Conotoxins amino acid sequences

Table 5. Amino acid sequences of major classes of conotoxins... 

Table 3 Comparison of the amino acid sequences and known targets of a-conotoxins...

Like the other paralytic toxins from Conus venom, a-conotoxins are small and very tightly folded, structural features which may be advantageous for rapid paralysis of prey (1). a-Conotoxins are typically 13 to 15 amino acids long with two disulfide bridges (see Table III). In addition to the five a-conotoxins shown, two new a-conotoxins (SIA and SIB) from C. striatus have recently been isolated, sequenced, and chemically synthesized. SIA is very unusual because it is 19 amino acid residues long and it contains 6 cysteine residues, three of which are contiguous near the amino terminus (C. Ramilo et al., unpublished results). 

When air oxidation of the reduced p-conotoxin GIIIB (18) was carried out in 0.1 M NHtOAc buffer (pH 7.5) at 0.01 mM peptide concentration and at 10 °C, three major products, isomers 15,16, and 17 were produced after 40 hours in a ratio of 1 4 3 (Figure 2). 86 The disulfide structures of each isomer were determined by enzymatic digestion followed by amino acid analyses, mass spectrometry, sequence analyses, as well as by the synthetic approach (Scheme 10). 

Cone snails, Conus spp., have been investigated because of their production of conotoxin peptides. From an evolutionary standpoint, the production of conotoxins is quite interesting due to their wide range of neurophysiological activities. The conotoxins are small peptides, 10-30 amino acids, with conformations constrained by multiple disulfide bonds that target a number of receptors in vertebrate and invertebrate nervous systems. Cone snails use these toxins to immobilize prey, which allows the relatively slow-moving cone snails to feed on fish and worms. The wide variety of conotoxins isolated and the hypervariability within peptide sequences has led some to hypothesize a combinatorial biosynthetic approach for the production of conotoxins.116117... 

The majority of conotoxin structures (115 of 125 structures in the PDB) have been determined by NMR spectroscopy as opposed to X-ray crystallography. They have significant structural diversity as a result of their diverse sequences and disulphide connectivities. Furthermore, conotoxins also have a large number of post-translational modifications, including the presence of hydroxyproline, y-carboxyglutamic acid and bromotryptophan residues, epimerisation, glycosyla-tion and amidation,144,145 that enhance their structural diversity. A recent analysis of NMR data for the unusual amino acids present in conotoxins3 provides a reference source for chemical shifts of post-translationally modified amino acids. 

Another member of the M-superfamily, conomarphin also contains 15 residues but is devoid of disulphide bonds and contains a d-amino acid.169 Although the mature toxin has no sequence similarity to other M-superfamily peptides, its signal sequence suggests it is part of the M-superfamily. Despite the lack of disulphide bonds, the peptide still has a well-defined structure characterised by a loop region in the middle of the peptide and a 3io helix at the C-terminus. Mutation of the d-Phe residue and subsequent structural analysis suggests that this residue is important in stabilising its well-defined structure. Conomarphin represents a new family of conotoxins, the biological activity of which remains to be determined.169... 

Conotoxin frameworks VI and VII have a C-C-CC-CC arrangement of Cys residues with a sequential disulfide pairing, which is also found in spider toxins and in the venom of scorpions. These particular frameworks form stable cystine knots with variable loop lengths. The sequence of amino acids within the loops defines the targeting and pharmacology of a particular framework VI/VII conotoxin. 

Sequence homology within conotoxin subtypes appears not to be related to receptor selectivity. For example, the cj-conotoxins found in several piscivorous cone snail species all paralyze fish and target the presynaptic calcium channels, yet they have up to 70% difference in their sequences. The only amino acids that are conserved are the Cys residues that provide the basic framework of these particular conotoxins. These findings indicate that the non-Cys amino acids are crucial in the physiological role of conotoxins. The King-Kong peptide (named from the aggressive response that it causes in lobsters) isolated from Conus textik has retained the... 

With the exception of a-conotoxin SII from Conus striatus, all a-conotoxins have the cysteine pattern, CC—C—C (Myers et al., 1993) (see Table 4). Peptides from the fish-hunting species, C geographus, C. striatus and C. magus, have the consensus core sequence CC(N/H)PACGXX(Y/F)XC and two disulfide bonds that connect Cys to Cys and Cys to Cys. Comparison with the a-conotoxins recently isolated from C. pennaceus, a mollusc-hunter, and C. imperialism a worm-hunter, indicates variations in the size of the intercysteine loops. The second loop has seven amino acid residues in a-conotoxins PnIA and PnIB from C pennaceus (Fainzilber et al., 1994), and only three residues in a-conotoxin Iml from C. imperialis (McIntosh et al., 1994), compared to five residues in the other a-conotoxins. 

---