This construct mimics the helical parts of two antimicrobial domains in the crystal structure of bovine lactoferrin. It shows high antimicrobial activity against a broad spectrum of Gram-positive and Gram-negative bacteria, fungi, and parasites including biowarfare agents such as Bacillus anthracis, Burkholderia pseudomallei, and Yersinia pestis. Further, sublethal concentrations of LFchimera inhibited biofilm formation, the invasiveness of HeLa cells by Yersinia spp., and prevented haemolysis of enteropathogenic Escherichia coli, demonstrating the versatility of these peptides.

Design of improved synthetic antifungal peptides with targeted variations in charge, hydrophobicity and chirality based on a correlation study between biological activity and primary structure of plant defensin γ-cores

Microbial resistance to available drugs is a growing health threat imposing the need for the development of new drugs. The scaffold of plant defensins, including their γ-cores, are particularly good candidates for drug design. This work aimed to improve the antifungal activity of a previous design peptide, named A₃₆,₄₂,₄₄γ_32-46VuDef (for short DD) against yeasts by altering its biochemical parameters. We explore the correlation of the biological activity and structure of plant defensins and compared their primary structures by superimposition with VuDef₁ and DD which indicated us the favorable position and the amino acid to be changed. Three new peptides with modifications in charge, hydrophobicity (RR and WR) and chirality (D-RR) were designed and tested against pathogenic yeasts. Inhibition was determined by absorbance. Viability of mammalian cells was determined by MTT. The three designed peptides had better inhibitory activity against the yeasts with better potency and spectrum of yeast species inhibition, with low toxicity to mammalian cells. WR, the most hydrophobic and cationic, exhibited better antifungal activity and lower toxicity. Our study provides experimental evidence that targeted changes in the primary structure of peptides based on plant defensins γ-core primary structures prove to be a good tool for the synthesis of new compounds that may be useful as alternative antifungal drugs. The method described did not have the drawback of synthesis of several peptides, because alterations are guided. When compared to other methods, the design process described is efficient and viable to those with scarce resources.

Effect of intramolecular disulfide bond of bovine lactoferricin on its molecular structure and antibacterial activity against Trueperella pyogenes separated from cow milk with mastitis

Background

Lactoferricin (Lfcin) is an antimicrobial activity center of lactoferrin, produced by hydrolysis from the N-terminal of lactoferrin. It was hypothesized that the intramolecular disulfide bond in Lfcin could affect its antibacterial function through influencing its molecular structure. To prove this hypothesis, bovine Lfcin (bLfcin) and its two derivatives, bLfcin with an intramolecular disulfate bond (bLfcin DB) and bLfcin with a mutation C36G (bLfcin C36G), were synthesized, purified, and identified. The circular dichroism spectra of the peptides were detected in solutions with different ionic and hydrophobic strength. The antibacterial activity of the peptides against Trueperella pyogenes, separated from cow milk with mastitis, were determined.

Results

The secondary structure of bLfcin DB showed more β-turn and less random coil than the other peptides in H₂O, similar ratios of secondary structures with bLfcin and bLfcin C36G under ionic conditions, and close percentages of secondary structure with bLfcin under hydrophobic conditions. The synthetic peptides exhibited strong antimicrobial activity against T. pyogenes isolates, T. pyogenes ATCC 19,411, and E. coli ATCC 25,922. The antimicrobial activities of the three peptides were greater against T. pyogenes than against E. coli, and bLfcin DB exhibited higher antibacterial activity compared with its derivatives.

Conclusions

The intramolecular disulfide bond could change the molecular structure of bLfcin under alternative ionic strengths and hydrophobic effects, and the formation of the disulfide bond is beneficial to executing the antibacterial function of bLfcin.

A review of the design and modification of lactoferricins and their derivatives

Lactoferricin (Lfcin), a multifunction short peptide with a length of 25 residues, is derived from the whey protein lactoferrin by acidic pepsin hydrolysis. It has potent nutritional enhancement, antimicrobial, anticancer, antiviral, antiparasitic, and anti-inflammatory activities. This review describes the research advantages of the above biological functions, with attention to the molecular design and modification of Lfcin. In this examination of design and modification studies, research on the identification of Lfcin active derivatives and crucial amino acid residues is also reviewed. Many strategies for Lfcin optimization have been studied in recent decades, but we mainly introduce chemical modification, cyclization, chimera and polymerization of this peptide. Modifications such as incorporation of D-amino acids, acetylation and/or amidation could effectively improve the activity and stability of these compounds. Due to their wide array of bio-functions and applications, Lfcins have great potential to be developed as biological agents with multiple functions involved with nutritional enhancement, as well as disease preventive and therapeutic effects.

Improved antibacterial activity of a marine peptide-N2 against intracellular Salmonella typhimurium by conjugating with cell-penetrating peptides-bLFcin6/Tat11

Salmonellae, gram-negative bacteria, are facultative intracellular pathogens that cause a number of diseases in animals and humans. The poor penetration ability of antimicrobial agents limits their use in the treatment of intracellular bacterial infections. In this study, the cell-penetrating peptides (CPPs) bLFcin₆ and Tat₁₁ were separately conjugated to the antimicrobial peptide N2, and the antibacterial activity and pharmacodynamics of the CPPs-N2 conjugates were first evaluated against Salmonellae typhimurium in vitro and in macrophage cells. The cytotoxicity, cellular uptake and mechanism of cellular internalization of the CPPs-N2 conjugates were also examined in RAW264.7 cells. Similar to N2, CPPs-N2 have two reverse β-sheets and three loops. The minimal inhibitory concentration (MIC) of CPPs-N2 was approximately 2 μM, which was higher than that of N2 (0.8 μM). The dose-time curves and cytotoxicity assay showed that both peptide conjugates were more effective than N2 alone at concentrations ranging from 0.25 to 1 × MIC, and they exhibited low cytotoxicity (9.78%-13.54%) at 100 μM. After 0.5 h incubation, the cell internalization ratio of B6N2 and T11N2 exceeded 28.3% and 93.5%, respectively, which was higher than that of N2. The uptake of B6N2 and T11N2 was reduced by low temperature (82.1%-91.7%), chlorpromazine (35.7%-75.1%), and amiloride (26.0%-52.1%), indicating that macropinocytosis and clathrin-mediated endocytosis may be involved. Approximately 98.85% and 91.35% of bacteria were killed within 3 h by T11N2 and B6N2, respectively, which was higher than the percentage killed by N2 (69.74%). Compared with the bactericidal activity of N2 alone, the bactericidal activity of T11N2 and B6N2 was increased by 53.7%-99.6% and 85.3-85.8%, respectively. Both CPPs-N2 conjugates may be excellent candidates for novel antimicrobial agents to treat infectious diseases caused by intracellular pathogens.

