Final answer:
Thrombin activates the conversion of the inactive plasma protein fibrinogen to the fibrous gel fibrin during the common pathway of coagulation, leading to the formation of a clot.
Step-by-step explanation:
Thrombin converts the inactive plasma protein fibrinogen into a fibrous gel called fibrin. This process is an essential step in the blood clotting mechanism, where a mesh is formed to trap more platelets and erythrocytes, leading to clot formation. The conversion happens during the common pathway of the coagulation process, where the activated factor X, via either the intrinsic or extrinsic pathway, allows prothrombinase to convert the inactive enzyme prothrombin (factor II) into the active enzyme thrombin. Subsequently, thrombin catalyzes the conversion of fibrinogen (factor I) into insoluble fibrin strands, which are later stabilized by factor XIII.