The conformation of cardiotoxins and neurotoxins from snake venoms

Folding and domain-domain interactions of the chaperone PapD measured by 19F NMR

The folding of the two-domain bacterial chaperone PapD has been studied to develop an understanding of the relationship between individual domain folding and the formation of domain-domain interactions. PapD contains six phenylalanine residues, four in the N-terminal domain and two in the C-terminal domain. To examine the folding properties of PapD, the protein was both uniformly and site-specifically labeled with p-fluoro-phenylalanine ((19)F-Phe) for (19)F NMR studies, in conjunction with those of circular dichroism and fluorescence. In equilibrium denaturation experiments monitored by (19)F NMR, the loss of (19)F-Phe native intensity for both the N- and C-terminal domains shows the same dependence on urea concentration. For the N-terminal domain the loss of native intensity is mirrored by the appearance of separate denatured resonances. For the C-terminal domain, which contains residues Phe 168 and Phe 205, intermediate as well as denatured resonances appear. These intermediate resonances persist at denaturant concentrations well beyond the loss of native resonance intensity and appear in kinetic refolding (19)F NMR experiments. In double-jump (19)F NMR experiments in which proline isomerization does not affect the refolding kinetics, the formation of domain-domain interactions is fast if the protein is denatured for only a short time. However, with increasing time of denaturation the native intensities of the N- and C-terminal domains decrease, and the denatured resonances of the N-terminal domain and the intermediate resonances of the C-terminal domain accumulate. The rate of loss of the N-terminal domain resonances is consistent with a cis to trans isomerization process, indicating that from an equilibrium denatured state the slow refolding of PapD is due to the trans to cis isomerization of one or both of the N-terminal cis proline residues. The data indicate that both the N- and C-terminal domains must fold into a native conformation prior to the formation of domain-domain interactions.

Characterization by infrared spectroscopy of the interaction of a cardiotoxin with phosphatidic acid and with binary mixtures of phosphatidic acid and phosphatidylcholine

The effect of cardiotoxin IIa from Naja mossambica mossambica, a small basic protein extracted from snake venom, on dimyristoylphosphatidic acid (DMPA) and on equimolar mixtures of DMPA and dimyristoylphosphatidylcholine (DMPC) has been studied by Fourier transform infrared spectroscopy. The interaction of cardiotoxin with DMPA dispersions decreases both the cooperativity of the phase transition of the lipid and the molecular order of the lipid acyl chains in the gel phase. This effect increases with the proportion of the toxin in the complexes and leads to the total abolition of the phase transition of DMPA at a lipid-to-protein molar ratio of 5. Small-angle X-ray results demonstrate that the structure of the lipid-protein complexes is poorly ordered and gives rise to broad diffusion peaks rather than to well-resolved diffraction patterns. Infrared spectra of oriented cardiotoxin-DMPA films show that the protein is not homogeneously oriented with respect to the bilayer surface. The destabilization of the gel-phase structure of DMPA by cardiotoxin also results in a deeper water penetration in the interfacial region of the lipid since more carbonyl ester groups appear to be hydrogen bonded in the presence of the toxin. The infrared results on the phosphate group vibrations also indicate clearly that the basic residues of cardiotoxin interact strongly with the phosphate group of DMPA that becomes partly ionized at a pH as low as 6.5. The results obtained on the interaction of cardiotoxin with an equimolar mixture of DMPA and DMPC clearly demonstrate the ability of this toxin to induce lateral phase separation in this mixture with one phase containing DMPA-rich domains perturbed by cardiotoxin while the second phase is composed of regions enriched in DMPC. Comparison of the results of the current study with those obtained on other basic proteins and polypeptides suggests that charge-induced phase separation occurs only when the charge density on certain regions of the protein structure is high enough to lead to efficient electrostatic interactions with anionic phospholipids. This condition occurs only when the conformation of the protein or polypeptide is well-ordered at the lipid interface.