Isolation and Characterization of Lactoferrin Peptides with Stimulatory Effect on Osteoblast Proliferation

Lactoferrin is reported to be a potential food protein with osteogenic activity. However, the activity of lactoferrin peptides is questionable. In the present study, we isolated and characterized peptides from lactoferrin with stimulatory effect on osteoblast proliferation. Peptides from the lactoferrin pepsin hydrolysate were purified using cation-exchange and gel-filtration chromatography. Effects of different hydrolysates and peptides on the proliferation of osteoblast MC3T3-E1 cells were compared by MTT assay. Results showed that fraction P5-a from Superdex Peptide 10/300 GL gel chromatography showed better activity. Tricine-sodium dodecyl sulfate polyacrylamide gel electrophoresis and high-performance liquid chromatography coupled to electrospray ionization tandem mass spectrometry confirmed that two peptides components of P5-a corresponded to fractions of 20-78 and 191-277 amino acids in Bos taurus lactoferrin molecule (GI: 221706349). These results will provide some theoretical and practical data for the preparation and application of osteogenic peptides in functional food industry.

Entry of a Six-Residue Antimicrobial Peptide Derived from Lactoferricin B into Single Vesicles and Escherichia coli Cells without Damaging their Membranes

Lactoferricin B (LfcinB) and shorter versions of this peptide have antimicrobial activity. However, the elementary processes of interactions of these peptides with lipid membranes and bacteria are still not well understood. To elucidate the mechanism of their antimicrobial activity, we investigated the interactions of LfcinB (4-9) (its sequence of RRWQWR) with Escherichia coli cells and giant unilamellar vesicles (GUVs). LfcinB (4-9) and lissamine rhodamine B red-labeled LfcinB (4-9) (Rh-LfcinB (4-9)) did not induce an influx of a membrane-impermeant fluorescent probe, SYTOX green, from the outside of E. coli cells into their cytoplasm, indicating that no damage occurred in their plasma membrane. To examine the activity of LfcinB (4-9) to enter E. coli cytoplasm, we investigated the interaction of Rh-LfcinB (4-9) with single cells of E. coli containing calcein using confocal microscopy. We found that Rh-LfcinB (4-9) entered the cytoplasm without leakage of calcein. Next, we investigated the interactions of Rh-LfcinB (4-9) with single GUVs of dioleoylphosphatidylglycerol (DOPG) and dioleoylphosphatidylcholine (DOPC) mixtures containing a fluorescent probe, Alexa Fluor 647 hydrazide (AF647), using the single GUV method. The results indicate that Rh-LfcinB (4-9) outside the GUV translocated through the GUV membrane and entered its lumen without leakage of AF647. Interaction of Rh-LfcinB (4-9) with DNA increased its fluorescence intensity greatly. Therefore, we can conclude that Rh-LfcinB (4-9) can translocate across lipid membrane regions of the plasma membrane of E. coli cells to enter their cytoplasm without leakage of calcein and its antimicrobial activity is not due to damage of their plasma membranes.

Antimicrobial Activity of Truncated and Polyvalent Peptides Derived from the FKCRRQWQWRMKKGLA Sequence against Escherichia coli ATCC 25922 and Staphylococcus aureus ATCC 25923

Peptides derived from LfcinB were designed and synthesized, and their antibacterial activity was tested against Escherichia coli ATCC 25922 and Staphylococcus aureus ATCC 25923. Specifically, a peptide library was constructed by systemically removing the flanking residues (N or C-terminal) of Lfcin 17–31 (¹⁷FKCRRWQWRMKKLGA³¹), maintaining in all peptides the ²⁰RRWQWR²⁵ sequence that corresponds to the minimal antimicrobial motif. For this research, also included were (i) a peptide containing an Ala instead of Cys ([Ala¹⁹]-LfcinB 17–31) and (ii) polyvalent peptides containing the RRWQWR sequence and a non-natural amino acid (aminocaproic acid). We established that the lineal peptides LfcinB 17–25 and LfcinB 17–26 exhibited the greatest activity against E. coli ATCC 25922 and S. aureus ATCC 25923, respectively. On the other hand, polyvalent peptides, a dimer and a tetramer, exhibited the greatest antibacterial activity, indicating that multiple copies of the sequence increase the activity. Our results suggest that the dimeric and tetrameric sequence forms potentiate the antibacterial activity of lineal sequences that have exhibited moderate antibacterial activity.

Antibacterial and detoxifying activity of NZ17074 analogues with multi-layers of selective antimicrobial actions against Escherichia coli and Salmonella enteritidis

NZ17074 (N1), an arenicin-3 derivative isolated from the lugworm, has potent antibacterial activity and is cytotoxic. To reduce its cytotoxicity, seven N1 analogues with different structures were designed by changing their disulfide bonds, hydrophobicity, or charge. The “rocket” analogue-N2 and the “kite” analogue-N6 have potent activity and showed lower cytotoxicity in RAW264.7 cells than N1. The NMR spectra revealed that N1, N2, and N6 adopt β-sheet structures stabilized by one or two disulfide bonds. N2 and N6 permeabilized the outer/inner membranes of E. coli, but did not permeabilize the inner membranes of S. enteritidis. N2 and N6 induced E. coli and S. enteritidis cell cycle arrest in the I-phase and R-phase, respectively. In E. coli and in S. enteritidis, 18.7–43.8% of DNA/RNA/cell wall synthesis and 5.7–61.8% of DNA/RNA/protein synthesis were inhibited by the two peptides, respectively. Collapsed and filamentous E. coli cells and intact morphologies of S. enteritidis cells were observed after treatment with the two peptides. Body weight doses from 2.5–7.5 mg/kg of N2 and N6 enhanced the survival rate of peritonitis- and endotoxemia-induced mice; reduced the serum IL-6, IL-1β and TNF-α levels; and protected mice from lipopolysaccharide-induced lung injury. These data indicate that N2 and N6, through multiple selective actions, may be promising dual-function candidates as novel antimicrobial and anti-endotoxin peptides.

