A 60-year-old male with angina comes to the emergency with severe chest pain unresponsive to sublingual nitroglycerin. An ECG shows ST segment elevation in the anterolateral leads, and thrombolytic therapy is initiated. If streptokinase is given to this patient, it may produce thrombolysis after binding to which of the following proteins?

The fibrinolytic activity of streptokinase is due to its ability to bind and cleave plasminogen, producing plasmin. Plasmin directly cleaves fibrin, both between and within the fibrin polymers, thus breaking up thrombi and potentially restoring blood flow to ischemic cardiac muscle. This same mechanism of fibrinolysis is shared by urokinase and tissue-plasminogen activator (tPA). Antithrombin III is a coagulation inhibitor that binds to and inactivates thrombin. Antithrombin III is anticoagulant, not fibrinolytic. Fibrin is not directly acted upon by streptokinase. It is indirectly cleaved through the action of plasmin. Protein C is a glycoprotein that modulates coagulation by inhibiting the procoagulant activities of factors V/Va and VIII/VIIIa. Protein C has no inherent fibrinolytic activity.