Delineation of an active fragment and poly(L-proline) II conformation for candidacidal activity of bactenecin 5

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.

Natural and synthetic peptides with antifungal activity

In recent years, the increase of invasive fungal infections and the emergence of antifungal resistance stressed the need for new antifungal drugs. Peptides have shown to be good candidates for the development of alternative antimicrobial agents through high-throughput screening, and subsequent optimization according to a rational approach. This review presents a brief overview on antifungal natural peptides of different sources (animals, plants, micro-organisms), peptide fragments derived by proteolytic cleavage of precursor physiological proteins (cryptides), synthetic unnatural peptides and peptide derivatives. Antifungal peptides are schematically reported based on their structure, antifungal spectrum and reported effects. Natural or synthetic peptides and their modified derivatives may represent the basis for new compounds active against fungal infections.

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.

Diversity in penaeidin antimicrobial peptide form and function

Penaeidins are a diverse family of two-domain antimicrobial peptides expressed in shrimp. Variation in penaeidin sequence results in functional diversity, which was discovered using synthetic reproductions of native penaeidins. An isoform of penaeidin class 3 from Litopenaeus setiferus (Litset Pen3-4) was synthesized using native ligation and compared directly with the synthetic penaeidin class 4 known to be expressed in the same organism. New antimicrobial activity data are included in this review that emphasize differences in effectiveness that are apparent from a direct comparison of two classes. A novel approach to intact penaeidin analysis is presented in the form of Fourier Transform Ion-Cyclotron Resonance Mass Spectrometry, which has implications for the identification of individual penaeidin isoforms without chemical modification or enzymatic cleavage. The new information included in this review helps gather the perspective on relevance of penaeidin diversity to antimicrobial function, the use of synthetic peptides as tools to evaluate specific immune functions and the application of high mass resolution, top-down sequencing methods to the intact analysis of individual penaeidin isoforms.

Novel molecules for intra-oral delivery of antimicrobials to prevent and treat oral infectious diseases

New molecules were designed for efficient intra-oral delivery of antimicrobials to prevent and treat oral infection. The salivary statherin fragment, which has high affinity for the tooth enamel, was used as a carrier peptide. This was linked through the side chain of the N-terminal residue to the C-terminus of a defensin-like 12-residue peptide to generate two bifunctional hybrid molecules, one with an ester linkage and the other with an anhydride bond between the carrier and the antimicrobial components. They were examined for their affinity to a HAP (hydroxyapatite) surface. The extent of the antimicrobial release in human whole saliva was determined using 13C-NMR spectroscopy. The candidacidal activity of the molecules was determined as a function of the antimicrobial release from the carrier peptide in human saliva. The hybrid-adsorbed HAP surface was examined against Candida albicans and Aggregatibacter actinomycetemcomitans using the fluorescence technique. The bifunctional molecules were tested on human erythrocytes, GECs (gingival epithelial cells) and GFCs (gingival fibroblast cells) for cytotoxicity. They were found to possess high affinity for the HAP mineral. In human whole saliva, a sustained antimicrobial release over a period of more than 40–60 h, and candidacidal activity consistent with the extent of hybrid dissociation were observed. Moreover, the bifunctional peptide-bound HAP surface was found to exhibit antimicrobial activity when suspended in clarified human saliva. The hybrid peptides did not show any toxic influence on human erythrocytes, GECs and GFCs. These novel hybrids could be safely used to deliver therapeutic agents intra-orally for the treatment and prevention of oral infectious diseases.

Synthesis of Conformationally Constrained Lysine Analogues

The synthesis of two conformationally constrained lysine analogues is reported. The synthesis of the novel analogue 1 based on the 3-aza-bicyclo[3.1.0]hexane system is accomplished from the known tricycle 3 in eight steps. The synthesis of the analogue 2 is accomplished in eight steps from 4-hydroxy proline. Both analogues are synthesized appropriately protected for Fmoc/Boc solid-phase peptide synthesis.

Distinct antifungal mechanisms: beta-defensins require Candida albicans Ssa1 protein, while Trk1p mediates activity of cysteine-free cationic peptides