Production and characterization of monoclonal antibodies against Naja naja atra cobrotoxin

Twelve monoclonal antibodies against cobrotoxin from Naja naja atra venom were tested for cross-reactivity with eight different snake toxins, binding to linear epitopes, prevention of cobrotoxin binding to acetylcholine receptor (AchR) in vitro, and protection in mice concomitantly given a lethal dose of cobrotoxin. The antibodies were highly specific, as evidenced by little reactivity with other snake toxins. None of the monoclonal antibodies bound to reduced cobrotoxin or synthesized 8-mer regions spanning the whole molecule, thus suggesting the recognition of conformational epitopes. The in vitro binding of toxin to AchR was competitively inhibited (23-79%) with a 1.66:1 mole ratio of antibody:AchR. Preincubation of monoclonal antibody with toxin before adding AchR (3:1 mole ratio of AchR:antibody) inhibited the in vitro binding of toxin to AchR by 20-80%. Monoclonal antibodies added after the preincubation of toxin with AchR did not dissociate the toxin-AchR complex. An antibody:toxin mole ratio of 2.5:1, with 6 micrograms of cobrotoxin, delayed the time to death of mice 3.7-23.8-fold over control mice. The monoclonal antibodies that most effectively prevented in vitro binding of toxin to AchR also provided the longest delay in time to death in mice.

Intercomparison of small biomolecules by gel filtration and small angle scattering

This paper describes how small angle neutron scattering (SANS) can be used to confirm that gel filtration results are free of dimerization effects. After characterization by analytical gel filtration, concentrated solutions (in heavy water, D2O) of a cobra neurotoxin, a cytotoxin, and a cytotoxin analog are studied by SANS. Small differences in shape are shown to be discernible by means of least-square fits to ellipsoidal models. The parallel axis theorem is then invoked to assess dimerization levels statistically. The results are briefly discussed on the basis of function in relation to structure.

Differing stabilities of snake venom cardiotoxins in acidic aqueous acetonitrile

1. Although snake venom cardiotoxins constitute a homologous family of proteins, subclasses with different structural and biological properties exist. 2. By using circular dichroism spectroscopy of twelve cardiotoxins belonging to two structural classes and one non-classified group, this investigation indicated that cardiotoxins differ in their stabilities towards denaturation in acidic aqueous acetonitrile, as used in some reversed-phase high performance liquid chromatography separations. 3. It was also shown that cardiotoxins of the structural class II are in general less stable towards this denaturation than class I and non-classified cardiotoxins.

Correlation between the surface hydrophobicities and elution orders of elapid neurotoxins and cardiotoxins on hydrophobic-interaction high-performance liquid chromatography

Hydrophilic and hydrophobic regions were predicted for Elapid neuro- and cardiotoxins. The contribution of these regions to the retention times of neuro- and cardiotoxins on hydrophobic-interaction HPLC was assessed from the known surface accessibilities of amino acid side-chains within these regions. Differences in retention times between neuro- and cardiotoxins on hydrophobic-interaction HPLC could be attributed to differences in hydrophobicity of regions 6-12 and 22-26 between these two types of toxins. Smaller differences in retention times between cardiotoxins were due to the variable hydrophobicities of regions 1-4 and 26-36.

The structural evolution of cobra venom cytotoxins

In order to analyze the evolutionary behavior of the cobra venom cytotoxins, their probable tertiary structure was predicted using computer graphics. The 41 amino acid sequences known show that the major evolutionary changes have taken place in two particularly exposed areas of the molecular surface. In each area, neighboring residue positions seem to have evolved interdependently, but there is no obvious interdependence between the two areas. Indeed, the relative evolution of these two areas prompts a subdivision of the sequence set into four groups. According to the known cytotoxin circular dichroism spectra, one of these four groups could be characterized by a difference in molecular secondary structure. Since the two variable areas have functional associations, it is suggested that their evolution may be governed by a target with several similar binding sites.

Reversed-phase and hydrophobic-interaction high-performance liquid chromatography of elapid cardiotoxins

The separation of proteins by hydrophobic-interaction HPLC and reversed-phase HPLC depends upon differences in the hydrophobicity of accessible surface groups. The elution order of a group of snake venom cardiotoxins was found to vary between these two HPLC methods. Circular dichroism spectroscopy showed that the eluant acetonitrile-trifluoroacetic acid used for reversed-phase HPLC altered the conformation of the toxins, whereas the salt-buffer eluting medium used for hydrophobic-interaction HPLC did not affect toxin conformation. The retention times of cardiotoxins on reversed-phase HPLC are therefore influenced by their conformational instability in the eluting medium which causes partial or complete unfolding. Hydrophobic interaction is clearly the preferred method with which to correlate the "surface hydrophobicity" of cardiotoxins and their biological effects.

