Conformation of cobrotoxin in aqueous solution as studied by nuclear magnetic resonance

Role of amino and carboxyl groups of cobrotoxin in the conformational stability and the interaction with acetylcholine receptor

To study the functional involvements of the common interaction of the Leu-1 alpha-amino group and Asp-58 in cobrotoxin, the lysine epsilon-amino groups of cobrotoxin were initially guanidinated with o-methylisourea. The alpha-amino group of Leu-1 was them modified with TNBS after the guanidination of cobrotoxin. Both modified derivatives displayed no significant changes in the secondary structure and antigenicity of cobrotoxin, whereas the binding affinity for nicotinic acetylcholine receptor (nAChR) was pronouncedly decreased when Leu-1 was modified. Six out of seven free carboxyl groups and the remaining buried Glu-21 carboxyl group of cobrotoxin were modified with glycine methyl ester in the absence and presence of guanidine HCl, respectively. Alternation in the beta-sheet secondary structure of cobrotoxin was observed with the carboxyl-group modified derivatives, which caused a decrease in the binding activity of the toxin molecule to the antibody and nAChR. Moreover, modification of the Glu-21 carboxyl group of cobrotoxin further reduced the nAChR binding activity, while the antigenicity remained unchange. Thus, our results conclude that the Glu-21 residue and the common interaction of the terminal Leu-1 alpha-amino group and the Asp-58 carboxyl group are related to the nAChR-binding activity of cobrotoxin, and the free carboxyl groups in cobrotoxin are conformation-essential.

Two-dimensional NMR studies and secondary structure of cobrotoxin in aqueous solution

The 1H-NMR spectra of cobrotoxin, a neurotoxic protein isolated from Formosan cobra Naja naja atra, have been studied by two-dimensional NMR techniques. Of 62 amino acid residues in cobrotoxin, the complete assignments of 58 residues have been made. The resonances from several of the remaining residues have been identified but not yet specifically assigned. The secondary structure of an antiparallel triple- and double-stranded beta-sheet has also been determined by observing the NOE.

Recognition of the acetylcholine receptor binding site of a long-chain neurotoxin by toxin-specific monoclonal antibodies

The present paper reports the preparation and characterization of two neutralizing monoclonal antibodies (Mabs), called MST1 and MST2, which bind at the central loop of a long-chain neurotoxin from cobra venom. The central loop is a critical region for the binding of the toxin to the nicotinic acetylcholine receptor. Some of the residues incorporated in the epitopes recognized by MST1 and MST2 have been identified on the basis of competition experiments using a set of 'chemical mutants' of the toxin. We show that MST1 and MST2 bind at the base and at the tip of the central loop of the toxin, respectively, however, only MST2 actually overlaps the acetylcholine receptor binding site. Accordingly, only MST2 is capable of recognizing all homologous toxins so far examined. MST2, therefore, mimicks, at least partially, the site by which the nicotinic acetylcholine receptor recognizes a long-chain neurotoxin.

A proton nuclear magnetic resonance study on the solution structure of crotamine

Proton nuclear magnetic resonance (NMR) spectra of crotamine, a myotoxic protein from a Brazilian rattlesnake (Crotalus durissus terrificus), have been analyzed. All the aromatic proton resonances have been assigned to amino acid types, and those from Tyr-1, Phe-12, and Phe-25 to the individual residues. The pH dependence of the chemical shifts of the aromatic proton resonances indicates that Tyr-1 and one of the two histidines (His-5 or His-10) are in close proximity. A conformational transition takes place at acidic pH, together with immobilization of Met-28 and His-5 or His-10. Two sets of proton resonances have been observed for Ile-17 and His-5 or His-10, which suggests the presence of two structural states for the crotamine molecule in solution.

Step-wise thermal denaturation of cobrotoxin, a snake venom neurotoxin from Naja naja atra: a proton nuclear magnetic resonance study

Temperature dependence of proton nuclear magnetic resonance spectra has been followed for cobrotoxin, a postsynaptic neurotoxin from Naja naja atra venom. Several aromatic amino-acid residues, including the functionally essential Trp-29 located at the tip of the central loop of the molecule, have been found to undergo a thermal structural transition above the global thermal denaturation temperature. It is suggested that a local structure around these residues behaves somehow independently of the rest of the molecule, and that such structural organization may be favorable for a conformational change of a neurotoxin molecule on binding to acetylcholine receptor.

Erabutoxin b. Structure/function relationships following initial protein refinement at 0.140-nm resolution

A study has been made, following high-resolution refinement at 0.14 nm, of the structure of erabutoxin b, prototype postsynaptic neurotoxin from snake venom. The detailed patterns of intramolecular van der Waal's interactions have been determined. From information, hitherto unavailable, about atomic temperature parameters, the relative mobilities in different regions of the molecule have been estimated. A detailed model of structure/function relationships in these neurotoxins, which bind to the acetylcholine receptor, has thus been established: the probable dynamic mode of toxin-receptor binding is described. The model identifies, and the binding mode depends on a unique structural feature of these protein toxins: the hydrophobic 'Trp' cleft. Charge-charge interactions are implicated in initial toxin orientation on the receptor surface. Possible reactive-site extension in short-chain toxins is described. Modifications in binding mode of long-chain toxins are considered. The relative mobilities of antigenic site residues are discussed.

