Cystine toxins

Comparisons between these toxins allow delineation of the variability of each position in the sequence. For instance, the residues which are extremely invariant (conservative) for both types of sea anemone toxin are the half-cystines, certain glycyl residues which are expected to be involved in )9-turns, and only a few other residues - Asp 5 or 6, Arg 13 or 14, and Tryp 30 or 31 (the numbering depends upon the toxin type) — expected to be important for folding or receptor binding. Rather surprising is the variation in the residues which NMR studies (22,23) have shown are involved in formation of the four stranded )9-pleated sheet. 

Schweitz et al. purified four related toxins (Rpl, RpII, RpIII, and RpIV) from sea anemone Radianthus paumotensis (Rp) and studied their pharmacological properties (9). During the course of initial NMR studies, the reported sequence of RpII was found to have errors, and was redetermined (8). Subsequently Metrione et al. determined the sequence of RpIII as well (10). T e other two Rp toxin sequences are yet to be determined. Sequences of the Rp, and several other sea anemone toxins, are shown in Table I. We have used a two letter code to denote the species consistently and this notation differs from the earlier designations of Norton and Wiithrich groups. In our notation. As la and Ax I correspond to ATX la and AP-A, respectively. From alignment of the cystines in these sequences, it is clear that Rp toxins have three disulfide bonds, as do the other toxins. 

The Rp toxins also have an additional residue at the N-terminus which extends the sheet one residue in that direction. This may be an indication that the structural difference arises from the substitution of F17 for L17 in RpII, because there is also leucine at position 17 in the A. xanthogrammica toxins. The structure of the core sheet is remarkably similar among the different toxins, considering that there is essentially no conservation of the residues involved in forming it, aside from the cystines. 

There are four disulfide bonds in short-chain (Type I) neurotoxins. This means that there are eight half-cystines. However, all Hydrophiinae toxins have nine halfcystines with one cysteine residue. An extra cysteine residue can be readily detected from the Raman spectrum as the sulfhydryl group shows a distinct S-H stretching vibration at 2578 cm" Some Laticaudinae toxins do not have a free cysteine residue as in the cases of L. laticaudata and L. semifasciata toxins. In long toxins (Type II) there are five disulfide bonds (Table III). 

BgK K+ channel-acting toxin from sea anemone Bunodosoma granulifera C (Cys) cysteine (reduced form) or half-cystine residue (oxidized form)... 

Many scorpion toxins, insect defensins, and enzyme inhibitors are cystine-rich polypeptides containing three to four disulfide bonds. In a large number of these toxins, two cystines are involved in the consensus Cys-(Xaa)1-Cys/Cys-(Xaa)3-Cys framework which is responsible for the common characteristic fold consisting of an a-helix and a two- or three-stranded antiparallel (3-sheet (a 3 3-fold or 3a 3 3-fold). For a review see ref[69]. The overall compact globular structures of these cystine-rich peptides contain the cystine stabilized a-helix motif (Section 6.1.5.1.2) which is further stabilized by a third disulfide bond between the N-terminus and the (3-strand adjacent to the helix and in some cases by an additional fourth disulfide bridge. Due to the presence of the cystine stabilized a-helix motif, a preferred initial formation of this motif followed by its stabilization via the additional disulfides was expected. However, in contrast to what was observed for the cystine peptides containing only the cystine stabilized a-helix motif, simple air oxidation is not successful. 

Cyanobacteria blooms can pose an extremely serious threat to human health (970-972), and some of the causative toxins contain halogen. The fresh water toxic cyanobacterium Oscillatoria agardhii produces oscillaginin A (916), which features the novel 3-amino-10-chloro-2-hydroxydecanoic acid, and is the source of the micro-cystins, which are heptatoxins (973). The prolific cyanobacterium Lyngbya majuscula from Curacao has furnished the novel barbamide (917) (974) and dechlorobarbamide (918) (975). Extensive biosynthetic studies show that the amino acids leucine, cysteine, and phenylalanine are involved in barbamide production (976-982). The chlorination of leucine is of great interest and may involve a radical mechanism (976, 980-983). 

Figure 17. Schematic diagrams of some representative topologically chiral proteins.79 (a) Condensed schematic drawing of the L subunit of the quinoprotein TV-MADH. The looped line represents the polypeptide backbone with N and C terminals. Cysteine (or half-cystine) residues are numbered, and their a-carbons are indicated by filled circles. Intrachain disulfide bonds are shown as dashed lines joining a pair of filled circles. The heavy line symbolizes an intrachain cofactor link, (b) Chromatium high potential iron protein (HiPIP), one of several Fe4S4 cluster-containing proteins, (c) Toxin II from the scorpion Androctonus australis Hector. Reprinted with permission from C. Liang and K. Mislow, J. Math. Chem. 1994,15,245. Copyright 1994, Baltzer Science Publishers.

Marine sponges contain a host of bioactive compounds, particularly small molecules, and also contain a range of peptides that are non-ribosomally synthesised, often containing non-native amino acids. However, there are examples of peptides of ribosomal origin, including, for example, asteropine A isolated from the sponge Asteropus simplex.133 This peptide comprises 36 residues and three disulphide bonds. It has potent sialidase inhibitory activity and thus has applications in the design of novel viral inhibitors. Structural analysis of asteropine A with NMR spectroscopy revealed a cystine-knot motif, similar to that already described for plant toxins. This observation emphasises the fact that the cystine-knot motif is extremely prevalent in disulphide-rich peptides.134 Asteropine A, discovered in 2006, was the first reported cystine-knot peptide isolated from marine invertebrates other than from cone snails, which are described in more detail below. 

Of the 15 spices marketed in India and screened by Saxena and Mehrotra (1989) for the mycotoxins, aflatoxin, rubratoxin, ochra-toxin A, citrinin, zearalenone and sterigmato-cystin, samples of coriander and fennel were found to contain the largest number of positive samples and mycotoxins. Other spices like cinnamon, clove, yellow mustard and Indian mustard did not contain detectable amounts of the mycotoxins tested. Aflatoxins are the most common contaminants in the majority of samples, levels being higher than the prescribed limit for human consumption. 

Three families of toxins mainly target the liver the micro-cystins, which include more than 80 variants, the nodular-ins, and the eylindrospermopsins. 

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. 

Dahlmann, J., Budakowski, W. R., and Luckas, B. (2003). Liquid chromatography-electrospray ionisation-mass spectrometry based method for the simultaneous determination of algal and cyanobacterial toxins in phytoplankton from marine waters and lakes followed by tentative structural elucidation of micro-cystins. J. Chromatogr. A 994,45-57. 

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