Key Enzyme of Blood Coagulation

Key enzyme of hemostasis or blood coagulation is:

Correct Answer: Thrombin

Thrombin is the key enzyme in hemostasis (blood coagulation). It is the central enzyme of the clotting cascade, responsible for the final critical steps of clot formation.

Role of Thrombin in Coagulation

• Thrombin is generated from Prothrombin (Factor II) by the action of Factor Xa (prothrombinase complex)
• Thrombin converts Fibrinogen (Factor I) → Fibrin (the structural scaffold of the clot)
• Thrombin also activates Factor XIII (Fibrin-stabilizing factor) which cross-links fibrin strands, making the clot stable
• Thrombin activates platelets and several other clotting factors (V, VIII, XI) — creating a positive feedback loop

Why Other Options Are Wrong

• Fibrinogen: It is a clotting factor (Factor I), not an enzyme — it is the substrate that thrombin acts on
• Thromboplastin: Also called Tissue Factor (Factor III); it initiates the extrinsic pathway but is not the key enzyme of hemostasis
• Plasmin: This is the enzyme responsible for clot lysis (fibrinolysis), the opposite of clotting

Memory Tip

Think of Thrombin as the conductor of the clotting orchestra — it activates the final common pathway and converts fibrinogen to fibrin to form the clot mesh.