Salivary histatin 5 (Hst 5) kills the fungal pathogen Candida albicans via a multistep process which includes binding to Ssa1/2 proteins on the cell surface and requires the TRK1 potassium transporter. Hst 5-induced membrane permeability to propidium iodide (PI) was nearly abolished in strain CaTK1 (TRK1/trk1), suggesting that Hst 5-induced influx of PI is via Trk1p. To explore the functional role of Trk1p in the mechanism of other antifungal peptides, we evaluated candidacidal activity and PI uptake in wild-type strain CaTK2 (TRK1/TRK1) and strain CaTK1 following treatment with lactoferricin 11 (LFcn 11), bactenecin 16 (BN 16), and virion-associated protein VPR 12. Strain CaTK1 was resistant to killing with these peptides (VPR 12 > LFcn 11 > BN 16), showing the requirement of Trk1p for fungicidal activity. In contrast, human neutrophil defensin 1 (HNP-1), human beta-defensin 2 (hBD-2), and hBD-3 effects on viability of and membrane permeability to PI were not different between mutant and wild-type strains, clearly showing that their candidacidal mechanism does not involve Trk1p as a functional effector. To test whether defensins require binding to Candida surface Ssa1/2 proteins for their activity, we measured the killing effectiveness in SSA1/2 mutant strains. Both hBD-2 and hBD-3, but not HNP-1, exhibited reduced killing of ssa1Delta and ssa2Delta strains compared to the wild type, showing that Ssa1 and Ssa2 proteins are required for their fungicidal activity. These results demonstrate that (i) Trk1p mediates candidacidal activities of cysteine-free peptides, but not of defensins, and (ii) hBD-2 and hBD-3, but not HNP-1, require Ssa1/2p for antifungal activity.

The interaction of cationic antimicrobial peptides with vesicles containing synthetic glycolipids as models of the outer membrane of gram-negative bacteria

Two simple lipid A analogues methyl 2,3-di-O-tetradecanoyl-alpha-D-glucopyranoside (GL1) and methyl 2,3-di-O-tetradecanoyl-alpha-D-glucopyranoside 4-O-phosphate (GL2) were synthesized and used for preparing mixed phosphocholine vesicles as models of the outer membrane of gram-negative bacteria. The interaction of these model membranes with magainin 2, a representative of the alpha-helical membrane active peptides, and apidaecin Ib and drosocin, two insect Pro-rich peptides which do not act at the level of the cellular membrane, were studied by CD and dye-releasing experiments. The CD spectra of apidaecin Ib and drosocin in the presence of GL1- or GL2-containing vesicles were consistent with largely unordered structures, whereas, according to the CD spectra, magainin 2 adopted an amphipathic alpha-helical conformation, particularly in the presence of negatively charged bilayers. The ability of the peptides to fold into amphipathic conformations was strictly correlated to their ability to bind and to permeabilize phospholipid as well as glycolipid membranes. Apidaecin Ib and drosocin, which are unable to adopt an amphipathic structure, showed negligible dye-leakage activity even in the presence of GL2-containing vesicles. It is reasonable to suppose that, as for the killing mechanism, the two classes of antimicrobial peptides follow different patterns to cross the bacterial outer membrane.

Susceptibility of oral bacteria to an antimicrobial decapeptide

Naturally occurring antimicrobial peptides have emerged as alternative classes of antimicrobials. In general, these antimicrobial peptides exhibit selectivity for prokaryotes and minimize the problems of engendering microbial resistance. As an alternative method to search for more effective broad-spectrum peptide antimicrobials, investigators have developed peptide libraries by using synthetic combinatorial technology. A novel decapeptide, KKVVFKVKFK (KSL), has been identified that shows a broad range of antibacterial activity. The purpose of this study was to test the efficacy of this antimicrobial peptide in killing selected strains of oral pathogens and resident saliva bacteria collected from human subjects. Cytotoxic activity of KSL against mammalian cells and the structural features of this decapeptide were also investigated, the latter by using two-dimensional NMR in aqueous and DMSO solutions. MICs of KSL for the majority of oral bacteria tested in vitro ranged from 3 to 100 microg ml(-1). Minimal bactericidal concentrations of KSL were, in general, within one to two dilutions of the MICs. KSL exhibited an ED(99) (the dose at which 99 % killing was observed after 15 min at 37 degrees C) of 6.25 microg ml(-1) against selected strains of Lactobacillus salivarius, Streptococcus mutans, Streptococcus gordonii and Actinobacillus actinomycetemcomitans. In addition, KSL damaged bacterial cell membranes and caused 1.05 log units reduction of viability counts of saliva bacteria. In vitro toxicity studies showed that KSL, at concentrations up to 1 mg ml(-1), did not induce cell death or compromise the membrane integrity of human gingival fibroblasts. NMR studies suggest that KSL adopts an alpha-helical structure in DMSO solution, which mimics the polar aprotic membrane environment, whereas it remains unstructured in aqueous medium. This study shows that KSL may be a useful antimicrobial agent for inhibiting the growth of oral bacteria that are associated with caries development and early plaque formation.

