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Figure 3<br />

The catalytic mechanism proposed <strong>by</strong> Veerapandian et al. [31] <strong>based</strong> on the x-ray<br />

structure of a difluoroketone (geminal-diol) inhibitor bound to endothiapepsin. A water<br />

molecule tightly bound to the aspartates in the native enzyme is proposed to<br />

nucleophilically attack the scissile-bond carbonyl. The resulting geminal-diol<br />

intermediate is stabilised <strong>by</strong> hydrogen bonds with the negatively charged carboxyl of aspartate 32.<br />

Fission of the scissile C-N bond is accompanied <strong>by</strong> transfer of a proton from Asp215 to<br />

the leaving amino group.<br />

B. Complementarity of the Inhibitor<br />

Page 328<br />

Optimizing the fit of a ligand to its binding site improves the potency <strong>by</strong> burying lipophilic residues and<br />

<strong>by</strong> maximizing the number of van der Waals contacts, hydrogen bonds, and charge—charge interactions.<br />

The principles that apply to ligand binding are similar to those involved in protein folding. Inhibitor<br />

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