Sequential individual resonance assignments in the 1H nuclear-magnetic-resonance spectrum of cardiotoxin VII2 from Naja mossambica mossambica

The assignment of the 1H nuclear magnetic resonance (NMR) spectrum of cardiotoxin VII2 from Naja mossambica mossambica is described and documented. The assignments are based entirely on the amino acid sequence and on two-dimensional NMR experiments at 500 MHz. Individual assignments were obtained at 45 degrees C for the backbone protons of 56 out of the total of 60 amino acid residues, the exceptions being the N-terminal dipeptide segment Leu-1--Lys-2--, Pro-8 and Pro-15. Complete assignments of the non-labile hydrogen atoms of the side chains were obtained for 37 residues, and for Asn-4 and Asn-19 the delta amide protons were also identified. For 19 long side chains the individual assignments include only the backbone and C-beta proton resonances; these are Gln-5, Pro-9, Pro-33, Pro-43, Leu-47, all three methionines, two arginines and nine lysines. The chemical shifts for the assigned resonances at 45 degrees C are listed for an aqueous solution at pH 3.6. A preliminary interpretation of the sequential connectivity patterns indicates that approximately 30 out of the total of 60 amino acid residues in cardiotoxin VII2 are in extended, beta-type secondary structures, and there is no indication for the formation of alpha-helical structure.

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.

Local conformational transition of toxin B from Naja naja as studied by nuclear magnetic resonance and circular dichroism

The 270-MHz proton NMR spectra of toxin B, a long neurotoxin from Naja naja, in 2H2O solution were analyzed. From the analysis of the discontinuous pH dependence of His-21 C-2 proton chemical shift a 23 degrees C, toxin B is found to exhibit a slow-exchange conformational transition between the states A and B with the transition midpoint at pH 5.75. Separate peaks due to the states A and B were also observed for His-21 C-4 proton, Tyr-25 ring protons, phenylalanine ring protons and two C-alpha protons. This local conformational transition as induced by the protonation of His-21 residue is further confirmed by the measurement of the pH dependence of circular dichroism. In state A, the pKa value of His-21 is 6.6-6.9, indicating that His-21 residue is not significantly perturbed. The His-21 residue in state B is accessible to solvent, as found by the analysis of hydrogen leads to deuterium exchange rate. The low pKa value (4.6-4.9) of His-21 in stat B is probably due to the formation of a hydrogen bond of the imidazole ring. The conformational transition rate at 23 degrees C is obtained as 2.5 s-1 at the transition midpoint (pH 5.75), by the saturation transfer method. The conformational transition at 60 degrees C corresponds to a fast exchange case for the Tyr-25 ring proton resonances and to an intermediate exchange case for the His-21 C-2 and C-4 proton resonances. The transition rate at the midpoint is about 300 s-1 at 60 degrees C.

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.

Proton-nuclear-magnetic-resonance study on molecular conformations of long neurotoxins. alpha-Bungarotoxin from Bungarus multicinctus and Toxin B from Naja naja

The 270-MHz proton NMR spectra were analyzed of the long neurotoxins alpha-bungarotoxin from Bungarus multicinctus and Toxin B from Naja naja. The aromatic proton resonances were completely assigned to individual nuclei for alpha-bungarotoxin and in part for toxin B. The pH dependences of proton chemical shifts were analyzed by the nonlinear least-square method, for obtaining pKa values and protonation shifts. The pKa values of Tyr-25, an invariant residue of neurotoxins, are 12.1 for alpha-bungarotoxin and 11.3 for toxin B, suggesting the presence of a strong hydrogen bond involving Tyr-25 in alpha-bungarotoxin. The Trp-29 residues of both toxins show a common titration shift due to the carboxylate group of Asp-31 and a similar structural arrangement of functionally invariant pair of Trp-29 and Asp-31 is implied. From the temperature dependences of the chemical shifts of His-68 and a methyl group of alpha-bungarotoxin, the local structure around His-68 near the tail part is shown to be more flexible than the other part. The six main-chain amide protons of alpha-bungarotoxin exchange most slowly with solvent deuterons and are found by interproton nuclear Overhauser effects to be in the beta-sheet near the aromatic ring of Tyr-25 residue. Hydrogen leads to deuterium exchange rates in 2H2O solution at 37 degrees C were measured of slowly exchanging amide protons of alpha-bungarotoxin, toxin B, and two short neurotoxins, namely cobrotoxin and erabutoxin b. The two long neurotoxins have amide protons with relatively long half-times spanning as long as 10-100 h, but the two short neurotoxins do not have amide protons with half-times longer than 3 h. The distributions of the half-times of amide proton exchange indicate the structural rigidity of neurotoxins in the order, alpha-bungarotoxin greater than toxin B greater than cobrotoxin approximately erabutoxin b, in agreement with the order of neurotoxicity as reported previously by Chicheportiche et al. and by Lee and Chen.

