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Role of the Streptokinase α-Domain in the Interactions of Streptokinase with Plasminogen and Plasmin*

The role of the streptokinase (SK) α-domain in plasminogen (Pg) and plasmin (Pm) interactions was investigated in quantitative binding studies employing active site fluorescein-labeled [Glu]Pg, [Lys]Pg, and [Lys]Pm, and the SK truncation mutants, SK-(55–414), SK-(70–414), and SK-(152–414). Lysine binding site (LBS)-dependent and -independent binding were resolved from the effects of the lysine analog, 6-aminohexanoic acid. The mutants bound indistinguishably, consistent with unfolding of the α-domain on deletion of SK-(1–54). The affinity of SK for [Glu]Pg was LBS-independent, and although [Lys]Pg affinity was enhanced 13-fold by LBS interactions, the LBS-independent free energy contributions were indistinguishable. α-Domain truncation reduced the affinity of SK for [Glu]Pg 2–7-fold and [Lys]Pg ≤2-fold, but surprisingly, rendered both interactions near totally LBS-dependent. The LBS-independent affinity of SK for [Lys]Pm, 3000-fold higher compared with [Lys]Pg, was reduced dramatically by α-domain truncation. Thermodynamic analysis demonstrates that the SK α-domain contributes substantially to affinity for all Pg/Pm species solely through LBS-independent interactions, and that the higher affinity of SK for [Lys]Pm compared with [Lys]Pg involves all three SK domains. The residual affinity of the SK βγ-fragment for all Pg/Pm species was increased by an enhanced contribution to complex stability from LBS-dependent interactions or free energy coupling between LBS-dependent and -independent interactions. Redistribution of the free energy contributions accompanying α-domain truncation demonstrates the interdependence of SK domains in stabilizing the SK-Pg/Pm complexes. The flexible segments connecting the SK α, β, and γ domains allow their rearrangement into a distinctly different bound conformation accompanying loss of the constraint imposed by interactions of the α-domain.