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Document<br />

D. Site-Directed Mutagenesis<br />

Page 308<br />

The complete synthesis of AP-A has been reported [60] but so far this approach has not been pursued to<br />

generate analogues. More productive has been the analysis of analogues produced <strong>by</strong> site-directed<br />

mutagenesis, following the successful cloning and expression of AP-B [59]. A series of single-site<br />

mutations [61,62] showed that R14A, K48A, and K49A had affinities for the cardiac sodium channel<br />

that were, respectively, only 3.2-, 2.9-, and 2.4-fold lower than that of native AP-B, while R12A had an<br />

8.5-fold lower affinity. These results suggested that amongst the cationic side chains only Arg12 was<br />

significant for activity. Recent data on double mutations [53] indicates that the situation is not quite that<br />

simple. For example, the R12S-R14Q double mutant had a 72-fold lower affinity for the neuronal<br />

sodium channel whereas the R12S and R14Q mutants individually had 5.9- and 2.4-fold lower affinities,<br />

respectively, which should combine to produce only a 14-fold effect. It appears that the presence of one<br />

cationic side chain in the Arg14 loop may be sufficient for activity and that its exact location can vary<br />

somewhat. It would be interesting to know if the same applies to the C-terminus, where Lys48 and<br />

Lys49 may be able to compensate for one another. However, the proposal that the cationic side chains of<br />

residues 12, 14, and 49 form a cluster [53] is not consistent with the solution structure of AP-B [47].<br />

A similar situation appears to exist in the ω-conotoxins, which possess 5–7 net positive charges.<br />

Substitution of individual cationic groups had a relatively minor effect on affinity for the voltage-gated<br />

calcium channel, whereas replacement of several had a significant effect, greater than that expected from<br />

the sum of the individual effects [63,64].<br />

A further outcome of analyses of the double mutants was that the cationic side chains at positions 12 and<br />

49 in AP-B seem to favor binding to the neuronal over the cardiac sodium channel. Thus, the R12S-<br />

R49Q double mutant, in which residues 12 and 49 in AP-A, had a 37-fold lower affinity for the neuronal<br />

channel but only a 5-fold lower affinity for the cardiac channel relative to native AP-B [53]. In fact, the<br />

affinity of this double mutant for the cardiac channel was lower than that of AP-A, implying that one or<br />

more of the other five differences between AP-A and AP-B might decrease affinity for the cardiac<br />

channel. It seems, therefore, that while AP-A is slightly less potent than AP-B on cardiac channels, it<br />

has greater selectivity for the cardiac channel when measured in sodium flux experiments. In voltageclamp<br />

experiments AP-A and AP-B favor the cardiac channel to similar degrees [53].<br />

V. Other Ligands for Site 3 on the Sodium Channel<br />

A. Sea Anemone Toxins<br />

Apart from the “long” sea anemone polypeptides that are the main focus of our interest, there are two<br />

other classes of anemone polypeptides that bind at or near<br />

http://legacy.netlibrary.com/nlreader/nlReader.dll?bookid=12640&filename=Page_308.html [4/5/2004 5:23:47 PM]

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