Snake venom toxins The amino acid sequences of three cytotoxin homologues from Naja mossambica mossambica venom

Predicted secondary structure of snake venom toxins from their primary structures

We have predicted the secondary structure of 38 snake venom toxins using the method of Chou and Fasman. Our predictions indicate that beta-chain and random coil structures predominate in these proteins. The conformations of long neurotoxins, short neurotoxins and cytotoxins are less similar than previously believed. Cytotoxins contain 40--50% of beta-structure and they form a notably homogeneous group. Short neurotoxins contain less beta-structure (13--30%) and more random coil than cytotoxins, and they also form a more heterogeneous group in terms of secondary structure. The characteristics of long neurotoxins are intermediate to the above mentioned groups. Experimental evidence supporting these propositions is discussed.

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.

Determination of the nuclear-magnetic-resonance solution structure of cardiotoxin CTX IIb from Naja mossambica mossambica

The NMR structure of cardiotoxin CTX IIb from Naja mossambica mossambica in aqueous solution was determined from a total of 593 nuclear Overhauser enhancement distance constraints and 135 dihedral angle constraints, which were collected using two-dimensional homonuclear 1H-NMR experiments. Structure calculations were performed with the program DIANA, using the redundant dihedral angle constraints strategy for improved convergence, followed by restrained energy minimization with the program FANTOM and a modified version of the program AMBER. The CTX IIb structure is represented by a group of 20 conformers with an average root-mean-square deviation relative to the mean solution structure of 0.072 nm for the backbone atoms, and 0.116 nm for all heavy atoms. The molecular structure of CTX IIb is characterized by a three-stranded beta-sheet made up of residues 20-26, 32-39 and 48-54, and a two-stranded beta-sheet composed of residues 1-5 and 10-14. A cluster of four disulfide bonds, 3-21, 14-38, 42-53 and 54-59, form the core of the molecule and crosslink the individual polypeptide strands. The NMR structure is similar to the previously reported X-ray crystal structure of the cardiotoxin CTX VII4 from the same species. Differences between the two structures were noted in the tips of the two loops formed by residues 6-9 and 27-31, which connect the beta-strand 1-5 with 10-14, and 20-26 with 32-39, respectively. For these loops the NMR data also indicate significantly increased dynamic disorder in the solution structure. These observations are discussed with respect to earlier suggestions by others that these two loops are essential structural elements for function and specificity of a wide variety of homologous toxins.

The ocular effects of spitting cobras: II. Evidence that cardiotoxins are responsible for the corneal opacification syndrome

Fractionation of H. haemachatus, N. nigricollis, N. nivea and N. melanoleuca venoms using Amberlite CG-50 and (NH4)HCO3 elution gradient chromatography yielded 11-13 fractions for each venom. One fraction, F X, from H. haemachatus, two fractions, F X and F XI, from N. nigricollis and one fraction, F VIII, from N. melanoleuca venoms possessed the whole of ocular activity of the venoms. The fractions were the only venom fractions that caused cardiac depressant activity; their effect was reversed by raising Ca++ concentration in the physiological solution; they did not influence the twitches of the phrenic nerve hemidiaphragm and guinea-pig ileum preparations. Further purification of the fractions on Sephadex G-50 followed by fractionation on Amberlite CG-50 yielded fractions free from phospholipase A2 activity but possessing the same ocular effects. Similarly, the cardiotoxin from commercial N. nigricollis venom caused the same ocular effects as the crude venom and its purified cardiotoxic fractions. All cardiotoxic fractions as well as N. nigricollis cardiotoxin, caused extensive chemosis, blepharitis and corneal opacification with corneal and subconjunctival neovascularization. On a weight basis, the cardiotoxins were weaker in their oculotoxic activity than the corresponding parent crude venoms possibly because of the potentiating effect of phospholipase A2 in the crude venoms. It is postulated that in spitting cobras the cardiotoxins are responsible for the corneal opacification syndrome. In other cobra venoms the stable binding of cardiotoxins with acidic proteins limits their possible ocular effects. Only in the venoms of the spitting species are the cardiotoxins present in an appropriately free form to cause the ocular opacification syndrome.