Killing of Staphylococcus aureus and Salmonella enteritidis and neutralization of lipopolysaccharide by 17-residue bovine lactoferricins: improved activity of Trp/Ala-containing molecules

Bovine lactoferricin (LfcinB) has potent antibacterial, antifungal and antiparasitic activities but is also hemolytic. Our objective was to identify LfcinB17-31 derivatives with reduced hemolysis and improved antimicrobial activity via substituting Cys3, Arg4, Gln7, Met10, and Gly14 with more hydrophobic residues. Two peptides, Lfcin4 and Lfcin5, showed higher activity against Staphylococcus aureus and Salmonella enteritidis and lower hemolytic activity than the parent peptide LfcinB17-31. These peptides permeabilized the outer and inner membranes of S. enteritidis; however, Lfcin5 did not permeabilize the inner membrane of S. aureus. Gel retardation and circular dichroism spectra showed that Lfcin4 and Lfcin5 bound to bacterial genomic DNA. Lfcin4 inhibited DNA, RNA and protein synthesis. Both peptides induced the peeling of membranes and the lysis of S. enteritidis. At doses of 10 and 15 mg/kg, Lfcin4 and Lfcin5 reduced the bacterial counts in infected thigh muscles by 0.03‒0.10 and 0.05‒0.63 log₁₀ CFU/g of tissue, respectively, within 10 h. Lfcin4 and Lfcin5 enhanced the survival rate of endotoxemic mice; reduced serum IL-6, IL-1β and TNF-α levels; and protected mice from lipopolysaccharide-induced lung injury. These data suggest that Lfcin4 and Lfcin5 may be antimicrobial and anti-endotoxin peptides that could serve as the basis for the development of dual-function agents.

Synthetic Peptides Derived from Bovine Lactoferricin Exhibit Antimicrobial Activity against E. coli ATCC 11775, S. maltophilia ATCC 13636 and S. enteritidis ATCC 13076

Linear, dimeric, tetrameric, and cyclic peptides derived from lactoferricin B–containing non-natural amino acids and the RWQWR motif were synthesized, purified, and characterized using RP-HPLC, MALDI-TOF mass spectrometry, and circular dichroism. The antibacterial activity of peptides against Escherichia coli ATCC 11775, Stenotrophomonas maltophilia ATCC 13636, and Salmonella enteritidis ATCC 13076 was evaluated. The minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) were determined. The synthetic bovine lactoferricin exhibited antibacterial activity against E. coli ATCC 11775 and S. enteritidis ATCC 13076. The dimeric peptide (RRWQWR)₂K-Ahx exhibited the highest antibacterial activity against the tested bacterial strain. The monomeric, cyclic, tetrameric, and palindromic peptides containing the RWQWR motif exhibited high and specific activity against E. coli ATCC 11775. The results suggest that short peptides derived from lactoferricin B could be considered as potential candidates for the development of antibacterial agents against infections caused by E. coli.

Structure activity relationship modelling of milk protein-derived peptides with dipeptidyl peptidase IV (DPP-IV) inhibitory activity

Quantitative structure activity type models were developed in an attempt to predict the key features of peptide sequences having dipeptidyl peptidase IV (DPP-IV) inhibitory activity. The models were then employed to help predict the potential of peptides, which are currently reported in the literature to be present in the intestinal tract of humans following milk/dairy product ingestion, to act as inhibitors of DPP-IV. Two models (z- and v-scale) for short (2-5 amino acid residues) bovine milk peptides, behaving as competitive inhibitors of DPP-IV, were developed. The z- and the v-scale models (p<0.05, R(2) of 0.829 and 0.815, respectively) were then applied to 56 milk protein-derived peptides previously reported in the literature to be found in the intestinal tract of humans which possessed a structural feature of DPP-IV inhibitory peptides (P at the N2 position). Ten of these peptides were synthetized and tested for their in vitro DPP-IV inhibitory properties. There was no agreement between the predicted and experimentally determined DPP-IV half maximal inhibitory concentrations (IC50) for the competitive peptide inhibitors. However, the ranking for DPP-IV inhibitory potency of the competitive peptide inhibitors was conserved. Furthermore, potent in vitro DPP-IV inhibitory activity was observed with two peptides, LPVPQ (IC50=43.8±8.8μM) and IPM (IC50=69.5±8.7μM). Peptides present within the gastrointestinal tract of human may have promise for the development of natural DPP-IV inhibitors for the management of serum glucose.

Discovery of a 9-mer Cationic Peptide (LTX-315) as a Potential First in Class Oncolytic Peptide

Oncolytic immunotherapies represent a new promising strategy in the treatment of cancer. In our efforts to develop oncolytic peptides, we identified a series of chemically modified 9-mer cationic peptides that were highly effective against both drug-resistant and drug-sensitive cancer cells and with lower toxicity toward normal cells. Among these peptides, LTX-315 displayed superior anticancer activity and was selected as a lead candidate. This peptide showed relative high plasma protein binding abilities and a human plasma half-life of 160 min, resulting in formation of nontoxic metabolites. In addition, the lead candidate demonstrated relatively low ability to inhibit CYP450 enzymes. Collectively these data indicated that this peptide has potential to be developed as a new anticancer agent for intratumoral administration and is currently being evaluated in a phase I/IIa study.

