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Page 282<br />

using BPTI residues 11–19 as a template [69]. The modeled dR-ATS 32-38:FXa complex is shown in<br />

Figure 11. Interestingly, these peptides do not inhibit trypsin even at 1000-fold higher concentrations<br />

than their FXa inhibitory concentrations. Antistasin, on the other hand, inhibits trypsin with a K i of 10<br />

nM.<br />

Preliminary reports have appeared describing a pentapeptide that is a potent (K i=3 nM) and selective<br />

inhibitor of Factor Xa (SEL2711) [70,71].<br />

Two possible modes of binding can be envisioned for this compound with either the methylpyridinium<br />

group occupying the S1 site in a “substratelike mode” or the p-amidinophenyl group in the S1 pocket,<br />

which would require a reversed binding reminiscent of the hirudin-thrombin interaction [59]. Figure 12<br />

shows the case for the amidinophenyl group in the Factor Xa S1 pocket [69]. In this mode of binding the<br />

methylpyridinium group easily fits the S4-aryl binding site and is well positioned for a π-cation<br />

interaction.<br />

Of interest from a drug-design viewpoint is the finding that cyclotheonamide, a compound isolated from<br />

a marine sponge and originally reported as a thrombin inhibitor, has been found to also inhibit Factor Xa<br />

with a K i of 50 nM [72]. Cyclotheonamide possesses a novel α-ketoamide transition state functionality<br />

and x-ray structures of cyclotheonamide with trypsin [73] and thrombin [74] provide templates for<br />

modeling this inhibitor into Factor Xa [69]. In the resulting fit (Figure 13) cyclotheonamide does not<br />

project functionality into the S4-cation-π site and would not be expected to show Factor Xa selectivity.<br />

VII. Defining the Requirements for Factor Xa Inhibition <strong>by</strong> Mutagenesis of BPTI<br />

It has been known for some time that many examples of naturally occurring Kunitz inhibitors exist, both<br />

isolated and as domains in larger proteins, which inhibit a variety of serine proteases [47]. This strongly<br />

suggests that this molecular framework is compatible with inhibition of this general class. The contact<br />

region between these inhibitors and their protease targets is known from a<br />

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