Structure-Function Analysis of the Streptokinase Amino Terminus (Residues 1-59)

Lakshmi V. Mundada, Mary Prorok, Melanie E. DeFord, Mariana Figuera, Francis J. Castellino, William P. Fay

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33 Scopus citations


Streptokinase (SK) binds to plasminogen (Pg) to form a complex that converts substrate Pg to plasmin. Residues 1-59 of SK regulate its capacity to induce an active site in bound Pg by a nonproteolytic mechanism and to activate substrate Pg in a fibrin-independent manner. We analyzed 24 SK mutants to better define the functional properties of SK-(1-59). Mutations within the αβ1 strand (residues 17-26) of SK completely prevented non-proteolytic active site induction in bound Pg and rendered SK incapable of protecting plasmin from inhibition by α 2-antiplasmin. However, when fibrin-bound, the activities of αβ1 strand mutants were similar to that of wild-type (WT) SK and resistant to α2-antiplasmin. Mutation of Ile1 of SK also prevented nonproteolytic active site induction in bound Pg. However, unlike αβ1 strand mutants, the functional defect of Ile1 mutants was not relieved by fibrin, and complexes of Ile1 mutants and plasmin were resistant to α2-antiplasmin. Plasmin enhanced the activities of αβ1 strand and Ile1 mutants, suggesting that SK-plasmin complexes activated mutant SK·Pg complexes by hydrolyzing the Pg Arg561-Val562 bond. Mutational analysis of Glu39 of SK suggested that a salt bridge between Glu 39 and Arg719 of Pg is important, but not essential, for nonproteolytic active site induction in Pg. Deleting residues 1-59 rendered SK dependent on plasmin and fibrin to generate plasminogen activator (PA) activity. However, the PA activity of SK-(60-414) in the presence of fibrin was markedly reduced compared with WT SK. Despite its reduced PA activity, the fibrinolytic potency of SK-(60-414) was greater than that of WT SK at higher (but not lower) SK concentrations due to its capacity to deplete plasma Pg. These studies define mechanisms by which the SK α domain regulates rapid active site induction in bound Pg, contributes to the resistance of the SK-plasmin complex to α2-antiplasmin, and controls fibrin-independent Pg activation.

Original languageEnglish (US)
Pages (from-to)24421-24427
Number of pages7
JournalJournal of Biological Chemistry
Issue number27
StatePublished - Jul 4 2003
Externally publishedYes

ASJC Scopus subject areas

  • Biochemistry
  • Molecular Biology
  • Cell Biology


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