ABSTRACT
BACKGROUND: Streptokinase, one of the most widely used thrombolytic medicines, is a favorable protein for site-specific PEGylation as it lacks any cysteine residues in its amino acid sequence; however, any changes in the protein's structure should be carefully planned to avoid undesired changes in its function. OBJECTIVES: This study aimed to design and produce novel di/tri-cysteine variants of streptokinase from previously developed cysteine analogues, Arg45, Glu263, and Arg319, as candidates for multiple site-specific PEGylation. METHODS: Using bioinformatics tools and site-directed mutagenesis, we incorporated concurrent mutations at Arg45, Glu263, and Arg319 (carried out in our previous study) to create di/tri-cysteine variants of streptokinase proteins (SK45-319cys, SK263-319cys, and SK45-263-319cys) and evaluated their kinetic activity parameters by a colorimetric method, using H-D-Val-Leu-Lys-pNA.2HCl (S2251) as substrate. RESULTS: Based on the kinetic results, SK263-319cys with 44% enzyme efficiency increment compared to wild-type SK was the superior protein in terms of activity; as well, SK45-319cys and SK45-263-319cys showed 17 and 22% activity enhancement, respectively. Docking of the mutant streptokinase proteins with µ-plasmin demonstrated that changes in intermolecular interactions caused by amino acid substitution could be the reason for activity difference. CONCLUSION: The novel mutant proteins created in this study exhibit remarkable biological activity and may be uniquely suitable for simultaneous PEGylation on two/three domains. As well, PEGylated derivates of these variants might prove to be more proficient proteins, compared to the singlecysteine analogs of streptokinase; because of their more surface coverage and increased molecular weight.
