draculin

Draculin (named after Count Dracula) is a glycoprotein found in the saliva of vampire bats. It is a single-chain polypeptide protein composed of 708 amino acids, weighing about 88.5 kDa when reduced and 83 kDa when non-reduced, and selectively inhibits FIXa and FXa. It functions as an anticoagulant, inhibiting coagulation factors IX (IXa) and X (Xa) by establishing rapid equilibrium with factor Xa, and is the first natural polypeptide which has been described to show immediate anti-IXa and anti-Xa properties. In addition, Draculin inhibits the conversion of prothrombin to thrombin, preventing fibrinogen from converting to fibrin. These two processes inhibit blood coagulation thus keeping the blood of the bitten victim from clotting while the bat is drinking. The activation of factor X is a common point between the intrinsic and extrinsic pathway of blood coagulation. Activated factor X (FXa) is the sole enzyme that catalyzes the conversion of prothrombine into thrombin, which is vital in the coagulation cascade. Draculin is a member of the Lactoferrin family of proteins that functions as an antibacterial protein in other mammals, but has been co-opted in bat evolution to function as an anticoagulant.

Draculin is a noncompetitive, tight-binding inhibitor of FXa. The inhibition upon contact with the blood of the victim is immediate. Draculin forms equimolar complexes with factor FXa. The formation of draculin-factor Xa is a two-stage process. The first reversible stage is characterized by the following constants: k1 = 1.117*106 M-1*sec-1, k-1 = 15.388*10-1 sec-1. The second irreversible (concentration-independent) stage is characterized by the forward reaction rate constant k2 = 0.072 sec-1. The dissociation constant is determined as the ratio k-1/k1 = 13.76 nM. Because of the immediate inhibition, the reaction is not readily reversible initially, but is a reversible reaction. It does not act on thrombin, trypsin or chymotrypsin and does not express fibrinolytic activity. The protein increases the lag phase as well as the height of the peak of thrombin generation when in plasma, leading to prolonged bleeding. The biological activity of Draculin is highly dependent on glycosylation of the native protein and can be severely affected by the salivation pattern of the animals.