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

Create successful ePaper yourself

Turn your PDF publications into a flip-book with our unique Google optimized e-Paper software.

Document<br />

Page 379<br />

For trypanosomal TIM we experimented with three different cocktails of 32 compounds (Table 4).<br />

Molecules were chosen in such a way that they would be compatible, soluble, cheap, and as varied as<br />

possible. Each compound was present at a concentration of 1 mM. The final cocktail solutions were<br />

clear and devoid of precipitate. Since this was a pilot experiment both subcocktails were checked at each<br />

stage of the dichotomic strategy. Only the soak with cocktail 1 revealed electron density that could not<br />

be accounted for <strong>by</strong> water molecules, hereafter called peak X. The soaks with cocktails 2 and 3 led to<br />

featureless difference Fourier maps. The quality of the data and refinement can be inspected from Table<br />

5, while Figure 9 illustrates the dichotomic search to identify peak X. An oxidized molecule of DTT,<br />

identified in the high-resolution structure of the native TIM crystals [24], served as an internal reference<br />

to judge the quality of the data and the noise level in the final difference Fourier maps.<br />

Peak X was found near His95 of the second subunit of the enzyme, i.e., the subunit where the flexible<br />

loop adopts the closed conformation in this crystal form. Its signal was somewhat weaker than that of<br />

DTT. The same density showed up when crystals were soaked with subcocktail 1B but not with 1A,<br />

narrowing down the list of potential ligands to sixteen compounds. However, the next round of the<br />

dichotomic search led to a problem that has not been solved thus far. Peaks of roughly the same shape as<br />

the original peak X appeared with both subcocktails 1BA and 1BB. Several strategies were followed to<br />

improved the quality of the maps. First, the model was further refined with all data while a bulk solvent<br />

scattering correction [75] was incorporated. Second, a variety of maps were calculated: (|F o|-|F c|) e iαc,<br />

(2|Fo|-|Fc|) eiαc, (3|Fo|-2|Fc|) eiαc and (|Fo|-|Fo,native|)eiαc. Third, all maps were SIGMAA-weighted [76].<br />

Since the shape of peak X varied substantially between the different maps it can be tentatively<br />

concluded that peak X did not originate from the presence of a compound but was noise. The lesson of<br />

this experiment seems to be that the crystallographic cocktail soaking approach should only be tried<br />

when high- resolution data can be obtained, probably better than 1.8 Å resolution.<br />

B. Glyceraldehyde-3-Phosphate Dehydrogenase: Docking<br />

In order to discover new ligands that would block GAPDH of T. brucei <strong>by</strong> occupying the adenosine<br />

binding region we used the program DOCK [77], version 3.5. This program characterizes a binding site<br />

<strong>by</strong> filling it with a set of overlapping spheres. The centers of these generated spheres constitute an<br />

irregular grid, called a “graph” <strong>by</strong> mathematicians. Docking of a ligand then consists of matching<br />

subsets of ligand interatomic distances onto subsets of the receptor graph. Finally, the quality of the fit<br />

between a docked ligand and the receptor is evaluated. Within DOCK 3.5 three methods are available<br />

for this evaluation: contact scoring, which measures shape complementarity; force-field scoring, which<br />

is an estimate of the enthalpy of the intermolecular interaction; and elec-<br />

http://legacy.netlibrary.com/nlreader/nlReader.dll?bookid=12640&filename=Page_379.html [4/5/2004 5:39:49 PM]

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

Saved successfully!

Ooh no, something went wrong!