Learnings from quantitative structure–activity relationship (QSAR) studies with respect to food protein-derived bioactive peptides: a review

QSAR studies may help to better understand structural requirements for peptide bioactivity and therefore to develop potent BAPs.

Antibacterial activity of synthetic peptides derived from lactoferricin against Escherichia coli ATCC 25922 and Enterococcus faecalis ATCC 29212

Peptides derived from human and bovine lactoferricin were designed, synthesized, purified, and characterized using RP-HPLC and MALDI-TOF-MS. Specific changes in the sequences were designed as (i) the incorporation of unnatural amino acids in the sequence, the (ii) reduction or (iii) elongation of the peptide chain length, and (iv) synthesis of molecules with different number of branches containing the same sequence. For each peptide, the antibacterial activity against Escherichia coli ATCC 25922 and Enterococcus faecalis ATCC 29212 was evaluated. Our results showed that Peptides I.2 (RWQWRWQWR) and I.4 ((RRWQWR)4K2Ahx2C2) exhibit bigger or similar activity against E. coli (MIC 4-33 μM) and E. faecalis (MIC 10-33 μM) when they were compared with lactoferricin protein (LF) and some of its derivate peptides as II.1 (FKCRRWQWRMKKLGA) and IV.1 (FKCRRWQWRMKKLGAPSITCVRRAE). It should be pointed out that Peptides I.2 and I.4, containing the RWQWR motif, are short and easy to synthesize; our results demonstrate that it is possible to design and obtain synthetic peptides that exhibit enhanced antibacterial activity using a methodology that is fast and low-cost and that allows obtaining products with a high degree of purity and high yield.

Synthetic cationic antimicrobial peptides bind with their hydrophobic parts to drug site II of human serum albumin

Background

Many biologically active compounds bind to plasma transport proteins, and this binding can be either advantageous or disadvantageous from a drug design perspective. Human serum albumin (HSA) is one of the most important transport proteins in the cardiovascular system due to its great binding capacity and high physiological concentration. HSA has a preference for accommodating neutral lipophilic and acidic drug-like ligands, but is also surprisingly able to bind positively charged peptides. Understanding of how short cationic antimicrobial peptides interact with human serum albumin is of importance for developing such compounds into the clinics.

Results

The binding of a selection of short synthetic cationic antimicrobial peptides (CAPs) to human albumin with binding affinities in the μM range is described. Competitive isothermal titration calorimetry (ITC) and NMR WaterLOGSY experiments mapped the binding site of the CAPs to the well-known drug site II within subdomain IIIA of HSA. Thermodynamic and structural analysis revealed that the binding is exclusively driven by interactions with the hydrophobic moieties of the peptides, and is independent of the cationic residues that are vital for antimicrobial activity. Both of the hydrophobic moieties comprising the peptides were detected to interact with drug site II by NMR saturation transfer difference (STD) group epitope mapping (GEM) and INPHARMA experiments. Molecular models of the complexes between the peptides and albumin were constructed using docking experiments, and support the binding hypothesis and confirm the overall binding affinities of the CAPs.

Conclusions

The biophysical and structural characterizations of albumin-peptide complexes reported here provide detailed insight into how albumin can bind short cationic peptides. The hydrophobic elements of the peptides studied here are responsible for the main interaction with HSA. We suggest that albumin binding should be taken into careful consideration in antimicrobial peptide studies, as the systemic distribution can be significantly affected by HSA interactions.

Recombinant expression, purification, and antimicrobial activity of a novel hybrid antimicrobial peptide LFT33

With great therapeutic potential against antibiotic-resistant bacteria, viruses, and even parasites, antimicrobial peptides (AMPs) have received increased interest as pharmaceutical agents in recent years. It is a worthy yet challenging work to carry out the implement and improvement of AMPs production using bioengineering techniques. In the present study, a novel hybrid peptide LFT33 was designed derived from LfcinB and thanatin. The cDNA fragment encoding LFT33 with preferred codons of Escherichia coli was chemically synthesized and ligated into the vector pET32a(+) to express the LFT33 fusion protein. The fusion protein was successfully expressed in soluble form in E. coli induced under optimized conditions. After purification by affinity chromatography, the fusion protein was cleaved successfully by enterokinase and released the peptide LFT33. About 0.5 mg of the recombinant LFT33 was obtained by reversed-phase high performance liquid chromatography from 1 l of culture medium. Mass spectrometry analysis of the purified recombinant LFT33 demonstrated that the molecular weight perfectly matched the calculated mass (4,195 Da). The recombinant peptide LFT33 caused an increase in antimicrobial activity (IC(50) = 16-64 μg/ml) against given strains and did not show hemolytic activity for human erythrocytes. The results indicated that the hybrid peptide LFT33 could serve as a promising candidate for pharmaceutical agents.

Conformational study of bovine lactoferricin in membrane-micking conditions by molecular dynamics simulation and circular dichroism

Lactoferricins are potent antimicrobial peptides released by pepsin cleavage of Lactoferrins. Bovine Lactoferricin (LfcinB) has higher activity than the intact bovine Lactoferrin, and is the most active among the other Lactoferricins of human, murine and caprine origin. In the intact protein the fragment corresponding to LfcinB is in an helical conformation, while in water LfcinB adopts an amphipathic β-hairpin structure. However, whether any of these structural motifs is the antibacterial active conformation, i.e., the one interacting with bacterial membrane components, remains to be seen. Here we present Circular Dichroism (CD) spectra and Molecular Dynamics (MD) simulations indicating that in membrane-mimicking solvents the LfcinB adopts an amphipathic β-hairpin structure similar to that observed in water, but differing in the dynamic behavior of the side-chains of the two tryptophan residues. In the membrane-mimicking solvent these side-chains show a high propensity to point towards the hydrophobic environment, rather than being in the hydrophobic core as seen in water, while the backbone preserves the hairpin conformation as found in water. These results suggest that the tryptophans might act as anchors pulling the stable, solvent-invariant hairpin structure into the membrane.

