Involvement of a nine-residue loop of streptokinase in the generation of macromolecular substrate specificity by the activator complex through interaction with substrate kringle domains

Abstract

The selective deletion of a discrete surface-exposed epitope (residues 254-262; 250-loop) in the β domain of streptokinase (SK) significantly decreased the rates of substrate human plasminogen (HPG) activation by the mutant (SK_del254-262). A kinetic analysis of SK_del254-262 revealed that its low HPG activator activity arose from a 5-6-fold increase in K_m for HPG as substrate, with little alteration in k_cat rates. This increase in the K_m for the macromolecular substrate was proportional to a similar decrease in the binding affinity for substrate HPG as observed in a new resonant mirror-based assay for the real-time kinetic analysis of the docking of substrate HPG onto preformed binary complex. In contrast, studies on the interaction of the two proteins with microplasminogen showed no difference between the rates of activation of microplasminogen under conditions where HPG was activated differentially by nSK and SK_del254-262. The involvement of kringles was further indicated by a hypersusceptibility of the SK_del254-262. plasmin activator complex to ε-aminocaproic acid-mediated inhibition of substrate HPG activation in comparison with that of the nSK.plasmin activator complex. Further, ternary binding experiments on the resonant mirror showed that the binding affinity of kringles 1-5 of HPG to SK_del254-262.HPG was reduced by about 3-fold in comparison with that of nSK.HPG. Overall, these observations identify the 250 loop in the β domain of SK as an important structural determinant of the inordinately stringent substrate specificity of the SK.HPG activator complex and demonstrate that it promotes the binding of substrate HPG to the activator via the kringle(s) during the HPG activation process.