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 center dot Pg* catalytic complex. SK center dot 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 Lys(414) deletion mutant (SK Delta K414) 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 ofSKfor [Glu] Pg was unaffected by deletion of Lys(414). 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, SK Delta K-414 exhibited a corresponding 17-fold affinity decrease for formation of the SK Delta K-414[Lys]Pg* complex. SK Delta K414 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.