Draculin, the anticoagulant factor in vampire bat saliva, is a tight-binding, noncompetitive inhibitor of activated factor X. Biochim. Biophys.


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September 1999


The kinetic mechanism of action of Draculin on activated Factor X (FXa) is established. Draculin inhibits activated Factor X within seconds of incubation at near equimolar concentration (2–6 times on molar basis). Fitting the data to the equation for a tight-binding inhibitor gives a value for Ki(Kd)=14.8±1.5 nM. The formation of the Draculin–FXa complex can be explained by a two-step mechanism, where for the first, reversible step, kon=1.117 (±0.169, S.E.M.)×106 M−1 s−1 and koff=15.388 (±1.672)×10−3 s−1, while for the second, irreversible step, which is concentration-independent, k2=0.072 s−1. Kd obtained from koff/kon=13.76 nM. Lineweaver–Burk plot shows a noncompetitive behavior. This noncompetitive mode of inhibition of Draculin is supported by the observation that Draculin, at concentrations giving complete inhibition, does not impair binding of p-aminobenzamidine to FXa. Moreover, under the same conditions, Draculin induces <14% decrease of the fluorescence intensity of the p-aminobenzamidine–FXa complex. We conclude that Draculin is a noncompetitive, tight-binding inhibitor of FXa, a characteristic so far unique amongst natural FXa inhibitors.


Fxa Inhibition, Draculin, Tight-Binding Inhibitor, Natural Anticoagulant, (Vampire Bat)

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Biochimica et Biophysica Acta (BBA) - Protein Structure and Molecular Enzymology, Vol. 1434, no. 1 (1999-09-14).