10.12.2012 Views

netLibrary - eBook Summary Structure-based Drug Design by ...

netLibrary - eBook Summary Structure-based Drug Design by ...

netLibrary - eBook Summary Structure-based Drug Design by ...

SHOW MORE
SHOW LESS

You also want an ePaper? Increase the reach of your titles

YUMPU automatically turns print PDFs into web optimized ePapers that Google loves.

Document<br />

A. Chemical Modification<br />

Page 305<br />

A number of chemical modification studies have been carried out on the sea anemone toxins. As<br />

discussed previously [24], the results of these studies have to be interpreted with some caution because<br />

of less than rigorous characterization of the reaction products in many cases. Nevertheless, in the Type 1<br />

toxins it appears that one or both of the Asp7 and Asp9 carboxylates are required for activity, as well as<br />

one or both of the Lys37 and Lys48 ε-ammonium groups [24,27,54]. The C-terminal carboxylate<br />

appears not to be essential, whereas the N-terminal ammonium appears to have some role, although the<br />

various studies give a confusing view of its importance. There are also conflicting data on the<br />

importance of His34 and His39 but it seems that at least one of them might be important. Both are<br />

located in the vicinity of other residues found to be necessary for activity, but His39 is in close contact<br />

with Asp7 and Lys37 and on this basis may be expected to be the more important.<br />

Although the available evidence points to a role for one or both of the Asp7 and Asp9 carboxylates in<br />

cardiotonic activity, it has not been established that either residue makes contact with the sodium<br />

channel. As indicated above, the carboxylate of Asp9 participates in a hydrogen bond to the backbone<br />

amide of Cys6, so its role may be structural. The carboxylate of Asp7 is close to the side chains of<br />

Lys37 and His39 and is exposed to the solvent, making it a more likely candidate for direct interactions<br />

with the sodium channel. The only evidence for its importance, however, is indirect, coming from the<br />

observation that its replacement <strong>by</strong> Asn in synthetic Sh I abolished toxicity to crabs [55].<br />

Considerable confusion has surrounded the role of Arg14, which is conserved throughout the Type 1 and<br />

Type 2 toxins. A recent study has shown, however, that modification of Arg14 in AP-A with 1,2cyclohexanedione<br />

under conditions where the positive charge is maintained did not affect positive<br />

inotropic activity [54]. This study also showed indirectly that any contact the Arg14 side chain makes<br />

with the sodium channel must be relatively loose: although the adduct is active, it is no longer<br />

susceptible to tryptic proteolysis, indicating that the modified side chain cannot be accommodated in the<br />

substrate binding site of the protease. The conclusion that the positive charge on Arg14, but not its exact<br />

spatial location, might be important for activity is consistent with the results of site-directed mutagenesis<br />

experiments discussed below.<br />

B. Selective Proteolysis<br />

When AP-A was treated with trypsin only the Arg14 to Gly15 peptide bond was cleaved [56]. The<br />

resulting derivative lacked cardiotonic activity but its binding affinity for the rat brain sodium channel<br />

was reduced <strong>by</strong> less than an order of magnitude (Llewellyn LE et al., unpublished results). Its overall<br />

structure, as<br />

http://legacy.netlibrary.com/nlreader/nlReader.dll?bookid=12640&filename=Page_305.html [4/5/2004 5:23:26 PM]

Hooray! Your file is uploaded and ready to be published.

Saved successfully!

Ooh no, something went wrong!