Morphological alterations accompanying the effect of peptaibiotics, α-aminoisobutyric acid-rich secondary metabolites of filamentous fungi, on Culex pipiens larvae

Sequences of Tolypins, Insecticidal Efrapeptin-Type Peptaibiotics from Species of the Fungal Genus Tolypocladium

A peptide mixture named tolypin, originally isolated from species of the fungal genus Tolypocladium, was structurally characterised and sequences compared to those reported for efrapeptins isolated from strains of Tolypocladium inflatum. Chiral amino acid analysis, direct infusion, and online HPLC electrospray ionization tandem mass spectrometry provided composition, molecular weights of peptides, and series of diagnostic fragment ions. Sequences deduced from ESI-MS revealed that tolypins C-G are identical to efrapeptins C-G. The results were corroborated by ESI-MS and HPLC of an authentic efrapeptin sample from Eli Lilly Research Laboratories (USA). Comparison of the HPLC elution profiles of efrapeptin and tolypin indicated a pronounced microheterogeneity of the former. A high-resolution HPLC of authentic efrapeptin has not been published before. Close relationship and partial identity of sequences of tolypins and efrapeptins, which had previously been postulated, were definitely proven. The geographical origin of the two most important T. inflatum strains used for sequencing of efrapeptins/tolypins could unambiguously be clarified. A new minor compound, designated tolypin H1, was sequenced. High proportions of helicogenic Aib (α-aminoisobutyric acid) and l-isovaline, N-terminal acetyl-l-pipecolic acid and the unusual, amide-bound C-terminal residue, named (S)-2-amino-1-(1,5-diazabicyclo[4.3.0]non-5-ene-5-ylium)-4-methylpentane corresponding to 1-[(2S)-2-amino-4-methylpentyl]-2,3,4,6,7,8-hexahydropyrrolo[1,2-a]pyrimidin-1-ium, define these peptides as linear, cationic peptaibiotics.

Development and Utilization of a Palladium-Catalyzed Dehydration of Primary Amides To Form Nitriles

A palladium(II) catalyst, in the presence of Selectfluor, enables the efficient and chemoselective transformation of primary amides into nitriles. The amides can be attached to aromatic rings, heteroaromatic rings, or aliphatic side chains, and the reactions tolerate steric bulk and electronic modification. Dehydration of a peptaibol containing three glutamine groups afforded structure–activity relationships for each glutamine residue. Thus, this dehydration can act similarly to an alanine scan for glutamines via synthetic mutation.

Synergistic combination of alkylphosphocholines with peptaibols in targeting Leishmania infantum in vitro

Anti-leishmanial treatment increasingly encounters therapeutic limitations due to drug toxicity and development of resistance. The effort for new therapeutic strategies led us to work on combinations of chemically different compounds that could yield enhanced leishmanicidal effect. Peptaibols are a special type of antimicrobial peptides that are able to form ion channels in cell membranes and potentially affect cell viability. We assayed the antileishmanial activity of two well studied helical peptaibols, the 16-residue antiamoebin and the 20-residue alamethicin-analogue suzukacillin, and we evaluated the biological effect of their combination with the alkylphosphocholine miltefosine and its synthetic analogue TC52. The peptaibols tested exhibited only moderate antileishmanial activity, however their combination with miltefosine had a super-additive effect against the intracellular parasite (combination index 0.83 and 0.43 for antiamoebin and suzukacillin respectively). Drug combinations altered the redox stage of promastigotes, rapidly dissipated mitochondrial membrane potential and induced concatenation of mitochondrial network promoting spheroidal morphology. These results evidenced a potent and specific antileishmanial effect of the peptaibols/miltefosine combinations, achieved with significantly lower concentrations of the compounds compared to monotherapy. Furthermore, they revealed the importance of exploring novel classes of bioactive compounds such as peptaibols and demonstrated for the first time that they can act in synergy with currently used antileishmanial drugs to improve the therapeutic outcome.

