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Table 3 Active Site Sequences for Factor Xa Substrates and Inhibitors<br />

Substrates P4 P3 P2 P1 P'1 P'2 P'3 P'4<br />

PT 271,272 Ile Glu Gly Arg Thr Ala Thr Ser<br />

PT 320,321 Ile Asp Gly Arg Ile Val Glu Gly<br />

FVII Pro Gln Gly Arg Ile Val Gly Gly<br />

FV Lys Lys Tyr Arg Ser Leu His Leu<br />

Inhibitors<br />

Antistasin Val Arg Cys Arg Val His Cys Pro<br />

Ecotin Val Ser Thr Met Met Ala Cys Pro<br />

TFPI-II Gly Ile Cys Arg Gly Tyr Ile Thr<br />

AcAP5 Cys Arg Ser Arg Gly Cys Leu Leu<br />

AcAP6 Cys Arg Ser Phe Ser Cys Pro Gly<br />

Page 271<br />

inhibitors [42]. A potent inhibitor of both Factor VIIa and Xa as well as trypsin, TFPI does not,<br />

however, have significant activity against leukocyte elastase, urokinase, activated Protein C, tissue<br />

factor plasminogen activator, thrombin, or kallikrein [53,54]. The second Kunitz domain from the Nterminus<br />

of TFPI has been identified as primarily responsible for the Factor Xa inhibition while both the<br />

first and second domains contribute to inhibition of Factor VIIa [42] (Figure 5). The proposed<br />

mechanism for this Factor-Xa-dependent inhibition of FVIIa/tissue factor involves the formation of a<br />

quarternary FXa-TFPI-FVIIa/TF complex [42]. The recombinant, isolated second domain, TFPI-II, has<br />

a K i for Factor Xa of 90 nM [50] compared to 3 pM [49] for the intact protein. The sequence of the<br />

P4–P5' region of the Factor Xa inhibitory second Kunitz domain (Table 3) has been incorporated into a<br />

prototypic Kunitz inhibitor, bovine pancreatic inhibitor (BPTI), to produce potent and selective Factor<br />

Xa inhibitors [75,76].<br />

B. Antistasin (ATS)<br />

Antistasin is one of several anticoagulants isolated from the Mexican leech, Haementeria officinalis<br />

[14]. It is a 119-amino-acid cysteine-rich protein with a primary structure that shows a two-fold<br />

sequence symmetry suggesting the molecule possesses two separate and distinct domains [51].<br />

Mutagenesis studies have shown that ATS binds to Factor Xa in a substratelike manner in the<br />

P3(Arg 32–P'3(Cys 37) regions and is cleaved only in the first domain [55].<br />

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