Designed low amphipathic peptides with alpha-helical propensity exhibiting antimicrobial activity via a lipid domain formation mechanism

Although several low amphipathic peptides have been known to exhibit antimicrobial activity, their mode of action has not been completely elucidated. In this study, using designed low amphipathic peptides that retain different alpha-helical content and hydrophobicity, we attempted to investigate the mechanism of these properties. Calorimetric and thermodynamic analyses demonstrated that the peptides induce formation of two lipid domains in an anionic liposome at a high peptide-to-lipid ratio. On the other hand, even at a low peptide-to-lipid ratio, they caused minimal membrane damage, such as flip-flop of membrane lipids or leakage of calcein molecules from liposomes, and never translocated across membranes. Interaction energies between the peptides and anionic liposomes showed good correlation with antimicrobial activity for both Escherichia coli and Bacillus subtilis. We thus propose that the domain formation mechanism in which antimicrobial peptides exhibit activity solely by forming lipid domains without membrane damage is a major determinant of the antimicrobial activity of low amphipathic peptides. These peptides appear to stiffen the membrane such that it is deprived of the fluidity necessary for biological functions. We also showed that to construct the lipid domains, peptides need not form stable and cooperative structures. Rather, it is essential for peptides to only interact tightly with the membrane interface via strong electrostatic interactions, and slight differences in binding strength are invoked by differences in hydrophobicity. The peptides thus designed might pave the way for "clean" antimicrobial reagents that never cause release of membrane elements and efflux of their inner components.

Study of the interaction of lactoferricin B with phospholipid monolayers and bilayers

Bovine lactoferricin (LfcinB) is an antimicrobial peptide obtained from the pepsin cleavage of lactoferrin. The activity of LfcinB has been extensively studied on diverse pathogens, but its mechanism of action still has to be elucidated. Because of its nonspecificity, its mode of action is assumed to be related to interactions with membranes. In this study, the interaction of LfcinB with a negatively charged monolayer of dipalmitoylphosphatidylglycerol has been investigated as a function of the surface pressure of the lipid film using in situ Brewster angle and polarization modulation infrared reflection absorption spectroscopy and on transferred monolayers by atomic force microscopy and polarized attenuated total reflection infrared spectroscopy. The data show clearly that LfcinB forms stable films at the air-water interface. They also reveal that the interaction of LfcinB with the lipid monolayer is modulated by the surface pressure. At low surface pressure, LfcinB inserts within the lipid film with its long molecular axis oriented mainly parallel to the acyl chains, while at high surface pressure, LfcinB is adsorbed under the lipid film, the hairpin being preferentially aligned parallel to the plane of the interface. The threshold for which the behavior changes is 20 mN/m. At this critical surface pressure, LfcinB interacts with the monolayer to form discoidal lipid-peptide assemblies. This structure may actually represent the mechanism of action of this peptide. The results obtained on monolayers are correlated by fluorescent probe release measurements of dye-containing vesicles made of lipids in different phases and support the important role of the lipid fluidity and packing on the activity of LfcinB.

Hydrolytic breakdown of lactoferricin by lactic acid bacteria

Lactoferricin is a 25-amino acid antimicrobial peptide fragment that is liberated by pepsin digestion of lactoferrin present in bovine milk. Along with its antibacterial properties, lactoferricin has also been reported to have immunostimulatory, antiviral, and anticarcinogenic effects. These attributes provide lactoferricin and other natural bioactive peptides with the potential to be functional food ingredients that can be used by the food industry in a variety of applications. At present, commercial uses of these types of compounds are limited by the scarcity of information on their ability to survive food processing environments. We have monitored the degradation of lactoferricin during its incubation with two types of lactic acid bacteria used in the yogurt-making industry, Streptococcus thermophilus and Lactobacillus delbrueckii ssp. bulgaricus, with the aim of assessing the stability of this milk protein-derived peptide under simulated yogurt-making conditions. Analysis of the hydrolysis products isolated from these experiments indicates degradation of this peptide near neutral pH by lactic acid bacteria-associated peptidases, the extent of which was influenced by the bacterial strain used. However, the data also showed that compared to other milk-derived bioactive peptides that undergo complete degradation under these conditions, the 25-amino acid lactoferricin is apparently more resistant, with approximately 50% of the starting material remaining after 4 h of incubation. These findings imply that lactoferricin, as a natural milk protein-derived peptide, has potential applications in the commercial production of yogurt-like fermented dairy products as a multi-functional food ingredient.

Serum stability of peptides

Hospitals worldwide have lately reported a worrying increase in the number of isolated drug-resistant pathogenic microbes. This has to some extent fueled at least academic interest in design and development of new lead components for novel drug design. Much of this interest has been focused on antimicrobial peptides and peptides in general, primarily due to their natural occurrence and low toxicity. However, issues have been raised regarding the stability of peptide therapeutics for systemic use. The focus of this chapter is assays for measuring peptide stability in the presence of serum, both in vitro and in vivo.

Multimerization and fusion expression of bovine lactoferricin derivative LfcinB15-W4,10 in Escherichia coli

Antimicrobial peptides are promising candidates for therapeutic and industrial application owing to their broad spectrum. In this work, a cost-effective method for expression of a potent antimicrobial peptide, bovine lactoferricin derivative LfcinB15-W4,10, has been developed. The oligonucleotide encoding the peptide was linked to generate different oligomeric oligonucleotide segments containing from one to nine but eight tandem copies which was inserted individually to the E. coli expression vector pET32a. The thioredoxin fusion peptides were successfully expressed and detected with different molecular weight on SDS gel, respectively. Among the monomer and other multimeric peptides, the tetramer was expressed at the highest level. After purification, more than 10 mg of tetramer with 99% purity was obtained from 1 l culture and exhibited similar antimicrobial activity as synthetic LfcinB15-W4,10 monomer. The expression system in this study provides a potential production method for lactoferricin derivatives and other antimicrobial peptides in research and industrial applications.