Mixed phospholipid-cardiotoxin monomolecular films studied by intrinsic polarized surface fluorescence

Surface fluorescence spectroscopy was used to study mixed phospholipid-cardiotoxin monomolecular films. Using polarized incident light we were able to detect the intrinsic fluorescence emission at a wavelength of 339 nm of the single Trp11 in the cardiotoxin molecule. Its fluorescence intensity increased continuously upon film compression up to a maximal value reached at 42 mN x m-1 in surface pressure. A relative polarization index can be used as an indication of the orientation of the indole ring. A transition at around 25 +/- 5 mN x m-1 in the orientation of the Trp11 relative to the plane of the interface was clearly evidenced during film compression, in agreement with our independent data showing a flipping of the whole toxin molecule (Bougis et al., (1981), Biochemistry 20, 4915-4920).

The selective S-alkylation of a methionine residue in an elapid venom cardiotoxin

1. The reaction of cardiotoxin with iodoacetamide or iodomethane at pH 3.0 afforded the corresponding methionine sulphonium derivatives. The major products were S-alkylated at Met-26 whilst the minor products were S-alkylated at both Met-24 and -26. 2. Reaction with iodoacetamide under denaturing conditions led to a reversal of the relative abundances of the two derivatives in the respective reaction mixtures. 3. The derivative S-methylated at Met-26 was about 5-fold less toxic than the parent cardiotoxin. That derivatised at both Met-24 and -26 was non-toxic, indicating the importance of Met-24. 4. The results are discussed in the light of a structural model, previous chemical modifications and 1H-NMR data. It appeared that Met-24 is important for the integrity of an important structural feature of cardiotoxin.

Properties of some 3-nitrotyrosyl elapid venom cardiotoxins

Nitration of the invariant Tyr-22 in Hemachatus haemachates cardiotoxin 12B did not greatly decrease lethality, and the haemolytic potency towards guinea-pig erythrocytes remained unchanged. This residue is thus non-essential for cardiotoxin to exert its biological action. Nitration of Naja haje annulifera and Naja melanoleuca cardiotoxins VII1 decreased but did not abolish the lethalities and haemolytic potencies. Thus Tyr-25 and Tyr-51 were concluded to have no direct functional role in cardiotoxin lethality. The pKa values of the phenolic hydroxyl groups of the tyrosine residues appeared to be important for certain properties of cardiotoxin in solution. No evidence could be produced to show that Tyr-51 is unreactive to nitration under normal (non-denaturing) conditions.

Circular dichroism study of the unfolding-refolding of a cardiotoxin from Taiwan cobra (Naja naja atra) venom

Circular dichroism spectroscopy has been used to study the unfolding-refolding process of a cardiotoxin from Taiwan cobra (Naja naja atra) venom upon addition of fluoroalcohols or sodium dodecyl sulfate (SDS) to its aqueous solution. In these experiments, the disulfide bridges remained intact. The unfolding process has been found to be reversible both for fluoroalcohols and for SDS unfolding. The reversibility of the unfolding-refolding process of cardiotoxin in aqueous mixtures of fluoroalcohols was dependent on the volume per volume ratio of alcohol to water. SDS did not unfold the secondary structures of cardiotoxin whereas its tertiary structure was affected. If the SDS concentration in aqueous solution exceeded the critical micelle concentration value of SDS, a quasi-refolded state of cardiotoxin was observed. The mechanism of unfolding-refolding is discussed in terms of molecular interactions which might govern the protein conformation in solution.

Preparation and characterization of monoclonal antibody specific for Naja nivea cardiotoxin VII1

Monoclonal antibodies against Naja nivea cardiotoxin VII1 were produced using the hybridoma technique. The antibodies of two clones were found to be identical by an avidity test, isoelectric focusing and immunodiffusion typing assay. The monoclonal antibody was focused at a pH range of 7.4-8.1 and belonged to the mouse sub-class IgG1. A dissociation constant of 0.26 nM demonstrated its high affinity to cardiotoxin. The monoclonal antibody had no effect on cardiotoxin lethality or lysis of red blood cells by the toxin and could therefore be assumed to bind to an antigenic site separate from the active centre.

The binding of snake venom cardiotoxins to heart cell membranes

Cobra venom cardiotoxins have the effect, inter alia, of causing systolic arrest of the heart. We have observed significant binding in vitro of 35S-labelled cardiotoxins to mouse heart cell membranes. Part of the binding was saturable and could be displaced with homologous unlabelled cardiotoxins but not by neurotoxins or cardiotoxins inactivated by chemical modification. The specifically bound component represented more than 70% of total binding at saturation. Inclusion of Triton X-100 and NaCl in the phosphate-buffered incubation medium prevented nonspecific adsorption to centrifuge tube walls, and gave lower but more reproducible specific binding results, respectively. An apparent dissociation constant of 5 . 10(-7) M and a binding density of 500 pmol toxin/mg membrane protein were derived from the saturation isotherms.

