The activation of prothrombin. Prothrombin fragment 1 (F1) consists of the γ-carboxyglutamic acid domain and first kringle domain, and fragment (F2) consists of the second kringle domain. Thrombin consists of disulfide-linked A and B chains, where the B chain contains the His (H), Asp (D), and Ser (S) residues of the serine protease active site. Meizothrombin (MZa) is produced by factor Xa cleavage after Arg320 of human prothrombin or after Arg317 of mouse prothrombin. Thrombin is produced by factor Xa cleavage after Arg271 of human MZa or after Arg268 of mouse MZa. Meizothrombin(desF1) (MZdesF1a) is produced by autocatalytic cleavage after Arg155 of human MZa or after Arg157 of mouse MZa. Fragment F2 and the A chain of human thrombin also can be separated by cleavage after Arg284; this cleavage site is replaced by Lys281 in mouse prothrombin.