A proton-magnetic-resonance study on the molecular conformation and structure-function relationship of a long neurotoxin, laticauda semifasciata III from Laticauda semifasciata

The 300-MHz and 500-MHz NMR spectra of a long neurotoxin laticauda semifasciata III (LS III) from Laticauda semifasciata have ben analysed. Comparison with the NMR spectra of alpha-cobratoxin from Naja naja siamensis, a homologous long neurotoxin to laticauda semifasciata III, allowed the assignment of all the aromatic protein resonances to specific amino acid residues. All the methyl proton resonances have been assigned to specific types of amino acid residues. The pH dependences of the aromatic and methyl proton chemical shifts were analyzed by the non-linear least-square method to give the pKa values and protonation shifts. The interproton nuclear Overhauser effect enhancements were measured in order to elucidate the spatial proximity of the methyl-bearing residues and aromatic residues. On the basis of these NMR data and using the crystal structure of alpha-cobratoxin by Walkinshaw et al., more than half of the methyl proton resonances have been assigned to specific amino acid residues. A hydrophobic core comprising the first loop, the central loop and the tail part of the molecule has been defined. This hydrophobic core may be common to all long neurotoxins and may protect the three-stranded antiparallel pleated beta-sheet structure, thus making the backbone structure of long neurotoxins more rigid than that of short neurotoxins. The positively charged surface of laticauda semifasciata III, which is responsible for binding to the acetylcholine receptor protein, is confirmed as the concave surface formed by the central and the third loop. The arrangement of the amino acid residues on this surface is similar to that of all other neurotoxins. Accordingly, the slow on-off rates of association of long neurotoxins with receptor is considered to arise from the rigid backbone structure. A small conformation change is thought to be associated with binding to the receptor protein.

The dynamic structures of proteins: short and long neurotoxins as examples

This review illustrates the way in which nuclear-magnetic-resonance studies of proteins in solution can add dynamic structural information to our knowledge of their crystal structures. The value of the study is illustrated by an analysis of the structure and the dynamics of structure within two series of neurotoxins. The dynamics may well control the binding rates of these and other proteins to receptors.

The 1H nuclear-magnetic-resonance spectra and spatial structure of the Naja mossambica mossambica neurotoxin III

The proton NMR spectra at 300 MHz of neurotoxin III from venom of Naja mossambica mossambica are reported. By the use of double resonance techniques, pH dependence chemical shifts, isotope labeling technique, and comparison with homologous neurotoxins all proton signals in the aromatic and methyl regions as well as epsilon-CH2 proton signals of some lysine residues have been assigned to individual amino acid residues and their spatial microenvironment has been determined. The results deduced on the solution structure of neurotoxin III are in complete agreement with the crystal structure of sea snake erabutoxins as well as with the previously established backbone folding and inter-residue interactions for the Naja naja oxiana short-chain neurotoxin in solution. In addition evidence has been obtained (a) that the conformation of the beta turn in the 31-34 segment depends on the ionization state of Asp-31 and His-32 side chain groups and (b) that an intricate electrostatic interaction exists in a system of ionogenic groups of the invariant Lys-27, Lys-47, Asp-31, Arg-33, Glu-38 and His-32 residues. These aspects of dynamic conformation are related to an interaction mechanism of a neurotoxin molecule and a nicotinic acetylcholine receptor.

Molecular conformation and function of erabutoxins as studied by nuclear magnetic resonance

The 270-MHz proton NMR spectra of erabutoxins a, b and c from Laticauda semifasciata in 2H2O solution were observed together with [15-N6-acetyllysine]erabutoxin b, [27-N6-acetyllysine]-erabutoxin b and [47-N6-acetyllysine]erabutoxin b. The lysine epsilon-methylene proton resonances of erabutoxin b are assigned to individual residues. The epsilon-methylene proton resonance of Lys-27 is significantly broad, indicating that the mobility of this residue is restricted. Upon acetylation of Lys-27 of erabutoxin b, the pKa values of three other lysine residues are lowered by about 0.2, indicating long-range interactions among lysine residues. All the methyl proton resonances are assigned to amino acid types, primarily by the spin-echo double-resonance method. The pH dependences of proton chemical shifts were analyzed by the nonlinear least-square method, for obtaining pKa values and protonation shifts. The interproton nuclear Overhauser effect enhancements were measured for elucidating the spatial proximity of methyl-bearing residues and aromatic residues. On the basis of these NMR data and with the crystal structures by Low et al. and by Petsko et al., the methyl proton resonances of all the valine, leucine, and isoleucine residues and Thr-45 have been identified. The microenvironments of Tyr-25, His-26, Trp-29, four lysines and eight methyl-bearing residues have been elucidated. The addition of the paramagnetic hexacyanochromate ion causes broadening of the proton resonances of Thr-45, Lys-47, Ile-50, Trp-29 and Ile-36 residues located on one end of the molecule of erabutoxin b. The positively charged invariant residues of Lys-47 and Arg-33 at this part of the molecule are probably involved in the binding to the receptor protein.