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

Studies on sea snake venom

Erabutoxins a and b are neurotoxins isolated from venom of a sea snake Laticauda semifasciata (erabu-umihebi). Amino acid sequences of the toxins indicated that the toxins are members of a superfamily consisting of short and long neurotoxins and cytotoxins found in sea snakes and terrestrial snakes. The short neurotoxins to which erabutoxins belong act by blocking the nicotinic acetylcholine receptor on the post synaptic membrane in a manner similar to that of curare. X-ray crystallography and NMR analyses showed that the toxins have a three-finger structure, in which three fingers made of three loops emerging from a dense core make a gently concave surface of the protein. The sequence comparison and the location of essential residues on the protein suggested the mechanism of binding of the toxin to the acetylcholine receptor. Classification of snakes by means of sequence comparison and that based on different morphological features were inconsistent, which led the authors to propose a hypothesis "Evolution without divergence."

Differential contribution of the conserved tyrosine residues to activity and structural stability of Ophiophagus hannah alpha-neurotoxins

Two alpha-neurotoxins, Oh-4 and Oh-7, from king cobra (Ophiophagus hannah) venom were subjected to Tyr modification with tetranitromethane. Selective nitration of Tyr4 in Oh-4 caused a slight decrease in lethal toxicity of 11% and a decrease in nicotinic acetylcholine receptor (nAchR)-binding activity of 28%, whereas nitration of Tyr4 in Oh-7 resulted in an approximately equal 60% decrease in lethality and nAchR-binding activity. When the Tyr23 in Oh-4 or Tyr22 in Oh-7 appears to be 'buried' in the toxin following further modification, the toxins lost their biological activity and conformational change concurrently. Nevertheless, the dinitrated Oh-4 retained a beta-sheet structure as revealed by CD spectra and exhibited a precipitin reaction with anti-Oh-4 sera. These results indicate that both Tyr4 and Tyr22 play a crucial role in the neurotoxicity of Oh-7, whereas intact Tyr23 is involved in the manifestation of the toxicity of Oh-4 to a greater extent. In contrast to Oh-4, the conformational stability of Oh-7 depends heavily upon the integrity of Tyr22.

Cardiotoxin VII4 from Naja mossambica mossambica. The refined crystal structure

The crystal structure of cardiotoxin VII4 from Naja mossambica mossambica was refined to 2.5 A resolution. Fifty ordered solvent sites were localized and included in the refinement. The final R factor is 0.197 (lambda/(2sin theta) less than 5 A; F greater than 3 sigma). The three-dimensional structure is characterized by two beta-sheets. Of particular interest is the two-stranded beta-sheet in the N-terminal region. This shows a large right-handed twist and, though strongly connected to the core of the molecule, and in particular to the C-terminal end, protrudes out of the bulk of the molecule. The segment of four amino acid residues connecting the two strands of this sheet is particularly exposed. It contains an invariant proline residue that has probably an important structural role, and is completely hydrophobic. Two other conserved hydrophobic zones were identified; the largest extends over the second and third loops, on one side only of the molecule. All side-chains of invariant hydrophobic character (except proline residues) belong to one of these three zones. Also discussed are the dimeric assembly and the rather loose packing in the crystal. The three-dimensional structure is compared with that of short and long alpha-neurotoxins. Comparison with two-dimensional nuclear magnetic resonance results on the 68% homologous cardiotoxin CT X IIb shows an excellent overall agreement. A few differences are probably genuine.

Molecular conformation of alpha-bungarotoxin as studied by nuclear magnetic resonance and circular dichroism

We have examined the circular dichroism and nuclear magnetic resonance spectra of a long neurotoxin, alpha-bungarotoxin, over a wide range of pH values and temperatures, and under high salt conditions. The observations are interpreted partly in terms of the known crystal structure of this polypeptide. We support earlier findings of a greater degree of beta-sheet structure in solution than has been reported by X-ray crystallography and, importantly, the invariant residue associated with neurotoxicity, Trp29, is shown to be in a similar environment to that found in alpha-cobratoxin and LS III from Laticauda semifasciata. The implications of this observation for structure/function relationships are outlined.

The modification of alpha-bungarotoxin by digestion with trypsin

Two modified alpha-bungarotoxins (alpha-BuTX)2 were obtained by trypsin digestion. On mild tryptic digestion, alpha-BuTX could be cleaved at a single site to give the des-tetrapeptide alpha-BuTX, designated as des-(71-74)-alpha-BuTX. Further cleavage at Arg36-Gly37 yielded a double-chained product held together by five disulfides and designated double-chained des-(71-74)-alpha-BuTX. The des-(71-74)-alpha-BuTX had a similar CD spectrum and antigenicity to that of native alpha-BuTX, but had a lower toxicity and neuromuscular-blocking potency. This indicates that the C-terminal tail of alpha-BuTX had some effect on the toxin's biological activities. The double-chained des-(71-74)-alpha-BuTX also retained full antigenicity, but changed moderately in CD spectrum and decreased dramatically in toxicity and neuromuscular-blocking potency. These results suggested that the invariant region at Arg36-Gly37, previously thought to be essential for the neurotoxicity of alpha-BuTX due to its cationic group, might also play a critical role in maintaining the native conformation of the toxin.

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.

Individual assignments of the amide proton resonances involved in the triple-stranded antiparallel pleated beta-sheet structure of a long neurotoxin, Laticauda semifasciata III from Laticauda semifasciata

THe characteristic feature of the crystal structure of erabutoxin b, a short neurotoxin from Laticauda semifasciata, and alpha-cobratoxin, a long neurotoxin from Naja naja siamensis, is the presence of a triple-stranded antiparallel pleated beta-sheet structure formed by the central and the third peptide loops. In the present study, we have studied the assignment of slowly exchangeable amide protons of Laticauda semifasciata III from L. semifasciata, using nuclear Overhauser effects (NOE) and spin-decoupling methods. The results show that nearly all of the slowly exchangeable amide protons are to be assigned to the back-bone amide protons, involved in the triple-stranded antiparallel pleated beta-sheet structure, indicating that this sheet is stable in 2H2O solution. In contrast, the amide protons in short neurotoxins are readily exchangeable under the same experimental condition, suggesting that long neurotoxins have a more rigid sheet structure than short ones. This rigidity may come from the hydrophobic and hydrogen bond interaction between the central loop and the tail, which is not present in short neurotoxins. Since the functionally important residues are located on this beta-sheet, the different kinetic properties of the neurotoxins are well correlated with the difference in the rigidity of the beta-sheet.