A catalytic switch and the conversion of streptokinase to a fibrin-targeted plasminogen activator

Plasminogen (Pg) activators such as streptokinase (SK) save lives by generating plasmin to dissolve blood clots. Some believe that the unique ability of SK to activate Pg in the absence of fibrin limits its therapeutic utility. We have found that SK contains an unusual NH₂-terminal "catalytic switch" that allows Pg activation through both fibrin-independent and fibrin-dependent mechanisms. Unlike SK, a mutant (rSKΔ59) fusion protein lacking the 59 NH₂-terminal residues was no longer capable of fibrin-independent Pg activation (k_cat/K_m decreased by >600-fold). This activity was restored by coincubation with equimolar amounts of the NH₂-terminal peptide rSK1-59. Deletion of the NH₂ terminus made rSKΔ59 a Pg activator that requires fibrin, but not fibrinogen, for efficient catalytic function. The fibrin-dependence of the rSKΔ59 activator complex apparently resulted from selective catalytic processing of fibrin-bound Pg substrates in preference to other Pg forms. Consistent with these observations, the presence (rSK) or absence (rSKΔ59) of the SK NH₂-terminal peptide markedly altered fibrinolysis of human clots suspended in plasma. Like native SK, rSK produced incomplete clot lysis and complete destruction of plasma fibrinogen; in contrast, rSKΔ59 produced total clot lysis and minimal fibrinogen degradation. These studies indicate that structural elements in the NH₂ terminus are responsible for SK's unique mechanism of fibrin-independent Pg activation. Because deletion of the NH₂ terminus alters SK's mechanism of action and targets Pg activation to fibrin, there is the potential to improve SK's therapeutic efficacy.