Conformational properties of secondary amino acids: replacement of pipecolic acid by N-methyl-l-alanine in efrapeptin C

The efrapeptins, a family of naturally occurring peptides with inhibitory activities against ATPases, contain several α,α-disubstituted α-amino acids such as α-aminoisobutyric acid (Aib) or isovaline (Iva) besides pipecolic acid (Pip), β-Ala, Leu, Gly, and a C-terminal heterocyclic residue. Secondary α-amino acids such as proline are known to stabilize discrete conformations in peptides. A similar influence is ascribed to N-alkyl α-amino acids. We synthesized two efrapeptin C analogs with replacement of Pip by N-methyl-L-alanine (MeAla) using a combination of solid- and solution-phase techniques in a fragment-condensation strategy to compare the conformational bias of both secondary amino acids. The solution conformation was investigated by vibrational circular dichroism (VCD) to probe whether the analogs adopt a 310 -helical conformation. The MeAla-containing analogs [MeAla(1,3) ]efrapeptin C and [MeAla(1,3,11) ]efrapeptin C inhibit ATP hydrolysis by the A3 B3 complex of A1 A0 -ATP synthase from Methanosarcina mazei Gö1.

Screening the biosphere: the fungicolous fungus Trichoderma phellinicola, a prolific source of hypophellins, new 17-, 18-, 19-, and 20-residue peptaibiotics

To investigate the significance of antibiotics for the producing organism(s) in the natural habitat, we screened a specimen of the fungicolous fungus Trichoderma phellinicola (syn. Hypocrea phellinicola) growing on its natural host Phellinus ferruginosus. Results revealed that a particular group of non-ribosomal antibiotic polypeptides, peptaibiotics, which contain the non-proteinogenic marker amino acid, α-aminoisobutyric acid, was biosynthesized in the natural habitat by the fungicolous producer and, consequently, released into the host. By means of liquid chromatography coupled to electrospray high-resolution time-of-flight mass spectrometry, we detected ten 20-residue peptaibols in the specimen. Sequences of peptaibiotics found in vivo were independently confirmed by analyzing the peptaibiome of an agar plate culture of T. phellinicola CBS 119283 (ex-type) grown under laboratory conditions. Notably, this strain could be identified as a potent producer of 39 new 17-, 18-, and 19-residue peptaibiotics, which display the same building scheme as the 20-residue peptaibols found in the specimen. Two of the 19-residue peptaibols are tentatively assigned to carry tyrosinol, a novel C-terminal residue, as deduced from high-resolution tandem mass-spectrometry data. For the new peptaibiotics produced by T. phellinicola, the name 'hypophellin(s)', based on the teleomorph name, is introduced.

Novel mycotoxin from Acremonium exuviarum is a powerful inhibitor of the mitochondrial respiratory chain complex III

A novel mycotoxin named acrebol, consisting of two closely similar peptaibols (1726 and 1740 Da), was isolated from an indoor strain of the mitosporic ascomycete fungus Acremonium exuviarum. This paper describes the unique mitochondrial toxicity of acrebol, not earlier described for any peptaibol. Acrebol inhibited complex III of the respiratory chain of isolated rat liver mitochondria (1 mg of protein mL(-1)) with an IC(50) of approximately 80 ng mL(-1) (50 nM) after a short preincubation, and 350 ng mL(-1) caused immediate and complete inhibition. Acrebol thus is a complex III inhibitor almost as potent as antimycin A and myxothiazol but completely different in structure. Similarly to myxothiazol but in contrast to antimycin A, acrebol decreased the level of mitochondrial superoxide anion detectable by chemiluminescent probe 3,7-dihydro-2-methyl-6-(4-methoxyphenyl)imidazol[1,2-a]pyrazine-3-one. Unlike other peptaibols, acrebol in toxic concentrations did not increase the ionic and solute permeability of membranes of isolated rat liver mitochondria, did not induce disturbance of the ionic homeostasis or the osmotic balance of mitochondria, and did not release apoptogenic proteins like cytochrome c from the intermembrane space of mitochondria. In boar spermatozoa, acrebol inhibited the respiratory chain and caused ATP depletion by activation of the oligomycin-sensitive F(0)F(1)-ATPase, which resulted in the inhibition of the progressive movement. In mouse insulinoma MIN-6 cells, whose energy supply solely depends on oxidative phosphorylation, acrebol induced necrosis-like death. The pathophysiological relevance of these findings is discussed.

