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

Plasminogen (Pg) activators such as streptokinase (SK) save lives by genera ting plasmin to dissolve blood clots. Some believe that the unique ability of SK to activate Pg in the absence of fibrin limits its therapeutic utilit y. We have found that SK contains an unusual NH2-terminal "catalytic switch " that allows Pg activation through both fibrin-independent and fibrin-depe ndent mechanisms. Unlike SK, a mutant (rSK Delta 59) fusion protein lacking the 59 NH2-terminal residues was no longer capable of fibrin-independent P g activation (k(cat)/k(m) decreased by >600-fold). This activity was restor ed by coincubation with equimolar amounts of the NH2-terminal peptide rSK1- 59. Deletion of the NH2 terminus made rSK Delta 59 a Pg activator that requ ires fibrin, but not fibrinogen, for efficient catalytic function. The fibr in-dependence of the rSK Delta 59 activator complex apparently resulted fro m selective catalytic processing of fibrin-bound Pg substrates in preferenc e to other Pg forms. Consistent with these observations, the presence (rSK) or absence (rSK Delta 59) of the SK NH2 terminal peptide markedly altered fibrinolysis of human clots suspended in plasma. Like native SK rSk produce d incomplete clot lysis and complete destruction of plasma fibrinogen; in c ontrast, rSK Delta 59 produced total clot lysis and minimal fibrinogen degr adation. These studies indicate that structural elements in the NH2 terminu s are responsible for SK's unique mechanism of fibrin-independent Pg activa tion. 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.