Translocating proline-rich peptides from the antimicrobial peptide bactenecin 7

The intracellular delivery of most peptides, proteins, and nucleotides to the cytoplasm and nucleus is impeded by the cell membrane. To allow simplified, noninvasive delivery of attached cargo, cell-permeant peptides that are either highly cationic or hydrophobic have been utilized. Because cell-permeable peptides share half of the structural features of antimicrobial peptides containing clusters of charge and hydrophobic residues, we have explored antimicrobial peptides as templates for designing cell-permeant peptides. We prepared synthetic fragments of Bac 7, an antimicrobial peptide with four 14-residue repeats from the bactenecin family. The dual functions of cell permeability and antimicrobial activity of Bac 7 were colocalized at the N-terminal 24 residues of Bac 7. In general, long fragments of Bac(1-24) containing both regions were bactericidal and cell-permeable, whereas short fragments with only a cationic or hydrophobic region were cell-permeant without the attendant microbicidal activity when measured in a fluorescence quantitation assay and by confocal microscopy. In addition, the highly cationic fragments were capable of traversing the cell membrane and residing within the nucleus. A common characteristic shared by the cell-permeant Bac(1-24) fragments, irrespective of their number of charged cationic amino acids, is their high proline content. A 10-residue proline-rich peptide with two arginine residues was capable of delivering a noncovalently linked protein into cells. Thus, the proline-rich peptides represent a potentially new class of cell-permeant peptides for intracellular delivery of protein cargo. Furthermore, our results suggest that antimicrobial peptides may represent a rich source of templates for designing cell-permeant peptides.

Conformation-dependent antibiotic activity of tritrpticin, a cathelicidin-derived antimicrobial peptide

Tritrpticin, a Trp-rich cationic antimicrobial peptide with a unique amino acid sequence (VRRFPWWWPFLRR), is found in porcine cathelicidin cDNA. Tritrpticin has a broad spectrum of antibacterial and antifungal activities and hemolytic activity comparable to that of indolicidin. To investigate the mechanism of the bacterial killing action of tritrpticin and to identify structural features important for bacterial cell selectivity, we designed several tritrpticin analogs with amino acid substitutions of the Pro and Trp residues. Circular dichroism studies revealed that the substitution of Pro-->Ala (TPA) or Trp-->Phe (TWF) leads to significant conformational changes in SDS micelles, converting the beta-turn to alpha-helix or to poly-L-proline II helix, respectively. Compared to tritrpticin, TPA retained most of its antimicrobial activity, but showed enhanced hemolytic and membrane-disrupting activities. In contrast, TWF showed a 2-4-fold increase in antimicrobial activity against Gram-negative bacteria, but a marked decrease in both hemolytic and membrane-disrupting activities. Taken together, our findings suggest that compared with the beta-turn and alpha-helical structures, the poly-L-proline II helix is crucial for effective bacterial cell selectivity in tritrpticin and its analogs.

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

Bactenecin 5 (Bac 5) is an antibacterial 43mer peptide isolated from bovine neutrophils. It consists of an Arg-rich N-terminal region and successive repeats of Arg-Pro-Pro-Ile (or Phe). We synthesized Bac 5(1-23) and several related peptides to clarify the roles these regions play in antibacterial activity. An assay of antibacterial activity revealed that such activity requires the presence of Arg residues at or near the N-terminus, as well as a chain length exceeding 15 residues. None of the peptides exhibited haemolytic activity. Polyproline II-like CD curves were observed for most of the peptides. Measurements of the membrane perturbation and fusion indicated that the perturbation and fusogenic activities of the peptides were, generally, parallel to their antibacterial activities. Amino acid substitution in the repeating region had some effect on antibacterial activity.

Solid-phase guanidinylation as a diversification strategy of poly-L-proline type II peptide mimic scaffolds

[reaction: see text] Solid-phase guanidinylation of proline-templated amino acids is studied as a diversification strategy of poly-L-proline type II scaffolds.

Poly-L-proline type II peptide mimics based on the 3-azabicyclo[3.1.0]hexane system

This paper describes conformational studies of proline-templated amino acids (PTAAs) based on the 3-azabicyclo[3.1.0]hexane system as well as conformational studies on short peptides composed of these PTAAs. NOE data, coupling constants, and molecular modeling are consistent with a flattened boat conformation for monomeric and oligomeric residues based on this bicyclic system. NMR studies on dimeric and trimeric oligomers are consistent with a populated poly-L-proline type II conformation in CDCl3 and D2O. Solution studies and molecular modeling predicts phi approximately -70 degrees, psi approximately 131 degrees, chi 1 approximately -57 degrees, and chi 2 approximately -158 degrees for oligomeric residues.

