Synthesis and immunological studies of alpha-conotoxin chimera containing an immunodominant epitope from the 268-284 region of HSV gD protein

Drugs from slugs. Part II--conopeptide bioengineering

The biological transformation of toxins as research probes, or as pharmaceutical drug leads, is an onerous and drawn out process. Issues regarding changes to pharmacological specificity, desired potency, and bioavailability are compounded naturally by their inherent toxicity. These often scuttle their progress as they move up the narrowing drug development pipeline. Yet one class of peptide toxins, from the genus Conus, has in many ways spearheaded the expansion of new peptide bioengineering techniques to aid peptide toxin pharmaceutical development. What has now emerged is the sequential bioengineering of new research probes and drug leads that owe their lineage to these highly potent and isoform specific peptides. Here we discuss the progressive bioengineering steps that many conopeptides have transitioned through, and specifically illustrate some of the biochemical approaches that have been established to maximize their biological research potential and pharmaceutical worth.

Antibody recognition and conformational flexibility of a plaque-specific beta-amyloid epitope modulated by non-native peptide flanking regions

Here we report on the synthesis, antibody binding, and QSAR studies of a series of linear and cyclic peptides containing a beta-amyloid plaque-specific epitope (Abeta(4-10); FRHDSGY). In these constructs, two or three alpha- l-Ala, alpha- d-Ala, or beta-Ala residues were introduced at both N- and C-termini of the epitope as non-native flanking sequences. Cyclization of the linear Abeta(4-10) epitope peptide resulted in reduced antibody binding. However, the antibody binding could be fully compensated by insertion of alanine flanks into the corresponding cyclic peptides. These results indicate that the modification of a beta-amyloid plaque-specific epitope by combination of cyclization and flanking sequences could generate highly antigenic peptides compared to the native sequence. A novel 3D QSAR method, which explicitly handles conformational flexibility, was developed for the case of such molecular libraries. This method led to the prediction of the binding conformation for the common FRHDSGY sequence.

Engineered proteins as specific binding reagents

Over the past 30 years, monoclonal antibodies have become the standard binding proteins and currently find applications in research, diagnostics and therapy. Yet, monoclonal antibodies now face strong competition from synthetic antibody libraries in combination with powerful library selection technologies. More recently, an increased understanding of other natural binding proteins together with advances in protein engineering, selection and evolution technologies has also triggered the exploration of numerous other protein architectures for the generation of designed binding molecules. Valuable protein-binding scaffolds have been obtained and represent promising alternatives to antibodies for biotechnological and, potentially, clinical applications.

Disulfide bond rearrangement during regioselective oxidation in PhS(O)Ph/CH3SiCl3 mixture for the synthesis of alpha-conotoxin GI

Rearrangement of disulfide bonds during the synthesis of alpha-conotoxin GI using PhS(O)Ph/CH(3)SiCl(3) oxidation procedure was observed. We have demonstrated that the protecting scheme (order of acetamidomethyl (Acm) and (t)Bu protecting groups) of the Cys residues as well as the reaction time influenced the ratio of the native and the mispaired compounds, while the temperature of the reaction mixture had no significant effect. However, in all cases the nonnative derivative was produced in high amount. The structure of the isomers was identified by the combination of enzymatic digestion and mass spectrometry measurements. We conclude that the air oxidation followed by the application of Tl(tfa)(3) for the regioselective formation of disulfide bonds leads up to the appropriate compound in the case of the synthesis of alpha-conotoxin GI, while the oxidation procedure using PhS(O)Ph/CH(3)SiCl(3) system resulted in the nonnative disulfide isomer.

[Natural alpha-conotoxins and their synthetic analogues in studies of nicotinic acetylcholine receptors]

alpha-Conotoxins, peptide neurotoxins from poisonous marine snails of the genus Conus that highly specifically block nicotinic acetylcholine receptors (AChRs) of various types, are reviewed. Preliminarily, the structural organization of AChRs of the muscular and neuronal types, their involvement in physiological processes, and their role in various diseases are briefly discussed. In this connection, the necessity of quantitative determination of AChR subtypes using neurotoxins and other approaches is substantiated. The chemical structure, spatial organization, and specificity of alpha-conotoxins are mainly discussed, taking into consideration the recent results on the ability of alpha-conotoxins to interact with muscular or neuronal hetero- and homooligomeric AChRs exhibiting a high species specificity. Particular emphasis is placed upon a thorough characterization of the surfaces of interaction of alpha-conotoxins with AChRs using synthetic analogues of alpha-conotoxins, mutations in AChRs, and pairwise mutations in both alpha-conotoxins and AChRs. The discovery in 2001 of the acetylcholine-binding protein from the pond snail Lymnaea stagnalis and the determination of its crystalline structure led to rapid progress in understanding the structural organization of ligand-binding domains of AChRs with which alpha-conotoxins also interact. We discuss the interaction of various alpha-conotoxins with acetylcholine-binding proteins, the recently reported X-ray structure of the complex of the acetylcholine-binding protein from Aplysia californica with the alpha-conotoxin analogue PnIA, and the application of this structure to the modeling of complexes of alpha-conotoxins with various AChRs.

