Factor Xa is a serine endoproteinase and a member of the S1 peptidase family. Factor Xa plays a critical role in the coagulation cascade by catalyzing the proteolytic conversion of prothrombin to active thrombin. Factor Xa′s prothrombin conversion activity is greatly enhanced in vivo when complexed with factor V, calcium ions and phospholipids on the activated platelet surface.

The zymogen form, Factor X, is activated in vivo by two different pathways. The intrinsic pathway utilizes a catalytic complex composed of factor IXa, factor VIII, phospholipids and calcium ions. The extrinsic pathway utilizes a complex of factor VII and tissue factor. The factor X zymogen is a 55 KDa glycoprotein with a light and heavy chain joined by a single disulfide.