Released ATP is an extracellular cytotoxic mediator in salivary histatin 5-induced killing of Candida albicans

Salivary histatins (Hsts) are antifungal peptides with promise as therapeutic agents against candidiasis. Hst 5 kills the fungal pathogen Candida albicans via a mechanism that involves release of cellular ATP in the absence of cytolysis. Here we demonstrate that released ATP has a further role in Hst 5 killing. Incubation of the cells with ATP analogues induced cell death, and addition of the ATP scavenger apyrase to remove extracellular ATP released during Hst 5 treatment resulted in a reduction in cell killing. Experiments using anaerobically grown C. albicans with decreased susceptibility to Hst 5 confirmed that depletion of cellular ATP as a result of ATP efflux was not sufficient to cause cell death. In contrast to Hst-susceptible aerobic cultures, anaerobically grown cells were not killed by exogenously applied ATP. These findings established that Hst binding, subsequent entry into the cells, and ATP release precede the signal for cytotoxicity, which is mediated by extracellular ATP. In a higher-eukaryote paradigm, released ATP acts as a cytotoxic mediator by binding to membrane nucleotide P2X receptors. Based on a pharmacological profile and detection of a C. albicans 60-kDa membrane protein immunoreactive with antibody to P2X(7) receptor, we propose that released ATP in response to Hst 5 activates candidal P2X(7)-like receptors to cause cell death.

Structural features and biological activities of the cathelicidin-derived antimicrobial peptides

Cathelicidins are a numerous group of mammalian proteins that carry diverse antimicrobial peptides at the C-terminus of a highly conserved preproregion. These peptides, which become active when released from the proregion, display a remarkable variety of sizes, sequences, and structures, and in fact comprise representatives of all the structural groups in which the known antimicrobial peptides have been classified. Most of the cathelicidin-derived peptides exert a broad spectrum and potent antimicrobial activity and also bind to lipopolysaccharide and neutralize its effects. In addition, some of them have recently been shown to exert other activities and might participate in host defense also by virtue of their ability to induce expression of molecules involved in a variety of biological processes. This review is aimed at providing a general overview of the cathelicidins and of the peptides derived therefrom, with emphasis on aspects such as structure, biological activities in vitro and in vivo, and structure/activity relationship studies.

CRAMP analogues having potent antibiotic activity against bacterial, fungal, and tumor cells without hemolytic activity

CRAMP-18 (GEKLKKIGQKIKNFFQKL) is the antibacterial sequence derived from CRMAP, a member of cathelicidin-derived antimicrobial peptides. To develop the novel antibiotic peptides useful as therapeutic drugs requires strong antibiotic activity against bacterial and fungal cells without hemolytic effect. To this goal, the analogues were designed to increase only net positively charge by Lys-substitution of positions 2, 9, 13, or 16 at the hydrophilic helix face of CRAMP-18 without any change at the hydrophobic helix face. In particular, Lys-substitution (K(2)-CRAMP-18) of position 2 in CRAMP-18 induced the enhanced antibiotic activity without any increase in hemolysis. Thus, this peptide may provide a useful template for the design novel antibiotic peptides for the treatment of infectious diseases. Additional CD spectra studies suggested that the alpha-helical structure of the peptides plays an important role in killing bacterial and fungal cells, but the increase of alpha-helical content is less connected with the enhanced antibiotic activity.

Animal antimicrobial peptides: an overview

Antibiotic peptides are a key component of the innate immune systems of most multicellular organisms. Despite broad divergences in sequence and taxonomy, most antibiotic peptides share a common mechanism of action, i.e., membrane permeabilization of the pathogen. This review provides a general introduction to the subject, with emphasis on aspects such as structural types, post-translational modifications, mode of action or mechanisms of resistance. Some of these questions are treated in depth in other reviews in this issue. The review also discusses the role of antimicrobial peptides in nature, including several pathological conditions, as well as recent accounts of their application at the preclinical level.

Structure of human salivary histatin 5 in aqueous and nonaqueous solutions

The solution structure of human salivary histatin 5 (D-S-H-A-K-R-H-H-G-Y-K-R-K-F-H-E-K-H-H-S-H-R-G-Y) was examined in water (pH 3.8) and dimethyl sulfoxide solutions using 500 MHz homo- and heteronuclear two-dimensional (2D) nmr. The resonance assignment of peptide backbone and side-chain protons was accomplished by 2D total correlated spectroscopy and nuclear Overhauser effect (NOE) spectroscopy. The high JNH-C alpha H values (> or = 7.4 Hz), absence of any characteristic NH-NH (i, i + 1) or C alpha H-C beta H (i, i + 3) NOE connectivities, high d delta/dT values (> or = 0.004 ppm K-1) and the fast 1H/2H amide exchange suggest that histatin 5 molecules remain unstructured in aqueous solution at pH 3.8. In contrast, histatin 5 prefers largely alpha-helical conformation in dimethyl sulfoxide solution as evident from the JNH-C alpha H values (< or = 6.4 Hz), slow 1H/2H exchange, low d delta/dT values (< or = 0.003 ppm K-1) observed for amide resonances of residues 6-24, and the characteristic NH-NH (i, i + 1) and C alpha H-C beta H (i, i + 3) NOE connectivities. All backbone amide 15N-1H connectivities fall within 6 ppm on the 15N scale in the 2D heteronuclear single quantum correlated spectrum, and the restrained structure calculations using DIANA suggest the prevalence of alpha-helical conformations stabilized by 19 (5-->1) intramolecular backbone amide hydrogen bonds in polar aprotic medium such as dimethyl sulfoxide. The interside-chain hydrogen bonding and salt-bridge type interactions that normally stabilize the helical structure of linear peptides in aqueous solutions are not observed. Histatin 5, unlike other naturally occurring antimicrobial polypeptides such as magainins, defensins, and tachyplesins, does not adopt amphiphilic structure, precluding its insertion into microbial membranes and formation of ion channels across membranes. Electrostatic (ionic type) and hydrogen bonding interactions of the positively charged and polar residues with the head groups of microbial membranes or with a membrane-bound receptor could be the initial step involved in the mechanism of antimicrobial activity of histatins.

