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

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Figure 6<br />

Schematic representation of the principal intermolecular<br />

interactions of a nonpeptide bicyclic inhibitor within the<br />

active site of thrombin.<br />

exposed to solvent. No specific interactions were observed with the catalytic triad. A bound water<br />

molecule hydrogen bonded to the Ser195 hydroxyl.<br />

Page 253<br />

The complex is stabilized <strong>by</strong> a network of hydrogen bonds as well as hydrophobic interactions. The<br />

Phe1-O and the Phe3-NH form hydrogen bonds with Gly216, and the Phe1-NH hydrogen bonds to the<br />

backbone carbonyl of Ser214. The Phe1 phenyl group occupies the S2 site, while Phe3 interacts within<br />

the S3 site.<br />

The retro-inhibitors contain a 4-guanidinobutanoyl group that extends into the S1 specificity site. Rather<br />

than forming two hydrogen bonds between the guanidine and Asp189 in a manner similar to PPACK,<br />

BMS-183507 forms only one, with the second hydrogen bond being directed to the carbonyl oxygen of<br />

Gly219. Binding affinity, as evidenced <strong>by</strong> loss of more than two orders of magnitude in affinity on<br />

addition of one or two methylene groups, was sensitive to chain length at this position.<br />

The allo-Thr hydroxyl oxygen accepts a hydrogen bond from the backbone NH of Gly219. This<br />

additional interaction accounts, at least in part, for the increase in affinity when compared to the<br />

inhibitor with Leu in this position. Comparison of the crystal structures of thrombin complexed with<br />

BMS-183507 and with hirudin reveals that the hirudin residue, Thr2, and the allo-Thr of BMS-183507<br />

interact differently with thrombin. The hirudin Thr2 binds at S2,<br />

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