Final answer:
The principal physiological inhibitor that AT III targets is thrombin. Antithrombin III is crucial for regulating coagulation by inactivating factor X and opposing the conversion of prothrombin to thrombin, preventing excessive clot formation.
Step-by-step explanation:
The principal physiological inhibitor that AT III (antithrombin III) targets is thrombin. It works by inhibiting the activity of several enzymes in the clotting process, primarily factor II, also known as prothrombin, and factor Xa. When prothrombin is converted into thrombin in the common pathway, the potential for clot formation increases. Here, antithrombin (AT III) plays a critical role in the negative regulation of coagulation, by inactivating factor X and opposing the conversion of prothrombin (factor II) to thrombin. This action is essential for balancing clot formation and preventing excessive blood clotting, which can lead to conditions such as thrombosis.