Conformational properties of the neurotoxins and cytotoxins isolated from Elapid snake venoms

The review will critically assess the information available on the conformation of homologous neurotoxins and cytotoxins isolated from Elapid snakes. Particular attention will be given to the dynamics of the molecules in solution because there is the possibility that defined intramolecular rearrangements are involved at the sites of action. Such properties will be then reconciled with the known X-ray crystallographic and sequence data in order to derive likely structure-activity relationships.

A possible orientation change of cardiotoxin molecule during its interaction with phospholipid monolayer

The monomolecular film technique was used in order to study the specific interactions of 4 cardiotoxins from Elapidae snake venom Naja mossambica mosambica with different phospholipids. The interaction, at pH 7.5, of cardiotoxin (10(-7) M) with both neutral and negatively charged phospholipids occurs up to a very high critical surface pressure (pi = 45 dynes/cm with the latest). The apparent molecular area of cardiotoxin molecule, during its insertion into a negatively charged phospholipid film, presents only two characteristic values: 1400 A2 for pi less than 20 dynes/cm and 420 A2 for pi greater than 30 dynes/cm, the transition occurring in a very narrow range of surface pressure (25 +/- 5 dynes/cm). Thus, according to the surface pressure, the cardiotoxin may present two different orientations: "flat" or "edgewise" and the transition between both could account for lytic activity of cardiotoxin.

Biosynthesis, secretion and in vivo isotopic labelling of venom of the Egyptian cobra, Naja haje annulifera

The venom glands of Elapidae differ from those of the Viperidae by lacking an expanded central lumen; the venom is stored in the tubular lumina as well as inside the cells in densely packed secretion granules. Using isotope tracer techniques, it was found that in the Egyptian cobra (Naja haje annulifera) venom is secreted both from pre-existing and from newly-formed granules. The rate of protein biosynthesis peaks at 4-9 days after venom was extracted (milked) from the glands. Highly labelled toxins (1-10 mCi/mmole protein) were isolated in good yield from the venom of snakes chronically intubated and infused i.p. with (3H)-amino acids. Repeated Fluothane (Halothane) anaesthesias and venom collections had no ill effect on venom yield. The radioactive venom and its component toxins retained full biological potency.

Reversed unfolding-refolding process of cobra neurotoxin

Circular dichroism and nuclear magnetic resonance spectroscopies have been used to study the unfolding process of cobrotoxin upon addition of fluoro alcohols/or sodium dodecyl sulfate to its aqueous solution. In each final unfolded state, the protein had its disulfide bonds intact. The unfolding process has been found to be reversible in the case of fluoro alcohol/water mixtures, while no such reversibility was found in the case of sodium dodecyl sulfate. However, when hexafluoro-2-propanol is added to the sodium dodecyl sulfate unfolded protein, refolding is induced. The mechanism of unfolding is discussed in terms of the different interactions which govern the protein conformation in solution.

1H nuclear-magnetic-resonance studies of the conformation of cardiotoxin VII2 from Naja mossambica mossambica

The membrane toxin VII2 from the venom of Naja mossambica mossambica was investigated in aqueous solution by one-dimensional and two-dimensional high-resolution nuclear magnetic resonance (NMR) techniques at 360 MHz. The spectral characterization included identification of the complete spin systems for several amino acid residues, nuclear Overhauser effect measurements, the use of chemically induced dynamic nuclear polarization and studies of the pH dependence of the NMR spectrum. Data from homologous toxins, in particular direct lytic factor 12B from Haemachatus haemachatus, were used to establish assignments of aromatic and methyl proton resonances. From these experiments a short, triple-stranded fragment of antiparallel beta structure could be determined, which includes the residues 23-27, 43-46 and 60-62. Furthermore, the nuclear Overhauser effect measurements indicate close proximity in the protein conformation of the aromatic rings of Trp-14, Tyr-25 and Tyr-59, and the side chain of Ile-46.

Refolding of reduced short neurotoxins: circular dichroism analysis

The four disulfide bonds of nine homologous short curare-like polypeptides are cleaved by reduced dithiothreitol. Air oxidation renaturations of the reduced compounds are followed by far-ultraviolet circular dichroism analysis, and the kinetics of refolding thus determined are compared. They indicate that three toxins refold 4--10 times more slowly than the six others. It is shown that a significant difference between the refolding kinetics still subsists when renaturations are made in the presence of various concentrations of thiol-disulfide exchange reagents or at various pH values. From an examination of the toxin sequences, it is proposed that a single additional amino acid insertion is responsible for the difference in the observed kinetics. This proposal is supported by temperature studies of renaturation kinetics.

