Factor X

Overview

Factor X, also known as Stuart-Prower factor, is an essential enzyme in the coagulation cascade, which is crucial for hemostasis. It is a serine protease that plays a pivotal role in the conversion of prothrombin to thrombin, a key step in the formation of a blood clot.

Structure

Factor X is synthesized in the liver and requires vitamin K for its activation. It circulates in the blood as an inactive zymogen. Upon activation, it becomes Factor Xa, which is the active form of the enzyme.

Activation

Factor X can be activated via two pathways:

• The intrinsic pathway, which is initiated by damage to the endothelium and involves several other clotting factors.
• The extrinsic pathway, which is triggered by external trauma that causes blood to escape from the vascular system.

Both pathways converge at the activation of Factor X, leading to the common pathway of coagulation.

Function

Once activated to Factor Xa, it forms a complex with Factor V, calcium ions, and phospholipids on the surface of platelets. This complex, known as the prothrombinase complex, catalyzes the conversion of prothrombin to thrombin. Thrombin then converts fibrinogen to fibrin, which forms the mesh that stabilizes the blood clot.

Clinical Significance

Deficiency or dysfunction of Factor X can lead to bleeding disorders. Conversely, excessive activity can result in thrombosis. Factor X inhibitors, such as rivaroxaban and apixaban, are used as anticoagulants to prevent and treat thromboembolic disorders.

In Vivo Coagulation

File:Factor X Coagulation in vivo.png

Diagram of Factor X in the coagulation cascade

In vivo, the regulation of Factor X activation is tightly controlled to maintain a balance between bleeding and clotting. The endothelial cells and antithrombin play roles in modulating its activity.

Related Pages

• Coagulation
• Thrombin
• Hemostasis
• Vitamin K
• Anticoagulant