Synthesis, microbicidal activity, and solution structure of the dodecapeptide from bovine neutrophils

The dodecapepetide sequence R-L-C-R-I-V-V-I-R-V-C-R with a disulfide bridge between the cysteine residues found in bovine neutrophils was synthesized by solid-phase procedures. Its antimicrobial activity against oral microorganisms such as Actinobacillus actinomycetemcomitans, Porphyromonas gingivalis, Streptococcus mutans, and Streptococcus gordonii was examined, and its structural features were examined by CD and determined by two-dimensional (2D) nmr. The strains P. gingivalis (W50 and 381), A. actinomycetemcomitans (Y4 and 67), S. gordonii (DL1), and S. mutans (GS5) are found to be highly sensitive to this peptide at 2-2.5 microM concentrations, suggesting that the dodecapeptide is a potent antibiotic for oral pathogens. The weak negative n-sigma* band observed at approximately 265-270 nm in the CD spectra of this peptide provides evidence for the presence of a disulfide bridge. The negative n-pi* band at approximately 200 nm and the positive pi-pi* band at 185 nm suggest a folded structure for this peptide. The negative n-pi* shifts from 200 to 206 nm with an increase in intensity in dipalmitoylphosphotidylcholine vesicles, suggesting that the peptide might associate to form higher order aggregates in lipid medium. The assignment of backbone and side-chain proton resonances has been accomplished by the combined analysis of 2D total correlated and nuclear Overhauser effect spectroscopy. The temperature dependence of amide NH chemical shifts and (1)H-(2)H exchange effect on amide NH resonances indicate the involvement of amide NH groups of Cys3, Ile5, Ile8, Val10, and Arg12 in intramolecular hydrogen bonding. The coupling constant (J(NH-C(alpha)H)) values, the set of medium-, short-, and long-range nuclear Overhauser effects, and the results of restrained structure calculation using the distance geometry algorithm for nmr applications provide evidence for a folded, loop-like structure with a type I (III) beta-turn involving Ile5, Val6, Val7, and Ile8, and two antiparallel beta-strands involving the N-terminal Arg1, Leu2, Cys3, and Val4 and the C-terminal Arg9, Val10, Cys11, and Arg12 residues. The structure of the dodecapeptide mimics the amphiphilic structure of large 30-35 residue defensins and the peptide appears to exhibit similar antimicrobial potency.

NMR analysis of human salivary mucin (MUC7) derived O-linked model glycopeptides: comparison of structural features and carbohydrate-peptide interactions

Two series of glycopeptides with mono- and disaccharides, [GalNAc and Galbeta (1-3)GalNAc] O-linked to serine and threonine at one, two or three contiguous sites were synthesized and characterized by 1H NMR. The conformational effects governed by O-glycosylation were studied and compared with the corresponding non-glycosylated counterparts using NMR, CD and molecular modelling. These model peptides encompassing the aa sequence, PAPPSSSAPPE (series I) and APPETTAAPPT (series II) were essentially derived from a 23-aa tandem repeat sequence of low molecular weight human salivary mucin (MUC7). NOEs, chemical shift perturbations and temperature coefficients of amide protons in aqueous and nonaqueous media suggest that carbohydrate moiety in threonine glycosylated peptides (series II) is in close proximity to the peptide backbone. An intramolecular hydrogen bonding between the amide proton of GalNAc or Galbeta (1-3)GalNAc and the carbonyl oxygen of the O-linked threonine residue is found to be the key structure stabilizing element. The carbohydrates in serine glycosylated peptides (series I), on the other hand, lack such intramolecular hydrogen bonding and assume a more apical position, thus allowing more rotational freedom around the O-glycosidic bond. The effect of O-glycosylation on peptide backbone is clearly reflected from the observed overall differences in sequential NOEs and CD band intensities among the various glycosylated and non-glycosylated analogues. Delineation of solution structure of these (glyco)peptides by NMR and CD revealed largely a poly L-proline type II and/or random coil conformation for the peptide core. Typical peptide fragments of tandem repeat sequence of mucin (MUC7) showing profound glycosylation effects and distinct differences between serine and threonine glycosylation as observed in the present investigation could serve as template for further studies to understand the multifunctional role played by mucin glycoproteins.