Comparison of NMR structures and model-membrane interactions of 15-residue antimicrobial peptides derived from bovine lactoferricin

LFB (FKCRRWQWRMKKLGA-HN2) is a 15-residue linear antimicrobial peptide derived from bovine lactoferricin, which has antimicrobial activity similar to that of the intact 25-residue disulfide-cyclized peptide. Previous alanine-scan studies, in which all of the residues in LFB were individually replaced with Ala, showed that the 2 tryptophan (Trp) residues of LFB were crucial to its antimicrobial activity. When either Trp6 or Trp8 was replaced with Ala (LFBA6 and LFBA8, respectively), these 2 peptides were almost devoid of antimicrobial activity. We determined the structures of LFB, LFBA6, and LFBA8 bound to membrane-mimetic SDS micelles using NMR spectroscopy, and studied their interactions with different phospholipid-model membranes. The membrane interactions of LFB exhibited little correlation with its antimicrobial activity, suggesting that the mechanism of action of LFB involves intracellular targets. However, the much higher antimicrobial activity of LFB compared with LFBA6 and LFBA8 might result, in part, from the formation of energetically favorable cation-pi interactions observed only in LFB. Information about the importance of Arg and Trp cation-pi interactions will provide insight for the future design of potent antimicrobial peptidomimetics.

Tryptophan- and arginine-rich antimicrobial peptides: Structures and mechanisms of action

Antimicrobial peptides encompass a number of different classes, including those that are rich in a particular amino acid. An important subset are peptides rich in Arg and Trp residues, such as indolicidin and tritrpticin, that have broad and potent antimicrobial activity. The importance of these two amino acids for antimicrobial activity was highlighted through the screening of a complete combinatorial library of hexapeptides. These residues possess some crucial chemical properties that make them suitable components of antimicrobial peptides. Trp has a distinct preference for the interfacial region of lipid bilayers, while Arg residues endow the peptides with cationic charges and hydrogen bonding properties necessary for interaction with the abundant anionic components of bacterial membranes. In combination, these two residues are capable of participating in cation-pi interactions, thereby facilitating enhanced peptide-membrane interactions. Trp sidechains are also implicated in peptide and protein folding in aqueous solution, where they contribute by maintaining native and nonnative hydrophobic contacts. This has been observed for the antimicrobial peptide from human lactoferrin, possibly restraining the peptide structure in a suitable conformation to interact with the bacterial membrane. These unique properties make the Arg- and Trp-rich antimicrobial peptides highly active even at very short peptide lengths. Moreover, they lead to structures for membrane-mimetic bound peptides that go far beyond regular alpha-helices and beta-sheet structures. In this review, the structures of a number of different Trp- and Arg-rich antimicrobial peptides are examined and some of the major mechanistic studies are presented.

Ultrastructural effects of antimicrobial peptides from bovine lactoferrin on the membranes of Candida albicans and Escherichia coli

Antimicrobial peptides allegedly exert their action on microbial membranes. Bovine lactoferrin enfold two antimicrobial domains, lactoferricin B (LFcin B) and lactoferrampin (LFampin). Effects of representative peptides thereof on the membranes of Candida albicans and Escherichia coli were investigated. Confocal laser scanning microscopy revealed that these peptides were internalized within a few minutes, concurrently with disrupting membrane integrity as indicated by freeze-fracture transmission electron microscopy. The most striking findings were induction of distinct vesicle-like structures in the membrane of C. albicans by the LFampin peptide, and detachment of the outer membrane and surface protrusions in E. coli by the LFcin B peptide.

Deletion of two C-terminal Gln residues of 12-26-residue fragment of melittin improves its antimicrobial activity

In our previous paper it was shown that the two C-terminal Gln residues of a C-terminal 15-residue fragment, Mel(12-26) (GLPALISWIKRKRQQ-NH2), of melittin and a series of individual substituted analogues might not involved in the interaction with bacterial membranes. In this paper, peptides with one and two Gln residues deletion, respectively, Mel(12-25) and Mel(12-24), were synthesized and characterized. Both of the deletion peptides showed higher antimicrobial activities than the parent peptide, Mel(12-26). If both of the Gln residues of Mel(12-26) were respectively replaced by a hydrophilic amino acid Gly, the antimicrobial activity increased slightly. If the Gln residue of Mel(12-25) was replaced by a hydrophobic amino acid Leu, the antimicrobial activity changed little, although the substituted peptide possessed much higher hydrophobicity and higher alpha-helical conformation percentage in 1,1,1,3,3,3-hexafluoro-2-propanol/water determined by circular dichroism spectroscopy (CD) than the parent peptide. These results indicated that the two C-terminal residues might be indeed not involved in the binding to bacterial membranes. The antimicrobial activity increasing with the residue deletion may be caused by the decrease of the translational and rotational entropic cost of the binding of the peptides to bacterial membranes because of the lower molecular weights of the deletion peptides.

Bulky nonproteinogenic amino acids permit the design of very small and effective cationic antibacterial peptides

The rate of multidrug-resistant infections is rapidly rising. Cationic antibacterial peptides are active against resistant pathogens and have low propensity for resistance development, but because of their unfavorable medicinal properties, cationic antibacterial peptides have been a limited clinical success. We have found that introduction of nongenetically coded amino acids and other lipophilic modifications opens the opportunity for development of extremely short and highly active antibacterial peptides with improved medicinal properties.

Anti-HSV activity of lactoferricin analogues is only partly related to their affinity for heparan sulfate

Earlier studies have shown that the heparan sulfate (HS) on the cell surface acts as a receptor for herpes simplex virus (HSV). We have recently shown that bovine lactoferricin (LfcinB), a small part of the milk protein lactoferrin, inhibits HSV-1 and HSV-2 infection, probably by blocking the entry of the virus. The human homologue (18-42), which shares 36% sequence similarity with LfcinB (17-41), displayed much lower antiviral activity. In the present study, a set of cyclic and linear human and bovine Lfcin derivatives were constructed to investigate the relation between their affinity to HS and chondroitin sulfate (CS) and their antiviral activity against HSV-1 and HSV-2. The lactoferrin (LF) proteins and several of the Lfcin derivatives exhibited similar affinity for HS, but the LF proteins possess a much higher antiviral activity than the smaller peptides. Our structure-activity relationship studies on the Lfcin derivates confirmed that affinity for HS, that was correlated to the net positive charge, is an important factor, but does not well predict the antiviral activity. Structural parameters such as hydrophobicity, molecular size, spatial distribution of charged and lipophilic amino acids, and the cyclic structure of Lfcin also seem to be important factors to govern antiviral activity against HSV.

