Lethal protein conformations

Plant natural products active against snake bite--the molecular approach

The article surveys the substances identified in plants reputed to neutralize the effects of snake venoms. Protective activity of many of them against the lethal action of the venom of the jararaca (Bothrops jararaca) snake was confirmed by biological assays. It was shown that all belong to chemical classes capable of interacting with macromolecular targets--receptors and enzymes. In a few cases it has been shown that exogenous natural micromolecules can mimic the biological activity of endogenous macromolecules. From the evidence presented, it can be inferred that micromolecules which neutralize the action of snake venoms mechanistically replace endogenous antitoxic serum proteins with venom neutralizing capacity such as produced by some animals.

Snake venom cardiotoxins-structure, dynamics, function and folding

Snake cardiotoxins are highly basic (pI > 10) small molecular weight (approximately 6.5 kDa), all beta-sheet proteins. They exhibit a broad spectrum of interesting biological activities. The secondary structural elements in these toxins include antiparallel double and triple stranded beta-sheets. The three dimensional structures of these toxins reveal an unique asymmetric distribution of the hydrophobic and hydrophilic amino acids. The 3D structures of closely related snake venom toxins such as neurotoxins and cardiotoxin-like basic proteins (CLBP) fail to show similar pattern(s) in the distribution of polar and nonpolar residues. Recently, many novel biological activities have been reported for cardiotoxins. However, to-date, there is no clear structure-function correlation(s) available for snake venom cardiotoxins. The aim of this comprehensive review is to summarize and critically evaluate the progress in research on the structure, dynamics, function and folding aspects of snake venom cardiotoxins.

Structure and chemical modifications of neurotoxin from Naja nigricollis studied by Raman spectroscopy

Raman spectroscopy was used to determine structural features of the native toxin alpha from Naja nigricollis, which contains only one Trp and one Tyr, and of chemically modified toxins having chromophores added to these two conserved aromatic amino acids. The percentages of secondary structure were determined by using amide I polypeptidic vibration analysis and are in agreement with X-ray structure [Low et al. (1976) Proc. Natl. Acad Sci. U.S.A. 73, 2991-2994] as well as with the geometry of the disulfide bridges estimated by using the v(S-S) vibrations. In the native toxin alpha, the single invariant tyrosine 25 appears to be buried in the structure and involved in a strong hydrogen bond. We have chemically modified these two invariant aromatic side chains by addition of chromophores. The presence of a (nitrophenyl)sulfenyl (NPS) chromophore bound to the Trp does not perturb the secondary structure of the toxin as shown by the analysis of the polypeptidic amide I vibrations; however, the environment of this Trp and the geometry of a disulfide bridge seem to be modified. The secondary structure is not affected by the presence of the NPS chromophore; therefore, the decrease in binding affinity observed after modification of Trp-29 by the reagent NPS-Cl [Faure et al. (1983) Biochemistry 22, 2068-2076] is due to an alteration of the environment of this aromatic amino acid and/or a steric hindrance and not to an overall modification of the toxin structure. The binding assays of [nitrotyrosyl]toxin show that after nitration the affinity toward the monoclonal antibody M alpha 1 is unchanged and that the affinity toward the cholinergic receptor (AcChR) from Torpedo marmorata remains high. We concluded that the structure of toxin alpha after adding the NO2 chromophore to Tyr-25 is the same as it is in native toxin.

Solution conformation of conotoxin GI determined by 1H nuclear magnetic resonance spectroscopy and distance geometry calculations

Conformational analysis of conotoxin GI, one of the neurotoxic peptides produced by a marine snail, genus Conus, was performed by a combination of nuclear magnetic resonance spectroscopy (NMR) and distance geometry calculations. The resulting conformers on minimization of the target function were classified into two groups. The difference in the structures of the conformers is mainly due to the difference in the orientation of the side chain of the tyrosyl residue. The results show that the solution structure of conotoxin GI satisfies the conformational requirements for the biological activity of an antagonist toward nicotinic cholinergic receptors elucidated in a series of studies on alkaloids. The structure is discussed on the basis of the results of comparison of the atomic arrangements of the active sites of snake venom peptides and molecular models based on the results of secondary structure prediction.

A proposal for the structure of conotoxin--a potent antagonist of the nicotinic acetylcholine receptor

Conotoxin, a thirteen residue neurotoxic peptide, is demonstrated, by circular dichroism measurements, to possess a high content of alpha-helix. The location of this helix in the sequence is severely limited by the structural preferences of individual amino acids and by the positioning of the two cystine links. Comparisons are made between the reactive site of elapidae snake venom postsynaptic neurotoxins and the surface of the conotoxin molecule.

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.

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.