The crystal structure of Z-Leu-Aib-Pro-Leuol, the synthetic, protected C-terminal tetrapeptide of trichovirin

The structure of the synthetic, protected tetrapeptide Z-Leu-Aib-Pro-Leuol(C29H46O6N4, Mw = 546.71) which contains the conformationally constrained residue α-aminoisobutyric acid (Aib), has been determined by X-ray crystallography. The title compound comprises the C-terminal tetrapeptide of trichovirin and has been chosen as a first step in a systematic analysis of the trichovirin structure-sequence relationships. Furthermore, the tetrapeptide has an interesting amino acid sequence, containing a strong helix former (Aib) and a helix breaker (Pro). The title compound crystallizes in P21 with a = 10.523(1) Å, b = 23.033(5) Å, c = 13.462(4) Å, β = 91.81(1)°, V = 3261(2) Å3, Z = 4, Dcalc = 1.135 g cm−3 (R = 0.054 for 3572 observed reflections). Both independent molecules in the asymmetric unit form a β-turn of type I with a 4 → 1 intramolecular hydrogen bond; the overall folding of each molecule fits approximately the pattern of a right-handed 310-helix, although the conformational angles φ, ψ of the Leu residues deviate significantly from the standard helical values. In the crystal the molecules are hydrogen-bonded head-to-tail forming infinitely long, helical columns along the a-axis; these columns are associated by hydrogen bonds forming layers parallel to the ac-plane.

Toxicity assessment of peptaibols and contaminated sediments on Crassostrea gigas embryos

Peptaibols are known membrane-modifying peptides that were recently detected in marine sediments and mussels collected from a shellfish farming area (Fier d'Ars, Atlantic coast, France). In this investigation, embryotoxicity bioassays with oysters (Crassostrea gigas) were performed to assess acute toxicity of alamethicin and different groups of peptaibols produced by a Trichoderma longibrachiatum strain isolated from marine environment. C. gigas embryos appeared very sensitive to all the metabolites examined with higher toxic effects for long-sequence peptides (EC50 ranging from 10 to 64 nM). D-shaped larvae with mantle abnormality were particularly noticed when peptaibol concentrations increased. Disturbances of embryogenesis were also observed following exposure to organic and aqueous extract of sediments from Fier d'Ars (EC50=42.4 and 6.6 g L(-1) dry weight, respectively). Although peptaibol concentrations measured in these sediments could explain only a part of the toxic effects observed, this study suggests that these mycotoxins can induce larval abnormalities in a population of exposed animals at environmentally realistic concentrations. Their detection in coastal areas devoted to bivalve culture should be taken into account.

Peptaibol production by sepedonium strains parasitizing boletales

Fungi of the genus Sepedonium (anamorphic ascomycetes) are known to infect fruiting bodies of Basidiomycetes of the order Boletales. We have characterized twelve Sepedonium isolates by intact-cell mass spectrometry (IC-MS) with the help of respective biomarkers and their metabolite spectra focusing on peptaibol production. A strain of mycoparasitic S. chalcipori was grown in solid-state fermentation, and tylopeptin production was found, suggesting an ascomycete origin of these peptaibols, which were first described in the basidiomycete Tylopilus neofelleus. In addition, the structures of two new peptaibols, chalciporin A (=Ac-Trp-Val-Aib-Val-Ala-Gln-Ala-Aib-Ser-Leu-Ala-Leu-Aib-Gln-Leuol) and chalciporin B (=Ac-Trp-Val-Aib-Val-Ala-Gln-Ala-Aib-Gln-Aib-Ala-Leu-Aib-Gln-Leuol) are presented. The IC-MS technique was applied for in situ peptaibol analysis of Sepedonium strains growing on Boletales, in particular S. chrysospermum infecting Xerocomus cf. badius. We found chrysospermins at the surface and within basidiomycete tissue, as well as in the cultivated parasite.

