Snake prothrombin activator that attacks the hemostatic system of prey. This non-catalytic subunit is functionally similar to blood coagulation factor V (PubMed:12362232, PubMed:23869089). It serves as a critical cofactor for the prothrombinase activity of the catalytic subunit, which is similar to the blood coagulation factor X (PubMed:12362232, PubMed:23869089). The complex converts prothrombin to thrombin by sequential cleavage at two positions, Arg-320 followed by Arg-271 (PubMed:23869089). Cleavage at Arg-320 produces an active intermediate known as meizothrombin (PubMed:23869089). Meizothrombin is the 'second' substrate for prothrombinase, and it docks in an altered manner to present the second cleavage site (271) (PubMed:23869089). Cleavage at Arg-271 releases active thrombin from its pro-fragment (PubMed:23869089). This order of events is reversed if the protease component of prothrombinase is used on its own, suggesting that the 271 site is inherently more accessible to proteolysis (PubMed:23869089). The complex converts prothrombin to thrombin in presence but also in the absence of membrane (PubMed:23869089). 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 pseutarin-C non-catalytic subunit (AC Q7SZN0) in a non-covalent manner. Expressed by the venom gland. Gamma-carboxyglutamate residues are formed by vitamin K dependent carboxylation. These residues are essential for the binding of calcium. Intravenous injection (23 ug/kg bodyweight) of the group C prothrombin activator causes death in rats through disseminated intravascular coagulopathy (PubMed:3075905), whereas pseutarin-C is not lethal even at 10 mg/kg in mice when injected intraperitoneally (PubMed:15351847). Is under preclinical trial by the Australian biopharmaceutical company QRxPharma Ltd, its subsidiary Venomics Pty Ltd (VPL) and the University of Queensland (UQ) under the name Haempatch (Q8009). Tested as a topical hemostatic agent to reduce blood loss resulting from surgery or trauma. 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.