Purification and properties of proline-rich antimicrobial peptides from sheep and goat leukocytes

We purified three proline-rich antimicrobial peptides from elastase-treated extracts of sheep and goat leukocytes and subjected two of them, OaBac5alpha and ChBac5, to detailed analysis. OaBac5alpha and ChBac5 were homologous to each other and to bovine Bac5. Both exhibited potent, broad-spectrum antimicrobial activity under low-concentration salt conditions. While the peptides remained active against Escherichia coli, Pseudomonas aeruginosa, Bacillus subtilis, and Listeria monocytogenes in 100 mM NaCl, they lost activity against Staphylococcus aureus and Candida albicans under these conditions. ChBac5 was shown to bind lipopolysaccharide, a property that could enhance its ability to kill gram-negative bacteria. Proline-rich Bac5 peptides are highly conserved in ruminants and may contribute significantly to their innate host defense mechanisms.

Salivary histatin 5 induces non-lytic release of ATP from Candida albicans leading to cell death

Salivary histatins are potent in vitro antifungal proteins and have promise as therapeutic agents against oral candidiasis. We performed pharmacological studies directed at understanding the biochemical basis of Hst 5 candidacidal activity. Three inhibitors of mitochondrial metabolism: carbonyl cyanide p-chlorophenylhydrazone, dinitrophenol, and azide inhibited Hst 5 killing of Candida albicans, while not inhibiting cellular ATP production. In contrast, Hst 5 caused a drastic reduction of C. albicans intracellular ATP content, which was a result of an efflux of ATP. Carbonyl cyanide p-chlorophenylhydrazone, dinitrophenol, and azide inhibited Hst 5-induced ATP efflux, thus establishing a correlation between ATP release and cell killing. Furthermore, C. albicans cells were respiring and had polarized membranes at least 80 min after ATP release, thus implying a non-lytic exit of cellular ATP in response to Hst 5. Based on evidence that transmembrane ATP efflux can occur in the absence of cytolysis through a channel-like pathway and that released ATP can act as a cytotoxic mediator by binding to membrane purinergic receptors, we evaluated whether extracellular ATP released by Hst 5 may have further functional role in cell killing. Consistent with this hypothesis, purinergic agonists BzATP and adenosine 5'O-(thiotriphosphate) induced loss of C. albicans cell viability and purinergic antagonists prevented Hst 5 killing.

A concise methodology for the stereoselective synthesis of O-glycosylated amino acid building blocks: complete 1H NMR assignments and their application in solid-phase glycopeptide synthesis

A facile strategy for the stereoselective synthesis of suitably protected O-glycosylated amino acid building blocks, namely, Nalpha-Fmoc-Ser-[Ac4-beta-D-Gal-(1-3)-Ac2-alpha or beta-D-GalN3]-OPfp and Nalpha-Fmoc-Thr-[Ac4-beta-D-Gal-(1-3)-Ac2-alpha or beta-D-GalN3]-OPfp is described. What is new and novel in this report is that Koenigs-Knorr type glycosylation of an aglycon serine/threonine derivative (i.e. Nalpha-Fmoc-Ser-OPfp or Nalpha-Fmoc-Thr-OPfp) with protected beta-D-Gal(1-3)-D-GalN3 synthon mediated by silver salts resulted in only alpha- and/or beta-isomers in excellent yields under two different reaction conditions. The subtle differences in stereoselectivity were demonstrated clearly when glycosylation was carried out using only AgClO4 at -40 degrees C which afforded a-isomer in a quantitative yield (alpha:beta = 5:1). On the other hand, the beta-isomer was formed exclusively when the reaction was performed in the presence of Ag2CO3/AgClO4 at room temperature. A complete assignment of 1H resonances to individual sugar ring protons and the characteristic anomeric alpha-1 H and beta-1 H in Ac4Galbeta(1-3)Ac2GalN3 alpha and/or beta linked to Ser/Thr building blocks was accomplished unequivocally by two-dimensional double-quantum filtered correlated spectroscopy and nuclear Overhauser enhancement and exchange spectroscopy NMR experiments. An unambiguous structural characterization and documentation of chemical shifts, including the coupling constants for all the protons of the aforementioned alpha- and beta-isomers of the O-glycosylated amino acid building blocks carrying protected beta-D-Gal(1-3)-D-GalN3, could serve as a template in elucidating the three-dimensional structure of glycoproteins. The synthetic utility of the building blocks and versatility of the strategy was exemplified in the construction of human salivary mucin (MUC7)-derived, O-linked glycopeptides with varied degrees of glycosylation by solid-phase Fmoc chemistry. Fmoc/tert-butyl-based protecting groups were used for the peptidic moieties in conjunction with acetyl sugar protection. The transformation of the 2-azido group into the acetamido derivative was carried out with thioacetic acid on the polymer-bound glycopeptides before the cleavage step. After cleaving the glycopeptide from the resin, the acetyl groups used for sugar OH-protection were removed with sodium methoxide in methanol. Finally, the glycopeptides were purified by reversed-phase high-performance liquid chromatography and their integrity was confirmed by proton NMR as well as by mass spectral analysis. Secondary structure analysis by circular dichroism of both the glycosylated and nonglycosylated peptides revealed that carbohydrates did not exert any profound structural effect on the peptide backbone conformation.