Modulating the affinity and the selectivity of engineered calmodulin EF-Hand peptides for lanthanides

A set of engineered peptides (33 amino acids long) corresponding to the helix-turn-helix (EF-Hand) motif of the metal-binding site I of the protein calmodulin from paramecium tetraurelia have been synthesized. A disulfide bridge has been introduced in the native sequence in order to stabilize a native-like conformation. The calcium-binding carboxylate residues in positions 20, 22, 24, and 31 were mutated into other amino acids and the influence of such mutations on the binding affinity of the peptides for calcium and lanthanides have been studied. It was shown that the binding affinity for terbium ions can be modulated with dissociation constants ranging from 40 nmolar to 40 mmolar. The study of the influence of the mutations on the terbium affinity showed that the residue in position 24 played a key role on the capability of the peptides to bind lanthanides and that the affinity could be enhanced by mutations on non-coordinating positions. Such peptides with high affinity for lanthanides may facilitate the development of new highly sensitive biosensors to monitor the metal pollution in the environment.

Synthesis, solution structure analysis and antibody binding of cyclic epitope peptides from glycoprotein D of Herpes simplex virus type I

Two cyclic peptides with a thioether bond have been synthesised corresponding to the 9-22 (9LKMADPNRFRGKDL(22)) sequence of glycoprotein D (gD-1) of Herpes simplex virus. The role of the secondary structure in protein-specific monoclonal antibody recognition was investigated. The sequence selected for this study comprises a strongly antigenic site adopting a beta-turn at residues 14Pro-(15)Asn. Thioether bond was formed between the free thiol group of cysteine or homocysteine inserted in position 11 and the chloroacetylated side chain of lysine in position 18. We report here the preparation of cyclic peptides containing Cys or Hcy in position 11, differing only in one methylene group. The linear precursor peptides were synthesised by Boc/Bzl strategy on MBHA resin, and the cyclisation was carried out in alkaline solution. The secondary structure of the peptides was studied by CD, FT-IR and NMR spectroscopy. The CD and FT-IR data have revealed fundamental changes in the solution conformation of the two compounds. The CH(2) group difference significantly resulted in the altered turn structure at the 12Ala and 13Asp as identified by NMR spectroscopy. The antibody binding properties of the cyclopeptides studied by gD-specific monoclonal antibody (A16) in direct and competition enzyme-linked immunosorbent assay (ELISA) were also not the same. We found that peptide LK[HcyADPNRFK]GKDL exhibited higher affinity to Mab A16 than peptide LK[CADPNRFK]GKDL, however, their reactivity was significantly lower compared to the linear ones. Our results clearly show the importance of secondary structure in an antibody binding and demonstrate that even a slight modification of the primary structure dramatically could influence the immune recognition of the synthetic antigens.

Conformational study of linear and cyclic peptides corresponding to the 276-284 epitope region of HSV gD-1

The results of conformational analysis of linear and cyclic peptides from the 276SALLEDPVG(284) sequence of glycoprotein D of Herpes simplex virus are presented. The epitope peptides were synthesized by SPPS and on resin cyclization was applied for preparation of cyclic compounds. Circular dichroism spectroscopy, Fourier-transform infrared spectroscopy and nuclear magnetic resonance (NMR) were used to determine of the solution structure of both linear and cyclic peptides. The results indicated that the cyclopeptides containing the core of the epitope (DPVG) as a part of the cycle have more stable beta-turn structure than the linear peptides or the cyclic analogues, where the core motif is not a part of the cycle. NMR study of H-SALLc(EDPVGK)-NH(2) confirm presence of a type I beta-turn structure which includes the DPVG epitope core.

