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Figure 5<br />

Predicted secondary structure of tissue factor pathway inhibitor (TFPI) showing the Factor<br />

Xa and Factor VIIa inhibitory domains. The arrows point to the P1 sites.<br />

Page 272<br />

bond, often imposed <strong>by</strong> cysteine crosslinks constraining the two ends of the cleavage site to be in close<br />

proximity even after cleavage [47]. Antistasin has cysteines at the P2(Cys 33) and P'3 (Cys 37) positions.<br />

C. Tick Anticoagulant Peptide (TAP)<br />

The tick anticoagulant peptide (TAP) is a 60-amino-acid polypeptide isolated from the soft tick<br />

Ornithodorus Moubata and is a potent (K i = 2–200 pM) and selective inhibitor of Factor Xa, both as the<br />

free enzyme and in the prothrombinase complex [13]. The TAP anticoagulant does not inhibit trypsin or<br />

other trypsinlike serine proteases and, importantly, is not cleaved <strong>by</strong> Factor Xa. The mechanism <strong>by</strong><br />

which TAP inhibits Factor Xa appears to be unique and it apparently does not utilize the substratelike<br />

binding modes characteristic of antistasin and the Kunitz inhibitors. Mutagenesis studies have shown<br />

that the primary interaction of TAP with Factor Xa occurs at the N-terminous where Arg 3 appears to<br />

play a key role [56]. The solution structure of TAP has been deter-<br />

http://legacy.netlibrary.com/nlreader/nlReader.dll?bookid=12640&filename=Page_272.html [4/5/2004 5:12:58 PM]

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