Bactericidal activity and poly-L-proline II conformation of the tandem repeat sequence of human salivary mucin glycoprotein (MG2)

The tandem repeat 23-residue sequence [TRS23 (145-167): T-T-A-A-P-P-T-P-S-A-T-T-P-A-P-P-S-S-S-A-P-P-E] of human salivary mucin glycoprotein MG2 was examined for its in vitro bactericidal activity against four oral microorganisms, Actinobacillus actinomycetemcomitans, Porphyromonas gingivalis, Streptococcus gordonii, and Streptococcus mutans. The conformational features of the proline-rich peptide were determined by circular dichroism (CD) and 600 MHz two-dimensional (2D) nuclear magnetic resonance (NMR) in aqueous solution. The strains of P. gingivalis (W50 and 381), A. actinomycetemcomitans (Y4 and 67), S. gordonii (DL1), and S. mutans (GS5) are highly sensitive to this peptide at 1.5-3.0 microM concentrations, suggesting that the proline-rich repeat sequence is a potent bactericidal agent for oral pathogens. The assignment of backbone and side-chain proton resonances was accomplished by the combined analysis of 2D total correlated spectroscopy and nuclear Overhauser effect spectroscopy. The temperature dependence of amide NH chemical shifts and the 1H-2H exchange effect on amide NH resonances suggest the absence of intramolecularly hydrogen-bonded NH groups. The coupling constant (JNH-CalphaH) values, conformational restriction offered by the proline residues (phi = -60 degrees +/- 15 degrees), the set of medium- and short-range nuclear Overhauser effects observed for this sequence, and the results of restrained structure calculation using DIANA, the distance geometry algorithm for NMR applications, provide evidence for the existence of a significant population of poly-L-proline II-type helices in aqueous solution. The CD spectra of the peptide in phosphate buffer (pH 7.2) and in methanol are reminiscent of the CD spectrum of the poly-L-proline II helical conformation and are consistent with the NMR data. The bactericidal activity of the proline-rich repeat sequence suggests that bacterial colonization, facilitated by the adsorbed salivary mucins on tooth surface, could be partly controlled and cleared by proteolytically degraded proline-rich peptides of MG2 in saliva before the colonized organisms turn into pathogens. It appears that the poly-L-proline II helix is the biologically active backbone conformation for bactericidal activity of the tandem repeat sequences of salivary MG2.

Candidacidal activity of salivary histatins. Identification of a histatin 5-binding protein on Candida albicans

Candida albicans is the predominant species of yeast isolated from patients with oral candidiasis, which is frequently a symptom of human immunodeficiency virus infection and is a criterion for staging and progression of AIDS. Salivary histatins (Hsts) are potent in vitro antifungal agents and have great promise as therapeutic agents in humans with oral candidiasis. The molecular mechanisms by which Hsts kill yeast cells are not known. We report here, that unlike other antimicrobial proteins, Hsts do not display lytic activities to lipid membranes, measured by release and dequenching of the fluorescent dye calcein. Analysis of the magnitude and time course of Hst-induced calcein release from C. albicans cells further showed that loss of cell integrity was a secondary effect following cell death, rather than the result of primary disruption of the yeast cell membrane. 125I-Hst 5 binding studies indicated that C. albicans expressed a class of saturable binding sites (KD = 1 microM), numbering 8.6 x 10(5) sites/cell. Both Hst 3 and Hst 4 competed for these binding sites with similar affinities, which is consistent with the micromolar concentration of Hsts required for candidacidal activity. Specific 125I-Hst 5 binding was not detected to C. albicans spheroplasts, which were 14-fold less susceptible to Hst 5 killing, compared with intact cells in candidacidal assays. In overlay experiments, 125I-Hst 5 bound to a 67-kDa protein detected in C. albicans whole cell lysates and crude membrane fractions, but not in the yeast cell wall fraction. Consistent with the overlay data, cross-linking of 125I-Hst 5 to C. albicans resulted in the appearance of a specific 73-kDa 125I-Hst 5-containing complex that was not detected in the cell wall. 125I-Hst 5-binding protein of similar size was also observed in susceptible S. cerevisiae strain TI#20. This is the first description of Hst 5 binding sites on C. albicans which mediate cell killing and identification of a 67-kDa yeast Hst 5-binding protein. The binding characteristics of Hst 5 are in agreement with the observed potency of its biological effect and provide crucial information to the use of Hst 5 as a therapeutic agent. The presence of a specific C. albicans Hst 5-binding protein provides further insight into the potential mechanism of yeast killing and suggests a basis for differential activity between yeast killing and the nontoxic nature of Hsts to humans.

