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Page 285<br />

number of x-ray structures of complexes. Appropriate site-specific and random mutagenesis, particularly<br />

of the P3-P4' residues of a prototypic Kunitz inhibitor, bovine pancreatic trypsin inhibitor (BPTI), has<br />

been shown to result in potent and selective Factor Xa inhibitors [75,76].<br />

A potent inhibitor of trypsin, kallikrein, and plasmin, BPTI does not inhibit Factor Xa. It binds to serine<br />

proteases such as trypsin in an extended substrate mode from residue 13 (P3) through 17 (P'2) [47]. A<br />

second loop from BPTI also extends into the active site bringing residues 34, 39, and 46 into contact<br />

with the protease-active site. In terms of spatial proximity of residues three clusters can be defined:<br />

cluster 1 (13,39); cluster 2 (11,17,19,34); and cluster 3 (16,18,20,46). While residue 39 is approximately<br />

in the same region of space as residue 13 (9.4 Å CB-CB) the CA rarrow.gif CB vectors are directed in<br />

different directions and substitution at 39 would not be expected to have a cooperative effect with<br />

residue 13. Residue 34 on the other hand is in a key position. It is centrally located between residues<br />

11,17, and 19 with CB-CB distances of 5.6, 5.7, and 6.5 Å respectively, and its CA rarrow.gif CB<br />

vector converges with the corresponding vectors from these residues to a common point in space. This<br />

residue is therefore expected to have a substantial cooperative effect with the other residues of cluster 2.<br />

Finally residue 46 is close to residue 20 (CB—CB of 6.5 Å) although the CA rarrow.gif CB vectors are<br />

approximately parallel and cooperative effects are expected to be minimal. The BPTI residues<br />

11,12,13,15–20, 34,39, and 46 were therefore the focus of the site-directed and random mutagenesis<br />

studies. Residue 14 is Cys in BPTI and was not modified in the mutants since it is required for structural<br />

reasons.<br />

A. Site Specific Mutagenesis<br />

As a starting point for the design of BPTI-<strong>based</strong> Factor Xa inhibitors, the second domain of TFPI (TFPI-<br />

II) was used as a template [75,76]. Table 5 shows the results of site-directed mutagenesis of BPTI.<br />

Mutant 50cl is a direct analog of TFPI-II with the exception of the Lys at position 46. The finding that<br />

4c2 and 4c10 are essentially equivalent in potency (K i 2.8 versus 1.8 nM) and are identical<br />

Table 5 Site-Directed BPTI Mutants with Factor Xa Inhibition<br />

K i(nM) 12 13 14 15 16 17 18 19 20 34 39 46<br />

r-TFPI-II 90 Gly Ile Cys Arg Gly Tyr Ile Thr Arg Lys Leu Glu<br />

50cl 205 Gly Ile Cys Arg Ala Tyr Ile Thr Arg Lys Leu Lys<br />

4c2 2.8 Gly Ile Cys Arg Ala Tyr Ile Thr Arg Val Leu Glu<br />

4c10 1.8 Gly Ile Cys Arg Ala Tyr Ile Thr Arg Val Leu Lys<br />

57c1 1.6 Gly Ile Cys Arg Ala Tyr Ile Ile Arg Val Leu Lys<br />

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