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

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Page 288<br />

Additionally, the binding of BPTI mutants also requires Tyr 99 to move because of potential severe<br />

interactions with the Cys 14–Cys 38 bridge of the BPTI mutants. While larger groups can be<br />

accommodated, structural requirements are fairly rigid in agreement with the limited mobility expected<br />

of the Tyr 99. The apparent requirement for a P2-Gly in the larger substrates may suggest that in these<br />

cases extended binding occurs that does not permit movement of Tyr 99.<br />

In the P1 position, as expected, a basic group is preferred. Interestingly, of the two naturally occuring<br />

basic amino acids, arginine is preferred over lysine. This is seen in both the BPTI mutants as well as the<br />

Arg rarrow.gif Lys switch in antistasin. This may be due to Ala 190 in the S1 pocket of FXa, which<br />

cannot orient and stabilize the BPTI lysine analog as Ser 190 does in trypsin. An interesting exception to<br />

the need for a basic group is in Ecotin where a methionine occupies this site. The x-ray of Ecotin with<br />

trypsin clearly shows this neutral residue in the P1 pocket, aligned very closely to that seen for lysine in<br />

the BPTI:trypsin complex, and proximal to the charged Asp 189 [77]. Apparently, extended binding over<br />

the rest of the site compensates for this energetically unfavorable situation.<br />

In the P'1 position, the natural cleavage sites use Thr and Ile while Ecotin has Met. In contrast to<br />

thrombin, FXa lacks the 60-insertion loop and can accommodate large groups at this position. The BPTI<br />

mutants, however, are forced to use a small residue (Ala) because of steric hindrance from the Cys 58-<br />

Cys 42 group residue 61 in the enzyme. The inhibitor TFPI-II has a Gly at P'1.<br />

The BPTI mutants, TFPI-II, and antistasin all show a preference for aromatic groups at P'2. In the BPTI<br />

panning experiments Tyr was selected more than 80% of the time at this position. It can be seen from<br />

Table 5 that the Arg to Tyr change at position 17 is one of three significant changes that converts wild<br />

type BPTI from a non-Factor Xa inhibitor to a ˜1.6 nM inhibitor. There is a possible hydrogen-bond<br />

interaction between Tyr 17 of the inhibitor and Gln 192 of the enzyme, which may explain the strong<br />

preference.<br />

While models suggest the P'3 residue is directed at solvent and the FXa thrombin cleavage sites have<br />

polar residues at this position (Thr, Glu), the BPTI mutant results show a clear preference for a<br />

hydrophobic group. It is possible that aromatic groups can pack to Phe 41 of the enzyme. Mutant results<br />

show Ile is favored over Phe, His, which in turn is selected over Tyr.<br />

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