In the clotting description, which entity catalyzes the conversion of prothrombin to thrombin?

• A. Prothrombin activator
• B. Thrombin
• C. Fibrin
• D. Platelets

Answer: (A)

The conversion of prothrombin to thrombin is driven by a prothrombin activator, also known as the prothrombinase complex. In the common pathway, factor Xa pairs with factor Va on a phospholipid surface in the presence of calcium to form prothrombin activator. This enzyme complex cleaves prothrombin (factor II) to active thrombin (IIa). Thrombin then goes on to convert fibrinogen to fibrin and to amplify the coagulation cascade by activating other factors and platelets. So the catalyst here is the prothrombin activator. Thrombin is the product of this step, not the catalyst; fibrin is formed later from fibrinogen, and platelets help provide a surface and cofactors but do not catalyze this specific conversion.