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

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Table 2 Naturally Occurring Inhibitors of Factor Xa<br />

Inhibitor Source K i Structural information<br />

TFPI Human 3 pM [49]<br />

90 nM [50]<br />

Ecotin Escherichia coli 50 pM [44] X-ray; complex with trypsin [46]<br />

TAP Ornithidoros moubata (tick) 135 pM [13] 2D-NMR [57,58]<br />

Antistasin Haementeria officinalis (leech) 61 pM [51] (X-ray in progress) [52]<br />

AcAP5 Ancylostoma caninum 43 pM [19] homology to Ascaris lumbricoides<br />

var.suum [79,80]<br />

Page 270<br />

Studies of these natural inhibitors can be useful in defining the active site requirements for Factor Xa<br />

inhibition, and importantly, can indicate the level of inhibition that may be necessary for an effective<br />

Factor Xa inhibitor, recognizing that TAP and antistasin have evolved to yield functional, in vivo<br />

antithrombotics. Table 3 shows the reactive-site sequences of these substratelike inhibitors as well as the<br />

cleavage site sequences recognized <strong>by</strong> Factor Xa in the activation of prothrombin (PT), Factor VII, and<br />

Factor V.<br />

A. Tissue Factor Pathway Inhibitor (TFPI)<br />

The mature tissue factor pathway inhibitor (TFPI) is a 276-residue protein consisting of three tandom<br />

domains with homology to the Kunitz-like protease<br />

http://legacy.netlibrary.com/nlreader/nlReader.dll?bookid=12640&filename=Page_270.html (1 of 2) [4/5/2004 5:12:45 PM]

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