Which substance does streptokinase form a complex with to convert plasminogen to plasmin?

Streptokinase works by forming a complex with plasminogen, which is a precursor to plasmin, the enzyme responsible for breaking down fibrin in blood clots. When streptokinase binds to plasminogen, this complex changes the conformation of plasminogen that ultimately converts it into active plasmin. This process is crucial for thrombolysis, the breakdown of clots, in conditions like myocardial infarction or pulmonary embolism.

The other substances listed - fibrinogen, thrombin, and factor X - play roles in the coagulation cascade and blood clot formation, but they are not involved in the specific mechanism by which streptokinase activates plasminogen. Understanding the specific interaction between streptokinase and plasminogen highlights the targeted approach of thrombolytic therapy in treating blood clots.