Binding of the COOH-terminal lysine residue of streptokinase to plasmin(ogen) kringles enhances formation of the streptokinase.plasmin(ogen) catalytic complexes

Streptokinase (SK) activates human fibrinolysis by inducing non-proteolytic activation of the serine proteinase zymogen, plasminogen (Pg), in the SK.Pg* catalytic complex. SK.Pg* proteolytically activates Pg to plasmin (Pm). SK-induced Pg activation is enhanced by lysine-binding site (LBS) interactions with kringles on Pg and Pm, as evidenced by inhibition of the reactions by the lysine analogue, 6-aminohexanoic acid. Equilibrium binding analysis and Lys]Pg activation kinetics with wild-type SK, carboxypeptidase B-treated SK, and a COOH-terminal Lys414 deletion mutant (SKDeltaK414) demonstrated a critical role for Lys414 in the enhancement of Lys]Pg and Lys]Pm binding and conformational Lys]Pg activation. The LBS-independent affinity of SK for Glu]Pg was unaffected by deletion of Lys414. By contrast, removal of SK Lys414 caused 19- and 14-fold decreases in SK affinity for Lys]Pg and Lys]Pm binding in the catalytic mode, respectively. In kinetic studies of the coupled conformational and proteolytic activation of Lys]Pg, SKDeltaK414 exhibited a corresponding 17-fold affinity decrease for formation of the SKDeltaK414.Lys]Pg* complex. SKDeltaK414 binding to Lys]Pg and Lys]Pm and conformational Lys]Pg activation were LBS-independent, whereas Lys]Pg substrate binding and proteolytic Lys]Pm generation remained LBS-dependent. We conclude that binding of SK Lys414 to Lys]Pg and Lys]Pm kringles enhances SK.Lys]Pg* and SK.Lys]Pm catalytic complex formation. This interaction is distinct structurally and functionally from LBS-dependent Pg substrate recognition by these complexes.