Delineation of the functional site of a snake venom cardiotoxin: preparation, structure, and function of monoacetylated derivatives

Toxin gamma, a cardiotoxin from the venom of the cobra Naja nigricollis, was modified with acetic anhydride, and the derivatives were separated by cation-exchange and reverse-phase chromatography. Nine monoacetylated derivatives were obtained, and those modified at positions 1, 2, 12, 23, and 35 were readily identified by automated sequencing. The overall structure of toxin gamma, composed of three adjacent loops (I, II, and III) rich in beta-sheet, was not affected by monoacetylation as revealed by circular dichroic analysis. Trp-11, Tyr-22, and Tyr-51 fluorescence intensities were not affected by modifications at Lys-12 and Lys-35, whereas Trp-11 fluorescence intensity slightly increased when Lys-1 and Lys-23 were modified. The cytotoxic activity of toxin gamma to FL cells in culture was unchanged after modification at positions 1 and 2, whereas it was 3-fold lower after modification at Lys-23 and Lys-35. The derivative modified at Lys-12 was 10-fold less active than native toxin. Using two isotoxins, we found that substitutions at positions 28, 30, 31, and 57 did not change the cytotoxic potency of toxin gamma. A good correlation between cytotoxicity, lethality, and, to some extent, depolarizing activity on cultured skeletal muscle cells was found. In particular, the derivative modified at Lys-12 always had the lowest potency. Our data show that the site responsible for cytotoxicity, lethality, and depolarizing activity is not diffuse but is well localized on loop I and perhaps at the base of loop II. This site is topographically different from the AcChoR binding site of the structurally similar snake neurotoxins.

1H nuclear-magnetic-resonance studies of the three-dimensional structure of the cardiotoxin CTXIIb from Naja mossambica mossambica in aqueous solution and comparison with the crystal structures of homologous toxins

Using the previously reported sequence-specific 1H-NMR assignments, structural constraints for the cardiotoxin CTXIIb from Naja mossambica mossambica were collected. These include distance constraints from nuclear Overhauser enhancement measurements both in the laboratory and in the rotating frame, dihedral angle constraints derived from spin-spin coupling constants, and constraints from hydrogen bonds and disulfide bridges. Structure calculations with the distance geometry program DISMAN confirmed the presence of the previously identified antiparallel beta-sheets formed by residues 1-5 and 10-14, and by 20-27, 35-39 and 49-55, and established the nature of the connections between the individual beta-strands. These include a right-handed crossover between the two peripheral strands in the triple-stranded beta-sheet, and a type I tight turn immediately preceding the beta-strand 49-55. The spatial arrangement of the polypeptide backbone in the solution structure of CTXIIb is closely similar to that in the crystal structure of the homologous cardiotoxin VII4 from the same species. In an Appendix the origin of the large pH dependence of two amide proton chemical shifts in CTXIIb is explained.

Snake venom toxins--II. The primary structures of cytotoxin homologues S3C2 and S4C8 from Aspidelaps scutatus (shield or shield-nose snake) venom

1. Cytotoxin homologues S3C2 and S4C8 from Aspidelaps scutatus were purified by gel filtration and ion exchange chromatography. 2. They consist of 63 amino acids including eight half-cystines. The toxicities of S3C2 and S4C8 were determined and LD50 values of 6.6 and 9.4 micrograms/g mouse were, respectively, found. 3. The complete primary structures of toxins S3C2 and S4C8 have been determined. The two toxins resemble the cytotoxin type toxins and in the cytotoxin homologues the ten structurally invariant amino acids of the neurotoxins and the cytotoxins are conserved.

Role of tyrosine and tryptophan residues in the structure-activity relationships of a cardiotoxin from Naja nigricollis venom

