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site 3 on the sodium channel. The first of these are the short anemone polypeptides [19,24] such as ATX
III [38] and PaTX [39], which are neurotoxic to crustacea. The polypeptide ATX III, which consists of
27 residues cross-linked by three disulfide bonds, and PaTX, which has 31 residues and four disulfides,
can be aligned such that 16 residues are identical. The three disulfides in ATX III are linked in a
1–5/2–4/3–6 pattern in the same way as in the long polypeptides [65], but the only other similarity at the
level of primary structure is a GCPXG sequence corresponding to residues 28–32 of AP-A. Although
neurotoxic to crustacea, ATX III is inactive as a positive inotrope [28], suggesting that it possesses an
appropriate structural scaffold to interact with site 3 but lacks key side chains required for interaction
with the cardiac channel. Nevertheless, the smaller size of ATX III makes it an attractive candidate for
further study, with the aim of engineering into it the ability to bind to the cardiac channel. The welldefined
solution structure for this toxin [65] provides and essential basis for such an effort.
The Anemonia sulcata polypeptides BDS I and II [40], which were claimed to have antihypertensive and
antiviral activity, also bind to site 3 on neuronal sodium channels and have weak negative inotropic
activity [41]. The points of similarity and difference between the solution structures of BDS I [40] and
the long anemone polypeptides have been discussed previously [40,41] and will not be reiterated here;
suffice to say that the overall folds are similar but the Arg14 loop in the long polypeptides is truncated in
BDS I and the molecule lacks several residues that have been shown to be important for activity.
B. Scorpion Toxins
The scorpion α-toxins have been shown to bind to site 3 on the voltage-gated sodium channel
[24,27,42]. These polypeptides contain up to 70 residues crosslinked by four disulfide bonds, but show
no sequence similarity to the anemone polypeptides. Possible structural similarities have been discussed
[24], and in a theoretical model of the anemone toxin Bg II, some of the cationic residues were in similar
locations to those in the crystal structure of the scorpion toxin Aah II [26].
It is clear that positively charged residues play an important role in the interactions of sea anemone
toxins and scorpion toxins with site 3 on the sodium channel (as indeed they do with other polypeptide
toxins binding to other ion channels) but this role may be relatively more important for the TTXsensitive
sodium channel in nerve and muscle than for the TTX-insensitive channel of the heart. For
example, Bg II, which is more positively charged than AP-A and AP-B (see above), has a higher affinity
for neuronal sodium channels [26], and replacement of Arg12 and Lys49 in AP-B with uncharged
residues favors its binding to the cardiac channel [53]. Similarly, the scorpion α-toxins bind more tightly
to the neuronal channel than the anemone toxins but, as with the Type 2
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