Manipulation of epitope function by modification of peptide structure: a minireview

We have explored various approaches to modify the immunrecognition of linear peptides representing sequential or continuous topographic B-cell or T-cell epitopes. For these studies, epitopes from herpes simplex virus (HSV) glycoprotein D (gD) and from mucin 1 and mucin 2 glycoproteins or T-cell epitopes from 16 kDa and 38 kDa proteins of Mycobacterium tuberculosis were selected. To increase antigenicity and immunogenicity we have prepared cyclic and chimaeric peptide variants as well as epitope peptides with altered flanking regions and epitope-carrier conjugates containing multiple epitope copies.

lambda-conotoxins, a new family of conotoxins with unique disulfide pattern and protein folding. Isolation and characterization from the venom of Conus marmoreus

Conotoxins are multiple disulfide-bonded peptides isolated from marine cone snail venom. These toxins have been classified into several families based on their disulfide pattern and biological properties. Here, we report a new family of Conus peptides, which have a novel cysteine motif. Three peptides of this family (CMrVIA, CMrVIB, and CMrX) have been purified from Conus marmoreus venom, and their structures have been determined. Their amino acid sequences are VCCGYK-LCHOC (CMrVIA), NGVCCGYKLCHOC (CMrVIB), and GICCGVSFCYOC (CMrX), where O represents 4-trans-hydroxyproline. Two of these peptides (CMrVIA and CMrX) have been chemically synthesized. Using a selective protection and deprotection strategy during disulfide bond formation, peptides with both feasible cysteine-pairing combinations were generated. The disulfide pattern (C(1)-C(4), C(2)-C(3)) in native toxins was identified by their co-elution with the synthetic disulfide-isomeric peptides on reverse-phase high pressure liquid chromatography. Although cysteine residues were found in comparable positions with those of alpha-conotoxins, these toxins exhibited a distinctly different disulfide bonding pattern; we have named this new family "lambda -conotoxins." CMrVIA and CMrX induced different biological effects when injected intra-cerebroventricularly in mice; CMrVIA induces seizures, whereas CMrX induces flaccid paralysis. The synthetic peptide with lambda-conotoxin folding is about 1150-fold more potent in inducing seizures than the mispaired isomer with alpha-conotoxin folding. Thus it appears that the unique disulfide pattern, and hence the "ribbon" conformation, in lambda-conotoxins is important for their biological activity.

Synthesis and antibody recognition of mucin 1 (MUC1)-alpha-conotoxin chimera

We synthesized and characterized new chimera peptides by inserting an epitope of the mucin 1 glycoprotein (MUC1) as a 'guest' sequence in the 'host' structure of alpha-conotoxin GI, a 13-residue peptide (ECCNPACGRHYSC) isolated from the venom of Conus geographus. The Pro-Asp-Thr-Arg (PDTR) sequence of MUC1 selected for these studies is highly hydrophilic and adopts a beta-turn conformation. The alpha-conotoxin GI also contains a beta-turn in the 8-12 region, which is stabilized by two disulphide bridges in positions 2-7 and 3-13. Thus, the tetramer sequence of alpha-conotoxin, Arg9-His-Tyr-Ser12, has been replaced by PDTR, comprising the minimal epitope for MUC1 specific monoclonal antibodies (MAbs) HMFG1 (PDTR) and HMFG2 (DTR). Synthesis of the chimera peptide was carried out by Fmoc strategy on (4-(2',4'-dimethoxyphenyl-aminomethyl)phenoxy) (Rink) resin and either 5,5'-dithio-bis-(2-nitrobenzoic acid) (DTNB) or air oxidation was applied for the formation of the first Cys3-Cys13 or Cys2-Cys7 disulphide bridge, respectively. For the second disulphide bridge, three different oxidation procedures (iodine in acetic acid, 10% DMSO/1 M HCl or tallium trifluoroacetate (Tl(tfa)3) in TFA) were utilized. The HPLC purified peptides were characterized by electrospray mass spectrometry (ES-MS) and amino acid analysis. The CD spectra of the bicyclic MUC1-alpha-[Tyr1]-conotoxin chimera peptide showed partially ordered conformation with turn character. In antibody binding studies, the RIA data showed that both the linear and the bicyclic forms of MUC1-alpha-[Tyr1]-conotoxin chimera were recognized by MAb HMFG1 specific for PDTR sequence, while no binding was observed between MAb HMFG2 and various forms of the chimera. MAb HMFG1, using synthetic epitope conjugates or native MUC1 as target antigens, recognizes the PDTR motif more efficiently in the linear than in the bicyclic compound, but no reactivity was found with the monocyclic forms of MUC1-alpha-[Tyr1]-conotoxin chimera, underlining the importance of certain conformers stabilized by double cyclization.