Enhanced antitumour activity of 15-residue bovine lactoferricin derivatives containing bulky aromatic amino acids and lipophilic N-terminal modifications

In a structure-antibacterial activity relationship study of a peptide fragment of bovine lactoferricin consisting of FKCRRWQWRMKKLGA (LFB 17-31), it was revealed that the two Trp residues were important for antibacterial activity. It has further been demonstrated that the size, shape and the aromatic character of the side chains were even more important than the Trp itself. In this study the antitumour effect of a series of LFB 17-31 derivatives are reported, in which the two Trp residues in position 6 and 8 were replaced with the larger non-coded aromatic amino acids Tbt, Tpc, Bip and Dip. The counterproductive Cys in position 3 was also substituted with these larger aromatic residues. In addition, the effect of introducing lipophilic groups of different size and shape in the N-terminal of the LFB 17-31 sequence was addressed. The resulting peptide derivatives were tested for activity against three human tumour cell lines and against normal human umbilical vein endothelial cells and fibroblasts. High antitumour activity by several of the peptides demonstrated that Trp successfully could be substituted by the bulky aromatic residues, and peptides containing the large and rigid Tbt residue in position 6 and/or 8 in LFB 17-31 were the most active candidates. The antitumour effect was even more increased by the Tbt-modified peptides when the three counterproductive amino acids Cys3, Gln7 and Gly14 were replaced by Ala. Enhanced antitumour activity was also obtained by modifying the N-terminal of LFB 17-31 with either long-chained fatty acids or bulky moieties. Thus, our results revealed that the size and shape of the lipophilic groups and their position in the peptide sequence were important for antitumour activity.

Tryptophan-rich antimicrobial peptides: comparative properties and membrane interactions

The interaction of several tryptophan (Trp)-rich cationic antimicrobial peptides with membranes was investigated. These peptides included tritrpticin, indolicidin, lactoferricin B (Lfcin B), and a shorter fragment of lactoferricin (LfcinB4-9). The average environment of the Trp residues of these peptides was assessed from their fluorescence properties, both the wavelength of maximal emission as well as the red edge effect. The insertion of the peptides into vesicles of differing composition was examined using quenching of the Trp fluorescence, with both soluble acrylamide and nitroxide-labelled phospholipids as well as by chemical modification of the Trp residues with N-bromosuccinimide. The results were consistent with the Trp side chains positioned mostly near the membrane-water interface. The extent of burial of the Trp side chains appears to be greater in vesicles containing phospholipids with the anionic phosphatidylglycerol headgroup. Leakage of the aqueous contents of liposomes was also measured using the 8-aminonaphthalene-1,3,6-trisulfonic acid--p-xylene-bis-pyridinium bromide assay. Tritrpticin, which demonstrated the greatest red edge shift, also displayed the largest amount of leakage from liposomes. Taken together, the results illustrate that cationic Trp-rich antimicrobial peptides preferentially disrupt large unilamellar vesicles with a net negative charge following their insertion into the interfacial region of the phospholipid bilayer.

Homology similarity analysis of sequences of lactoferricin and its derivatives

A new method, homology similarity analysis (HSA), was developed to investigate homology pattern similarities of selected segments within sequences of peptides. This new approach facilitated elucidation of the structure-function relationships of lactoferricin derivatives. Helix propensity of positions 4-9 in the lactoferricin sequence was the most important in determining the antimicrobial activity of lactoferricin against Escherichia coli, followed by cationic charge pattern at positions 4-9 and 1-3. The pattern similarity of segments within sequences could be a useful tool for representing the distribution attributes of amino acid residue properties to the structure-function relationships of proteins and peptides, especially when used in conjunction with principal component similarity analysis followed by the regression version of artificial neural networks.

Important structural features of 15-residue lactoferricin derivatives and methods for improvement of antimicrobial activity

This review focuses on important structural features affecting the antimicrobial activity of 15-residue derivatives of lactoferricins. Our investigations are based on an alanine-scan of a 15-residue bovine lactoferricin derivative that revealed the absolute necessity of two tryptophan residues for antimicrobial activity. This "tryptophan-effect" was further explored in homologous derivatives of human, caprine, and porcine lactoferricins by the incorporation of one additional tryptophan residue, and by increasing the content of tryptophan in the bovine derivative to five residues. Most of the resulting peptides display a substantial increase in antimicrobial activity. To identify which molecular properties make tryptophan so effective, a series of bovine lactoferricin derivatives were prepared containing non-encoded unnatural aromatic amino acids, which represented various aspects of the physicochemical nature of tryptophan. The results clearly demonstrate that tryptophan is not unique since most of the modified peptides were of higher antimicrobial potency than the native peptide. The size and three-dimensional shape of the inserted "super-tryptophans" are the most important determinants for the high antimicrobial activity of the modified peptides. This review also describes the use of a "soft-modeling" approach in order to identify important structural parameters affecting the antimicrobial activity of modified 15-residue murine lactoferricin derivatives. This QSAR-study revealed that the net charge, charge asymmetry, and micelle affinity of the peptides were the most important structural parameters affecting their antimicrobial activity.