Peptaibols: stable markers of fungal development in the marine environment

Different peptaibols were observed in both fresh and frozen marine sediments collected from a marine area devoted to bivalve culture (Fier d'Ars, Atlantic coast, France). The identification of the peptaibols was based on a three-step mass-spectrometric analysis: observation of doubly charged ions with a characteristic isotopic profile, cleavage and observation of C- and N-terminal fragments, and partial sequencing of the N-terminal segments. The MS characteristics indicated numerous similarities between the peptaibols detected and those produced by different strains of Trichoderma species isolated from fresh sediments. Peptaibols were also detected in mussel samples collected at the same site. This constitutes the first observation of contamination of the marine human-food chain by fungal metabolites. Since peptaibols were readily observed both in fresh sediments and in samples kept frozen for several years, these compounds can be considered as stable markers of the development of Trichoderma in the marine environment.

Synthesis, and structural and biological studies of efrapeptin C analogues

A series of analogues of efrapeptin C (1), with variations in the central tripeptide epitope (positions 6-8), were prepared by a combination of solid- and solution-phase peptide syntheses. The conformations of the modified compounds 2-6 were investigated by circular-dichroism (CD) spectroscopy to differentiate between 3(10)- and alpha-helical secondary structures. The inhibitory activities of the new compounds towards F(1)-ATPase from E. coli were determined. The modified congeners 3-5 were less active by one order of magnitude compared to 1 (K(i) 10 microM), and 6 was completely inactive. Our experiments demonstrate that the flexible, central tripeptide epitope, comprising positions 6-8 in 1, is crucial for molecular recognition, even slight sequence modifications being hardly tolerated.

New Sequences, Constituents, and Producers of Peptaibiotics: An Updated Review

To date, 18 genera of imperfect and ascomycetous fungi have been recognized to produce ca. 700 individual sequences of peptaibiotics. These are linear polypeptide antibiotics which i) have a molecular weight between 500 and 2,200 Dalton, thus containing 5-21 residues; ii) show a high content of alpha-aminoisobutyric acid; iii) are characterized by the presence of other nonproteinogenic amino acids and/or lipoamino acids; iv) possess an acylated N-terminus, and v) have a C-terminal residue that, in most of them, consists of a free or acetylated amide-bonded 1,2-amino alcohol, but might also be an amine, amide, free amino acid, 2,5-dioxopiperazine, or sugar alcohol. From April 2003 until present, ca. 300 new individual sequences of peptaibiotics have been published in the literature, but most of them have not yet been included in databases. To summarize these new sequences and novel constituents, as well as to introduce fungal species hitherto unknown as producers of peptaibiotics, the relevant literature is reviewed. Furthermore, ecophysiological and taxonomic aspects of the producing fungi are discussed.

Peptaibiotics and Peptaibols: An Alternative to Classical Antibiotics?

The development and antimicrobial properties of peptaibiotics and peptaibols are discussed. Also, the role of emerging peptaibol analogues, of alamethicin, e.g., harzianins HC, trichotoxin, and antiamoebin, is outlined.

Peptaibiomics: an advanced, rapid and selective analysis of peptaibiotics/peptaibols by SPE/LC-ES-MS

"Proteomics" and "peptidomics" are used as technical terms to define the analysis and study of all proteins and peptides expressed in an organism or tissue. In analogy we propose the name peptaibiomics for the analysis of a group of fungal peptide antibiotics (peptaibiotics) containing the characteristic amino acid Aib (alpha-aminoisobutyric acid). In analogy to the peptidome the complete expression of peptaibiotics by fungal multienzyme complexes should be named the peptaibiome. Peptaibiotics are defined as peptides containing Aib and exerting a variety of bioactivities. They comprise the sub-groups of N-acetylated peptaibols, characterized also by a C-terminal amide-linked 2-amino alcohol, and lipopeptaibols having in place of an acetyl group a lipophilic fatty acid acyl group. Furthermore, lipoaminopeptides are also known with long-chain fatty acid on the N-termini, a lipoamino acid in position three and a strongly basic secondary or tertiary amine form a subgroup of mixed forms which could not be integrated in one of these three previously mentioned groups. Here we present a specific and rapid screening method on the peptaibiome applicable directly onto filamentous fungi cultured in a single Petri dish. The method comprises solid-phase extraction (SPE) of peptaibiotics followed by on-line reversed-phase HPLC coupled to an ion trap electrospray tandem mass spectrometer (ES-MS). The presence of these peptides is indicated by characteristic mass differences of Deltam = 85.1 Da representing Aib-residues which can be observed in the b-series of acylium fragment ions resulting from ES-MS. Partial sequences can be deduced from the data and compared with structures compiled in electronic peptaibol data bases. The judgement is possible whether or not structures are novel, already known or related to known structures. Suitability of the method is demonstrated with the analysis of strains of Trichoderma and its teleomorph Hypocrea. New sequences of peptaibiotics are presented and those being related to established 10- to 18-residue peptaibols trichovirin, trichogin and trichotoxin, which have been described in the literature.