This paper is an attempt to localize the critical area determining toxicity in a snake cardiotoxin. Toxin gamma is a single-chain polypeptide of 60 amino acids, which has been isolated from the venom of the African spitting cobra, Naja nigricollis. Three aromatic residues, namely, Trp-11, Tyr-22, and Tyr-51, have been individually modified by chemical means. The structure of the native toxin and of each derivative has been carefully investigated by circular dichroism, fluorescence, proton magnetic resonance spectroscopy, and two specific monoclonal antibodies. None of the chemical modifications alters the overall structure of the toxin, which in all cases remains folded into three adjacent loops (I, II, and III) rich in beta-pleated sheet emerging from a small globular region containing four disulfide bridges. A number of subtle changes, however, have been detected in the structure of each derivative compared with that of the native toxin. In particular, nitration of Tyr-51 provoked a structural perturbation in the globular region. Nitration of Tyr-22 induces a more substantial change in the beta-sheet area of the molecule. Thus, the strong inter-ring NOE that is observed in the native toxin between Tyr-22 and Tyr-51 vanishes in the Tyr-22 derivative, and significant changes are observed in the globular region. In contrast, no alteration of the beta-sheet structure of loops II and III has been detected after modification of Trp-11. All changes observed for this derivative remain located in the vicinity of the indole side chain of Trp-11 in loop I. The biological consequences of the modifications were measured: the lethal potency in vivo in mice and the cytotoxic activities in vitro on FL-cells. Lethal activities correlate with cytotoxicity: Tyr-51 modified toxin is equally potent as native toxin, whereas Tyr-22 and Trp-11 derivatized toxins are characterized by substantially lesser activities, the Trp-11 derivatized toxin being the least potent. We conclude that (1) Tyr-51 is not involved in the functional site of the toxin, although it is in interaction with the core of the molecule, (2) Tyr-22 may play a dual structural and functional role, and (3) Trp-11 is in, or in close proximity to, the functional site of the toxin. These data indicate the importance of loop I in determining toxicity of the cardiotoxin.

Monitoring the purification by high-performance liquid chromatography of cardiotoxins from Naja mossambica mossambica using phase-sensitive two-dimensional nuclear magnetic resonance

High-resolution phase-sensitive two-dimensional proton nuclear magnetic resonance was used to monitor the preparation by high-performance liquid chromatography of homogeneous proteins from the venom of Naja mossambica mossambica. This resulted in the characterization of a heterogeneous protein preparation VII2, which had been used in earlier structural studies by NMR, as well as a homogeneous protein CTXIIb and a nearly homogeneous protein fraction CTXIIa, which are now both subject to further investigations of their solution conformations.

Characterization of elapidae snake venom components using optimized reverse-phase high-performance liquid chromatographic conditions and screening assays for alpha-neurotoxin and phospholipase A2 activities

The vast majority of Elapidae snake venoms, genus Naja, includes three classes of toxic polypeptides: alpha-neurotoxins, phospholipases A2, and cardiotoxins. A new experimental approach using reverse-phase high-performance liquid chromatography in particular has been developed, allowing their respective resolution, identification, and quantitation from milligram quantities of venom. First, definition of optimal chromatographic conditions for Naja mossambica mossambica toxins has been ascertained. Different column packing and solvent systems were compared for their efficiency, with particular attention to the ionic strength of the aqueous solvent. A medium-chain alkyl support (octyl) in conjunction with a volatile ammonium formate (0.15 M, pH 2.70)/acetonitrile solvent system was found to be particularly effective. All the components known until now from this venom could be resolved in a single step, and the elution order was alpha-neurotoxins, phospholipases A2, and cardiotoxins with a total recovery of absorbance and toxicity. Then, with these suitable conditions, we describe a new major cardiotoxin molecule in this venom by hydrophobic and not ionic-charge discrimination. Second, specific assays were designed to detect alpha-neurotoxin and phospholipase A2 activities in chromatographic fractions: alpha-neurotoxin activity was determined by competition for the binding of a radiolabeled alpha-neurotoxin to the acetylcholine receptor of the ray electric organ, and phospholipase A2 activity was defined by the enzymatic activity of these toxins with a fluorescent phospholipid as substrate. Finally, the applicability of these new methods to study other Naja snake venoms was demonstrated.

Structure-function relationships for cardiotoxins interacting with phospholipids

Four cardiotoxins (CTX I-IV) from Naja mossambica mossambica were compared for their ability to interact with phospholipid vesicles and their capacity to bind erythrocytes. It is concluded that the affinity of the toxins always increases in the order: I approximately equal to II less than III less than IV. The binding is specific for charged lipids even in lipid mixtures. Proteolytic attack of the free and lipid-bound cardiotoxin indicates that at least the first loop Leu1-Thr13 is at the lipid contact. Tryptic and synthetic peptides constitutive of this loop are shown to interact with lipids. Arg5 residue increases the affinity toward the bilayer. The Raman spectra of lipid-bound cardiotoxin indicate a secondary and tertiary structure mainly similar to that of the free toxin. On charged lipids cardiotoxins induce a decrease of the enthalpy and an increase of disorder without change in the transition temperature; at saturating amounts of toxin the transition is abolished. In binary mixtures of phosphatidylcholine and charged lipids the observed effects can be accounted by a phase separation induced by the toxin.