Delineation of conformational preferences in human salivary statherin by 1H, 31P NMR and CD studies: sequential assignment and structure-function correlations

Membrane-induced solution structure of human salivary statherin, a 43 amino acid residue acidic phosphoprotein, has been investigated by two-dimensional proton nuclear magnetic resonance (2D 1H NMR) spectroscopy. NMR assignments and structural analysis of this phosphoprotein was accomplished by analyzing the pattern of sequential and medium range NOEs, alphaCH chemical shift perturbations and deuterium exchange measurements of the amide proton resonances. The NMR data revealed three distinct structural motifs in the molecule: (1) an alpha-helical structure at the N-terminal domain comprising Asp1-Tyr16, (2) a polyproline type II (PPII) conformation predominantly occurring at the middle proline-rich domain spanning Gly19-Gln35, and (3) a 3(10)-helical structure at the C-terminal Pro36-Phe43 sequence. Presence of a few weak dalphaN(i,i+2) NOEs suggests that N-terminus also possesses minor population of 3(10)-helical conformation. Of the three secondary structural elements, helical structure formed by the N-terminal residues, Asp1-Ile11 appears to be more rigid as observed by the relatively very slow exchange of amide hydrogens of Glu5-Ile11. 31P NMR experiments clearly indicated that N-terminal domain of statherin exists mainly in disordered state in water whereas, upon addition of structure stabilizing co-solvent, 2,2,2-trifluorethanol (TFE), it showed a strong propensity for helical conformation. Calcium ion interaction studies suggested that the disordered N-terminal region encompassing the two vicinal phosphoserines is essential for the binding of calcium ions in vivo. Results from the circular dichroism (CD) experiments were found to be consistent with and complimentary to the NMR data and provided an evidence that non-aqueous environment such as TFE, could induce the protein to fold into helical conformation. The findings that the statherin possesses blended solvent sensitive secondary structural elements and the requirement of non-structured N-terminal region under aqueous environment in calcium ion interaction may be invaluable to understand various physiological functions of statherin in the oral fluid.

Structure and property of model peptides of proline/arginine-rich region in bactenecin 5

Bactenecin 5 (Bac 5), a cationic antibacterial peptide, contains a repeating region of Arg-Pro-Pro-X (X = hydrophobic residue). To investigate the structure and property of a Pro/Arg-rich region, we synthesized a series of repeating peptides, Ac-(Arg-Pro-Pro-Phe)n-NHCH3 (n = 2, 4, 6, 8 and 10) (PR2, PR4, PR6, PR8 and PR10) as models. The circular dichroism (CD) study suggested that the peptides with longer repeats, PR6, PR8 and PR10, formed a conformation similar to poly(proline)-II in aqueous solution. The CD spectra did not change in the presence of dipalmitoyl-DL-alpha-phosphatidylcholine (DPPC), but they changed in the presence of DPPC/ dipalmitoyl-DL-3-phosphatidylglycerol (DPPG). The gamma-helix, which is very similar in conformation to the poly(proline)-II helix, had the lowest energy conformation for the peptides by energy calculations. Peptides PR6, PR8 and PR10 caused slight leakage of fluorescent dye entrapped in DPPC vesicles, and in the presence of DPPC/DPPG, these peptides showed a considerable level of dye-leakage activity. In contrast, the shorter peptides PR2 and PR4 showed no activity. The same tendency was found in measurements of membrane-fusion activity. Judging from these results, the repeating region of Bac 5 may make a framework to hold a conformation resembling the poly(proline)-II structure in aqueous solution. In addition, this region may interact with acidic lipids, resulting in a change in conformation of the peptide.

Beta-sheet antibiotic peptides as potential dental therapeutics

Small, cysteine-rich, beta-sheet peptide antibiotics are found throughout the Animalia. Though broad spectrum in potential, they may exert selective antimicrobial effects under certain conditions. We have explored the antimicrobial properties of two families of beta-sheet peptide antibiotics, defensins and protegrins, against periodontopathic bacteria. The rabbit defensin NP-1 was active against facultative Gram-negative bacteria associated with early onset periodontitis, including Actinobacillus actinomycetemcomitans and the Capnocytophaga spp. Porcine protegrins showed even greater activity against those organisms, as well as against anaerobic bacteria associated with adult periodontitis, including Porphyromonas gingivalis Prevotella intermedia and Fusobacterium nucleatum. Based on these observations, we believe that protegrin-like beta-sheet peptide antibiotics may be useful dental therapeutics.