Snake prothrombin activator that attacks the hemostatic system of prey. This non-catalytic subunit is functionally similar to blood coagulation factor V. It serves as a critical cofactor for the prothrombinase activity of the catalytic subunit, which is similar to the blood coagulation factor X. The complex converts prothrombin to thrombin by sequential cleavage at two positions, Arg-320 followed by Arg-271. Cleavage at Arg-320 produces an active intermediate known as meizothrombin. Meizothrombin is the 'second' substrate for prothrombinase, and it docks in an altered manner to present the second cleavage site (271). Cleavage at Arg-271 releases active thrombin from its pro-fragment. 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. Heterodimer of a light and a heavy chains; non-disulfide-linked. The interaction between the two chains is calcium-dependent. Found in its active form associated with omicarin-C catalytic subunit (AC Q58L95) (By similarity). Expressed by the venom gland. In physiological conditions, blood coagulation factor V and factor Va are inactivated by activated protein C (APC) through proteolytic degradation of the heavy chain. However, omicarin-C non-catalytic subunit (factor V-like protein) retains its full activity even at high concentration of APC. This has two explanations: this protein has only one of the three cleavage sites present in factor V that are targeted by the APC for inactivation, and the binding with the catalytic subunit protect the cleavage site from inactivation (By similarity). 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 multicopper oxidase family.