Conformation of snake toxic polypeptides studied by a method of prediction and circular dichroism

Short and long neurotoxins as well as cardiotoxins belong to three distinct families of homologous toxic polypeptides extracted from cobra venoms. A study of their conformation was undertaken by using the method of Chou and Fasman for prediction of secondary structures of proteins. To improve the reliability of this method, an averaging scheme was developed. The data obtained showed that all toxins have a predominant trend for beta-sheet nucleation. Moreover, predicted beta-sheet strands fitted well those actually observed from X-ray data. Thus, it seems that all toxins share similarities in their secondary structure. This proposition was supported by a comparative study of the CD spectra of a set of toxins. Nevertheless, the present data suggest also that each type of toxins possesses localized structural individualities which might be responsible for the biological and/or immunological specificities.

Naja haje (Egyptian cobra) venom. Purification, some properties and the amino acid sequences of four toxins (CM-7, CM-8, CM-9, and CM-10b)

Four toxins (CM-7, CM-8, CM-9 and CM-10b) were purified from Naja haje haje (Egyptian cobra) venom by gel filtration on Sephadex G-50 followed by ion-exchange chromatography on CM-cellulose. They each contain 60 amino acid residues and are cross-linked by four intrachain disulphide bridges. The complete primary structure of the four toxins have been elucidated. The toxicities, the immunochemical properties, the sequences and invariant amino acid residues opf toxins CM-7, CM-8, CM-9 and CM-10b resemble the corresponding properties of the cytotoxin group.

Molecular evolution of snake venom toxins

Phylogenetic trees were constructed for 62 venom toxins of snakes of Proteroglyphae suborder using matrix method. The resulting tree from Minimum Spanning Tree-Cluster Analysis technique had the lowest "percent deviation" (8.55). The taxonomic relationship of these toxins agrees very well with zoological opinions. However, the appearance of the tree did not directly provide a plausible evolutionary model for the toxins. A model was derived from nodal ancestral sequence calculations, comparisons between intra- and intergenerical rates of amino acid change, and generally held ideas about protein evolution. According to the model, short neurotoxin is the ancient form of snake venom toxins. The courses of evolution leading to the present intraspecific homologous toxins are explained by gene duplication and allelomorphism.

Naja mossambica mossambica venom. Purification, some properties and the amino acid sequences of three phospholipases A (CM-I, CM-II and CM-III)

Three phospholipases A, CM-I, CM-II and CM-III, were purified from Naja mossambica mossambica venom by gel filtration on Sephadex G-50 followed by ion-exchange chromatography on CM-cellulose. They comprise each 118 amino acid residues and are close-linked by seven intrachain disulphide bridges. The complete primary structure of the three phospholipases A have been elucidated. The sequences and the invariant amino acid residues of CM-I, CM-II and CM-III resemble those of phospholipases A from other snake venoms and also from porcine pancreas. However, the letality (LD50 values) of the three phospholipases A from Naja mossambica mossambica venom, differ among themselves, and are also much higher than the LD100 value encountered for notexin from Notechis scutatus scutatus venom.

Snake venom toxins. The amino-acid sequences of three toxins (9B, 11 and 12A) from Hemachatus haemachatus (Ringhals) venom

Three toxins (9B, 11 and 12A) were purified from the venom of Hemachtus haemachatus as described previously. Whereas toxin 11 and 12A comprise 61 amino acid residues, toxin 9B contains 63 residues. All three toxins are cross-linked by four intrachain disulphide bridges. The complete amino acid sequences of these toxins have elucidated. The properties of the toxins were compared with those of the cytotoxin group. The toxicities, the sequences and some of the invariant residues of toxin 11 and 12A resemble the corresponding properties of the cytotoxin group. However their immunochemical properties indicate that they are distinct from both the cytotoxin and neurotoxin groups. The sequence of toxin 9B shows that it is related to the cytotoxins, but its toxicity is much lower than those encountered among members of this group.

