Which anticoagulant protein is activated by heparin binding?

• A. Protein C
• B. Antithrombin III
• C. Plasmin
• D. Thrombomodulin

Answer: (B)

The key idea is that heparin works by boosting the activity of antithrombin III. When heparin binds to antithrombin III, it changes its shape and greatly increases its ability to inhibit the enzymes that drive clot formation, especially thrombin and Factor Xa. This makes antithrombin III the anticoagulant protein that is activated in the presence of heparin.

Protein C is activated not by heparin, but when thrombin binds to thrombomodulin on endothelial cells, which then activates protein C to help slow coagulation. Plasmin is the enzyme that degrades fibrin clots, activated by other pathways such as tissue plasminogen activator. Thrombomodulin is the endothelial receptor that partners with thrombin to activate protein C, not something activated by heparin.