Snake prothrombin activator that attacks the hemostatic system of prey. This protein is functionally similar to blood coagulation factor Xa. The procoagulant activity of hopsarin-D is approximately 10-fold lower than that of trocarin-D and FXa. Selective cleavage of Arg-|-Thr and then Arg-|-Ile bonds in prothrombin to form thrombin. Heterodimer of a light chain and a heavy chain; disulfide-linked. Expressed by the venom gland. The vitamin K-dependent, enzymatic carboxylation of some glutamate residues allows the modified protein to bind calcium. Is classified in the group D of snake venom prothrombin activators, since it requires the mammalian factor Va for maximal activity for the cleavage of prothrombin. 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. Belongs to the peptidase S1 family. Snake venom subfamily. Lacks the Cys residue in position 216 that is replaced by a Ser residue, resulting of a loss a disulfide bond. This may contribute to the lower procoagulant activity.