Snake prothrombin (F2) activator that attacks the hemostatic system of prey. This catalytic subunit is functionally similar to blood coagulation factor Xa. For prothrombin activation, it requires a non-catalytic subunit present in the venom, which is similar to coagulation factor Va, to be fully active. In contrast to the 8 other snake venoms tested, this protein is the only one to also activate factor VII (F7). However, in contrast to prothrombin activation, the factor Va-like subunit is not essential for this activation. Selective cleavage of Arg-|-Thr and then Arg-|-Ile bonds in prothrombin to form thrombin. Activated by calcium and negatively charged phospholipids. Heterodimer of a light and a heavy chains; disulfide-linked. Is associated with oscutarin-C non-catalytic subunit (AC Q58L91) in a non-covalent manner (By similarity). Expressed by the venom gland. Gamma-carboxyglutamate residues are formed by vitamin K dependent carboxylation. These residues are essential for the binding of calcium. Is classified in the group C of snake venom prothrombin activators, since it does not require the mammalian factor Va for the cleavage of prothrombin as the venom contains its own non-catalytic factor Va-like molecule. In contrast to blood coagulation factors that circulate as inactive zymogen in plasma, venom prothrombin activators are always found in the active form in the venom. Hence, catalytic and non-catalytic subunits are found naturally in venom as stable complexes. Belongs to the peptidase S1 family. Snake venom subfamily.