Conductance studies on trichotoxin_A50E and implications for channel structure

Trichotoxin_A50E is an 18-residue peptaibol whose crystal structure has recently been determined. In this study, the conductance properties of trichotoxin_A50E have been investigated in neutral planar lipid bilayers. The macroscopic current-voltage curves disclose a moderate voltage-sensitivity and the concentration-dependence suggests the channels are primarily hexameric. Under ion gradients, shifts of the reversal potential indicate that cations are preferentially transported. Trichotoxin displays only one single-channel conductance state in a given experiment, but an ensemble of experiments reveals a distribution of conductance levels. This contrasts with the related peptaibol alamethicin, which produces multiple channel levels in a single experiment, indicative of recruitment of additional monomers into different multimeric-sized channels. Based on these conductance measurements and on the recently available crystal structure of trichotoxin_A50E, which is a shorter and straighter helix than alamethicin, a tightly-packed hexameric model structure has been constructed for the trichotoxin channel. It has molecular dimensions and surface electrostatic potential compatible with the observed conductance properties of the most probable and longer-lived channel.

Sequences of polypeptide antibiotics stilboflavins, natural peptaibol libraries of the mold Stilbella flavipes

From the culture broths of the mold Stilbella flavipes CBS 146.81, a mixture of polypeptides could be isolated by adsorption on XAD polystyrene resin and purified by Sephadex LH-20 chromatography. Using preparative thin-layer chromatography (TLC) three groups of peptides, named stilboflavins (SF) A, B, and C could be separated. Each of the groups showed microheterogeneity when investigated by high-performance liquid chromatography (HPLC). Employing on-line HPLC-electrospray ionization tandem mass spectrometry in the positive and negative ionization mode, together with gas chromatography-selected ion monitoring mass spectrometry, enantioselective GC and quantitative amino acid analysis, the sequences of stilboflavins A and B could be determined. Exchange of Glu in stilboflavins A peptides (acidic) against Gln in stilboflavins B peptides (neutral) is the rational for different polarity of the peptide groups and their separatability by TLC. Since SF A and B are bioactive N-acetylated 20-residue peptides with a high proportion of alpha-aminoisobutyric acid and C-terminal bonded amino alcohols (either leucinol, isoleucinol or valinol) the peptides belong to the group of peptaibol antibiotics.

New sequences and new fungal producers of peptaibol antibiotics antiamoebins

Mixtures of the microheterogeneous 16-mer peptaibol antibiotics called antiamoebins (AAM) have been isolated from the culture broths of strains of the filamentous fungi Stilbella erythrocephala ATCC 28144, Stilbella fimetaria CBS 548.84 and Gliocladium catenulatum CBS 511.66. Sequences were determined using on-line HPLC together with positive- and negative-ion electrospray ionization mass spectrometry. Some characteristic features are recognized in the mass spectrometric fragmentation pattern of AAM. From a sample originally used for sequencing AAM (from Hindustan Antibiotics, Ltd., Pimpri, Poona-411018, India), and a sample of AAM commercially available (from Sigma Chemicals, St. Louis, MO, USA) HPLC elution profiles and sequences were assigned. Further, sequences of AAM previously isolated from Emericellopsis synnematicola CBS 176.60 and Emericellopsis salmosynnemata CBS 382.62 were determined. The peptide designated AAM I was the most abundant in all isolates and its structure could be confirmed. AAM II was detectable as a minor component (1.9%) only in the original sample of AAM, but not in the other isolates. The structures of AAM III, IV and V, which had previously been partly assigned, were definitely established, and the new sequences AAM VI-XVI were elucidated. AAM showing Phe1/Leu1 or Phe1/Val1 exchange, respectively, are produced in amounts only by S. erythrocephala. Sequences, HPLC elution profiles ('fingerprints') and relative amounts of peptides of all isolates were correlated.