Circular dichroism of elapidae protein toxins

The effect of temperature variation on the circular dichroic spectra of four Elapid snake toxins (one short neurotoxin, two long neurotoxins and one cytotoxin) was determined and the results compared. The three neurotoxins have in common a delicately balanced conformation which can be perturbed independently of the predominant secondary structuring. Despite the many differences in sequence between long and short neurotoxins, the nature of this balance seems to be the same in both types and it is considered to involve an equilibrium between two defined conformers. Such properties could not be discerned in the cytotoxin studied, but related work suggests that they could nevertheless be present in some cases. In terms of global structure, long neurotoxins may have a greater proportion of 'random chain' inherent than short neurotoxins, the latter having extensive beta-sheet. The important feature of all toxins is deemed to be a conformational freedom defined by the secondary structuring and characteristic disulphide bridge formation. Possible relevance to toxicity of such a property is discussed.

Penetrability of proteins through the digestive system of Sarcophaga falculata blowfly

On the background of the previous findings concerning the passage of a low molecular basic protein from cobra snake venom through the gut of a blowfly (Primor, N., Teitelbaum, Z. and Zlotkin, E. (1980) Biochim, Biophys, Acta 627, 71--81) several additional proteins were tested in the present study. With the aid of toxicity tests, radioiodinated proteins, gel filtration chromatography, and the usage of Sarcophaga falculata flies as test animals, the following information was obtained: 1. Out of the five species (representing four orders) of insects tested, only flies demonstrated the lethality by oral application of cobra snake venom. 2. The absence of oral toxicity of flies to phospholipase A2 (highly lethan by injection of cobra venom with molecular weight of about 13,000) was due to its impermeability through the digestive system. 3. About 2.2% of the orally applied low molecular weight basic neurotoxin (Mr 7000) from cobra venom crossed the gut and was found in the insects' hemolymph. 4. Bovine serum albumin (Mr 68,000) and an immunoglobulin (Mr 180,000), under the present experimental conditions, were found to be gut impermeable.

Reverse turns in peptides and proteins

The evidence that reverse turns frequently occur as structural components of proteins, as well as of linear and cyclic peptides, is overwhelming. This review summarizes and examines critically the experimental evidence derived from physical methods such as 1H and 13C nuclear magnetic resonance spectroscopy, spin-lattice relaxation time, circular dichroism, IR spectroscopy, and X-ray crystallography. Secondly, theoretical evidence obtained from energy calculations, which rely on empirical energy functions, and correlative methods, which rely on algorithms based on the frequency of occurrence of amino acids, is evaluated. In particular, those theoretical studies for which complementary physical studies have been completed are emphasized. Finally, examples of structure-function relationships involving reverse turns and their biological recognition are demonstrated.

The 1H nuclear-magnetic-resonance spectra of Neurotoxin I and cardiotoxin Vii4 from Naja mossambica mossambica

Two toxins from the venom of Naja mossambica mossambica, neurotoxin I and cardiotoxin VII4, were investigated in aqueous solution by high-resolution 1H nuclear magnetic resonance (NMR) techniques at 360 MHz. The spectral characterization of the proteins included determination of the number of slowly exchanging amide protons which can be observed in 2H2O solution, measurement of the amide proton chemical shifts and exchange rates, characterization of the aromatic spin systems and the internal mobilities of aromatic rings, and studies of the pH dependence of the NMR spectra. For numerous resonances of labile and non-labile protons quite outstanding pH titration shifts were observed. It is suggested that these NMR parameters provide a useful basis for comparative structural studies of different proteins in the large group of homologous snake toxins. As a first application the NMR data presently available in the literature on neurotoxin II from Naja naja oxiana, toxin alpha from Naja nigricollis and erabutoxin a and b from Laticauda semifasciata have been used to compare these three proteins with neurotoxin I from Naja mossambica mossambica. This preliminary comparative study provides evidence that the same type of spatial structure prevails for these four homologous neurotoxins and that the folding of the backbone corresponds quite closely to that observed in the crystal structure of erabutoxin b. A second application is the comparison of cardiotoxin VII4 from Naja mossambica mossambica with the neurotoxins. The experimental data indicate that the folding of the polypeptide backbone is closely similar, but that the cardiotoxin molecule is markedly more flexible than the neurotoxins.