Chimerism reveals a role for the streptokinase β-domain in nonproteolytic active site formation, substrate, and inhibitor interactions

Streptokinase (SK) and staphylokinase form cofactor-enzyme complexes that promote the degradation of fibrin thrombi by activating human plasminogen. The unique abilities of streptokinase to nonproteolytically activate plasminogen or to alter the interactions of plasmin with substrates and inhibitors may be the result of high affinity binding mediated by the streptokinase β-domain. To examine this hypothesis, a chimeric streptokinase, SKβswap, was created by swapping the SK β-domain with the homologous β-domain of Streptococcus uberis Pg activator (SUPA or PauA, SK uberis), a streptokinase that cannot activate human plasminogen. SKβswap formed a tight complex with microplasminogen with an affinity comparable with streptokinase. The SKβswap-plasmin complex also activated human plasminogen with catalytic efficiencies (k_cat/K_m = 16.8 versus 15.2 μM^-1 min^-1) comparable with streptokinase. However, SKβswap was incapable of nonproteolytic active site generation and activated plasminogen by a staphylokinase mechanism. When compared with streptokinase complexes, SKβswap-plasmin and SKβswap-microplasmin complexes had altered affinities for low molecular weight substrates. The SKβswap-plasmin complex also was less resistant than the streptokinaseplasmin complex to inhibition by α₂-antiplasmin and was readily inhibited by soybean trypsin inhibitor. Thus, in addition to mediating high affinity binding to plasmin(ogen), the streptokinase β-domain is required for nonproteolytic active site generation and specifically modulates the interactions of the complex with substrates and inhibitors.