Primary structures of cardiotoxin analogues II and IV from the venom of Naja jaja atra

Cardiotoxin analogues II and IV were isolated from the venom of Naja naja atra by gel filtration on Sephadex G-50 followed by CM-cellulose chromatography. The venom contains at least four cardiotoxin analogues that account for about 54% of the weight of the lyophilized crude venom. These four cardiotoxin analogues, named cardiotoxin analogues I, II, III, and IV, show strong cytotoxicity to Yoshida sarcoma cells but the lethal toxicity is one-order less. These toxins contain 60 amino acid residues in a single peptide chain. Cardiotoxin analogue IV differs from cardiotoxin analogue II only by the presence of arginine in place of a leucine residue at position 1. A comparison of the amino acid sequences of these toxins with that of cobrotoxin, a neurotoxin containing 62 amino acid residues obtained from the same snake venom, shows that about 20 amino acid residues, including 8 half cystine residues, are identical, assuming 3 residues deletion and 2 residues insertion in the cardiotoxin molecule.

The amino acid sequence of three non-curarimimetictoxins from Naja nivea venom

Three toxins of the non-curarimimetic type have been isolated from the venom of the Cape cobra Naja nivea. The basic and hydrophobic amino acids are dominant in all three toxins. They comprise 60 amino acid residues with 4 intrachain disulphide linkages. The toxins have been characterized with respect to their linear structures and immunochemical properties. Toxicity and hemolytic data suggest a much higher affinity for receptors on the heart cell membrane than for that of the red cell.

Snake venom toxins. The amino-acid sequences of three toxins (CM-8, CM-11 and CM-13a) from Naja haje annulifera (Egyptian cobra) venom

Three toxins (CM-8, CM-11, and CM-13a) were purified from the venom of Naja haje annulifera by gel filtration on Sephadex G-50 and by ion-exchange chromatography on CM-cellulose. Whereas toxin CM-8 and CM-11 comprise 60 amino acid residues, toxin CM-13a contains 61 residues. All three toxins are cross-linked by four intrachain disulphide bridges. The complete amino acid sequences of these toxins have been elucidated. The reduced and S-carboxymethylated toxins were digested with trypsin and chymotrypsin and the peptides purified by ion-exchange chromatography, gel filtration and chromatography or electrophoresis on paper. The Edman procedure, either through the use of the automatic sequencer or by manual manipulation, was employed to obtain the sequence of the intact toxins and the pure peptides. The chymotryptic digests provided the necessary overlapping peptides which allowed the alignment of the tryptic peptides. The properties of the three toxins were compared with those of the cytotoxin group. The toxicities the serological properties, the sequences and the invariant amino acid residues of toxin CM-8 and CM-11 resemble the corresponding properties of the cytotoxin group. The sequence and serological properties of toxin CM-13a show that it is related to the cytotoxin group, but its toxicity is much lower than those encountered in the cytotoxin group.

Studies of an acidic cardiotoxin isolated from the venom of Mojave rattlesnake (Crotalus scutulatus)

A major lethal protein was isolated from the venom of Mojave rattlesnake (Crotalus scutulatus) by successive purification in DEAE column chromatography and isoelectric focusing. This homogeneous and monomeric form of toxin is designated as "Mojave toxin". Unlike basic neurotoxins or cytotoxins isolated from venoms of cobras, kraits and sea snakes, the Mojave toxin is an acidic protein with an isoelectric point of 4.7. The toxin is also different from crotoxin (from Crotalus durissus terrificus) which consists of both acidic and basic components. The molecular weight determined by Sephadex G-75 column chromatography resulted in a value of about 22 000. A singel protein band with a molecular weight of about 12 000, was observed after sodium dodecyl sulfate gel electrophoresis of the reduced Mojave toxin. Isoelectric focusing gel in the presence of 8 M urea also showed a single protein band, suggesting that the toxin is composed of subunits. Unlike the neurotoxic nature of the basic proteins from the venoms of Elapidae and sea snakes (Hydrophiidae) and crotoxin, Mojave toxin is cardiotoxic rather than neurotoxic. It is very likely that venoms of all rattlesnakes from North and Central America contain Mojave toxin as the common toxin.