Detection of new sequences of peptaibol antibiotics trichotoxins A-40 by on-line liquid chromatography-electrospray ionization mass spectrometry

Using high-performance liquid chromatography (HPLC) coupled to electrospray ionization mass spectrometry (ESI-MS) the sequences of the microheterogeneous peptide mixture of the 18-residue "peptaibol" antibiotics trichotoxins A-40, isolated from the mold Trichoderma viride strain NRRL 5242, were reinvestigated. The structures of two major and one minor component [J. Chromatogr., 296 (1984) 236] could be confirmed and hitherto not known sequences of a further major and two minor peptides could be determined. It is demonstrated that ESI-MS in the positive ionization mode is advantageously completed by applying negative ionization. The methods used make possible the sequence determination of components of peptaibols without previous isolation and allow, in certain cases, sequencing of peptides which are incompletely or not resolved by HPLC.

Structures of trichovirins II, peptaibol antibiotics from the mold Trichoderma viride NRRL 5243

From the culture broth of the mold Trichoderma viride NRRL 5243 a mixture of polypeptides, named trichovirins (TV), could be isolated and purified by chromatography on XAD-2 adsorber resin and Sephadex LH-20 gel. Chromatography on silica gel using chloroform/methanol 8:2 as eluent provided a mixture of peptides named TV I. Subsequent elution with chloroform/methanol 1:1 yielded a second group of peptides named TV II. That group could be separated into individual components by repetitive HPLC on an octadecylsilyl and a fluorocarbon stationary phase. The sequences of 12 peptides of TV II could be determined by electrospray ionization tandem mass spectrometry of isolated peptides and gas chromatography-mass spectrometry of methanolysates. The N-termini of the 18-mer peptides are acetylated and the C-termini consist of leucinol. Owing to the presence of alpha-aminoisobutyric acid (Aib) residues and the bactericidal and hemolytic activity, the peptides belong to the family of peptaibol antibiotics.

The structure and function of antiamoebin I, a proline-rich membrane-active polypeptide

BACKGROUND

Antiamoebin is a member of the peptaibol family of polypeptides and has a unique antibiotic activity: it acts as an antiamoebic agent, but does not effectively haemolyze erythrocytes even though it does exhibit membrane-modifying activity.

RESULTS

The structure of antiamoebin I has been determined by X-ray crystallography at 1.4 A resolution. The molecule forms a helical structure, which, as a result of the presence of a number of proline and hydroxyproline residues, has a deep bend in the middle. Circular dichroism spectroscopy, single-channel conductance studies and fluorescence diffusion studies suggest a mode of ion transport that is entirely different from that of the other two members of the peptaibol family (alamethicin and zervamicin) whose structures and functions have been examined in detail.

CONCLUSIONS

The structure of the polypeptide has been determined and a functional model for its mode of action in membranes is presented. Although under some conditions antiamoebin may form ion channels, unlike the closely related alamethicin and zervamicin polypeptides, its major membrane-modifying activity appears to be as an ion carrier.

Proteolytically stable peptides by incorporation of alpha-Tfm amino acids

A series of model peptides containing alpha-trifluoromethyl-substituted amino acids in five different positions relative to the predominant cleavage site of the serine protease alpha-chymotrypsin was synthesized by solution methods to investigate the influence of alpha-Tfm substitution on the proteolytic stability of peptides. Proteolysis studies demonstrated absolute stability of peptides substituted to the P1 position and still considerable proteolytic stability for peptides substituted at the P2 and P'2 positions compared with the corresponding unsubstituted model peptide. Comparison with peptides containing the fluorine-free disubstituted amino acid alpha-aminoisobutyric acid allowed to separate electronic from steric effects. Furthermore, the absolute configuration of the alpha-Tfm-substituted amino acid was found to exert considerable effects on the proteolytic stability, especially in P'1 substituted peptides. Investigations of this phenomenon using empirical force field calculations revealed that in the (S,R,S)-diasteromer the steric constraints exhibited by the alpha-Tfm group can be outweighed by an advantageous interaction of the flourine atoms with the serine side chain of the enzyme. In contrast, a favourable interaction between substrate and enzyme is impossible for the (S,S,S)-diastereomer.