Towards a structure-function analysis of bovine lactoferricin and related tryptophan- and arginine-containing peptides

The iron-binding protein lactoferrin is a multifunctional protein that has antibacterial, antifungal, antiviral, antitumour, anti-inflammatory, and immunoregulatory properties. All of these additional properties appear to be related to its highly basic N-terminal region. This part of the protein can be released in the stomach by pepsin cleavage at acid pH. The 25-residue antimicrobial peptide that is released is called lactoferricin. In this work, we review our knowledge about the structure of the peptide and attempt to relate this to its many functions. Microcalorimetry and fluorescence spectroscopy data regarding the interaction of the peptide with model membranes show that binding to net negatively charged bacterial and cancer cell membranes is preferred over neutral eukaryotic membranes. Binding of the peptide destabilizes the regular membrane bilayer structure. Residues that are of particular importance for the activity of lactoferricin are tryptophan and arginine. These two amino acids are also prevalent in "penetratins", which are regions of proteins or synthetic peptides that can spontaneously cross membranes and in short hexapeptide antimicrobial peptides derived through combinatorial chemistry. While the antimicrobial, antifungal, antitumour, and antiviral properties of lactoferricin can be related to the Trp/Arg-rich portion of the peptide, we suggest that the anti-inflammatory and immunomodulating properties are more related to a positively charged region of the molecule, which, like the alpha- and beta-defensins, may act as a chemokine. Few small peptides are involved in as wide a range of host defense functions as bovine and human lactoferricin.

Enhanced antitumor activity and selectivity of lactoferrin-derived peptides

A number of peptide analogs derived from the N-terminal alpha-helical region of bovine lactoferrin (LFB 14-31), were designed in order to investigate how deviating numbers and positions of positively charged residues and numbers of aromatic residues affected their activity against prokaryotic, normal and transformed eukaryotic cells. Most of the LFB derivatives were highly active against both Escherichia coli and Staphylococcus aureus. The peptides were more active against the tumor cell lines MethA, HT-29 and MT-1 than normal eukaryotic cells. The peptides that were most active against the tumor cell lines had all cationic residues concentrated in one sector of the helical structure. These peptides were less selective against the tumor cell lines than against normal fibroblasts. Quantitative structure-activity relationship studies showed that certain structural parameters affected toxicity against the tumor cell lines more than against fibroblasts. Peptides encompassing these parameters were slightly less active against tumor cells, but gained significant selectivity.

Simple parameterization of non-proteinogenic amino acids for QSAR of antibacterial peptides

The antibacterial activity of bovine lactoferricin-(17-31)-pentadecapeptide against Escherichia coli and Staphylococcus aureus is sensitive to substitution of the Trp residues, and synthetic peptides with phenylalanine and any of eight non-proteinogenic aromatic amino acids greatly affected antibiotic activity. Using simple size-related descriptors for the new amino acids it is possible to develop quantitative structure-activity relationships (QSARs) that can be used as tools in the search for more active peptides.

Synthesis of a 2-arylsulphonylated tryptophan: the antibacterial activity of bovine lactoferricin peptides containing Trp(2-Pmc)

A modified tryptophan, beta-[2-(2,2,5,7,8-pentamethylchroman-6-sulphonyl)-indol-3-yl]alanine, Trp(2-Pmc) = Tpc has been synthesized. Replacement of tryptophan in a bovine lactoferricin model peptide with the modified tryptophan resulted in peptides with a substantially increased antibacterial activity against Escherichia coli and Staphylococcus aureus. The most active peptides against each bacterial strain displayed minimal inhibitory concentrations of 7.5 microg/ml.

The effects of charge and lipophilicity on the antibacterial activity of undecapeptides derived from bovine lactoferricin

We have investigated the effects of charge and lipophilicity on the antibacterial activity of an undecapeptide (FKCRRWQWRMK) derived from the sequence of bovine lactoferricin. We prepared ten analogues that were modified by the incorporation of Ala, Tyr, Trp, Met and Arg residues, which are amino acids known to be important for the antibacterial activity of longer derivatives of lactoferricins. All undecapeptides contained the native Trp residues in positions 6 and 8, and the Arg residues in positions 5 and 9. Generally, the Gram-positive bacterium Staphylococcus aureus was more susceptible to these undecapeptides than the Gram-negative bacteria, and a higher antibacterial activity was observed against Escherichia coli than against Pseudomonas aeruginosa. The only exception was the peptide Undeca 9 (RRWYRWAWRMR-NH2), which was almost equally active against all three test strains, displaying minimal inhibitory concentrations of 10 microg/ml (5.8 microM), 7.5 microg/ml (4.4 microM) and 5 microg/ml (2.9 microM) against Escherichia coli, Pseudomonas aeruginosa and Staphylococcus aureus, respectively. The peptides Undeca 6 (YRAWRWAWRWR-NH2) and Undeca 7 (YRMWRWAWRWR-NH2) were the two most active undecapeptides against Staphylococcus aureus, both displaying a minimal inhibitory concentration of 2.5 microg/ml (1.5 microM). The study showed that a level was reached in which undecapeptides having a net charge above +4 and containing three or four Trp residues all displayed a high antibacterial activity. All undecapeptides prepared were essentially non-haemolytic, but undecapeptides containing more than three Trp residues displayed 50% haemolysis of human red blood cells at concentrations above 400 microg/ml (>230 microM).

The role of tryptophan in the antibacterial activity of a 15-residue bovine lactoferricin peptide

Bovine lactoferricin is a 25-residue antibacterial peptide isolated after gastric cleavage of the iron transporting protein lactoferrin. A 15-residue fragment, FKCRRWQWRMKKLGA of this peptide sustains most of the antibacterial activity. In this truncated sequence, the two Trp residues are found to be essential for antibacterial activity. The anchoring properties of Trp, as have been observed in membrane proteins, are believed to be important for the interaction of Trp containing antibacterial peptides with bacterial cell membranes. We have investigated the molecular properties which make Trp important for the antibacterial activity of the 15-residue peptide by replacing Trp with natural and unnatural aromatic amino acids. This series of peptides was tested for antibacterial activity against Echerichia coli and Staphylococcus aureus. We found that neither the hydrogen bonding ability nor the amphipathicity of the indole system are essential properties for the effect of Trp on the antibacterial activity of the peptides. Replacement of Trp with residues containing aromatic hydrocarbon side chains gave the most active peptides. We propose that aromatic hydrocarbon residues are able to position themselves deeper into the bacterial cell membrane, making the peptide more efficient in disrupting the bacterial cell membrane. From our results the size, shape and aromatic character of Trp seem to be the most important features for the activity of this class of Trp containing antibacterial peptides.