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Page 307<br />

(the exception is ATX Ib [19,24], which has Pro 3 and Thr21, but it is not known if this has the same<br />

local structure). Residues 3 and 21 are juxtaposed in the β sheet [46–48] and the effect of switching<br />

Ser3/Thr21 to Pro3/Ile21 can be assessed <strong>by</strong> comparing the structures of AP-A [46] and AP-B [47]. It<br />

appears that this leads to a distortion of the sheet between the second and third residues but causes no<br />

significant perturbations to the overall structure. In the calculated structures of AP-A a Va12 NH<br />

rarrow.gif Leu22 CO hydrogen bond was found but this was not present in the AP-B structures,<br />

presumably as a result of the local differences. Comparing Af I and Af II, the combination of the<br />

Ala3/Thr21 to Pro3/Ile21 switch plus the addition of an extra Gly at the N-terminus of Af II would be<br />

expected to alter the local conformation at the N-terminus. The fact that the cardiac stimulatory activities<br />

of the two are the same [58] therefore implies that the N-terminus is not important functionally. This<br />

inference is conditional on the effect on activity of the only other difference between Af I and Af II,<br />

Asn12 rarrow.gif Ser, being minimal. In the Type 1 toxins, Ser is found more often than Asn at position<br />

12, while in the Type 2 toxins, replacement of Asn12 <strong>by</strong> Tyr increases toxicity in mice <strong>by</strong> a factor of<br />

two [24,27]. Thus, it is possible that the presence of Ser in Af II slightly favors cardiac activity while the<br />

changes at the N-terminus might reduce it slightly; the main conclusion to be drawn, however, is that<br />

neither change has a significant effect. The lack of effect of changes at the N-terminus is consistent with<br />

the results from expression of AP-B [59], where a Gly-Arg extension at the N-terminus had no effect on<br />

activity.<br />

At seven locations AP-B differs from AP-A. Two of these, Ser3 rarrow.gif Pro and Thr21 rarrow.gif<br />

Ile, were discussed above. Two more, Leu24 rarrow.gif Phe and Thr42 rarrow.gif Asn, are<br />

conservative changes located in or at the start of loops that are not in the immediate vicinity of the<br />

sodium channel binding surface and would not be expected to have a direct effect on activity. This<br />

leaves Ser12 rarrow.gif Arg, Val13 rarrow.gif Pro, and Gln49 rarrow.gif Lys, which do have<br />

functional significance, as discussed in the following section.<br />

It is important to note that there are several residues that are common to all of the long toxins and serve<br />

to maintain the biologically active conformations of these molecules. The clearest examples of residues<br />

in this category are the six half-cystines, although we believe that the Gly10-Pro11 sequence may<br />

influence the structure and flexibility of the Arg14-containing loop [46] (at the other end of this loop<br />

Thr17 and/or Ser19-Gly20 may also be important). Also conserved, Trp33 is probably important<br />

structurally even though its surroundings are different in the Type 1 and Type 2 toxins.<br />

It is also interesting that in ATX Ia, which is a potent crustacean neurotoxin but a poor mammalian<br />

cardiac stimulant, Lys37 and both histidines are missing, suggesting that one or more of these side<br />

chains may be important in promoting specificity for the mammalian cardiac sodium channel.<br />

http://legacy.netlibrary.com/nlreader/nlReader.dll?bookid=12640&filename=Page_307.html [4/5/2004 5:23:45 PM]

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