Role of the streptokinase alpha-domain in the interactions of streptokinase with plasminogen and plasmin

The role of the streptokinase (SK) alpha-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 alpha-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. alpha-Domain truncation reduced the affinity of SK for [Glu]Pg 2-7-fold and [Lys]Pg