Final answer:
Thrombin serves as the catalyst to form fibrin and is formed from prothrombin. It is essential for the final steps in the formation of a fibrin clot.
Step-by-step explanation:
The enzyme that serves as the catalyst to form fibrin and is formed from prothrombin is thrombin. Once factor X has been activated, prothrombinase converts prothrombin into thrombin, which then converts fibrinogen into fibrin protein strands. Factor XIII stabilizes the fibrin clot. Thrombin is essential for the final steps in the formation of a fibrin clot.