Improving the therapeutic potential of staphylokinase, a potent thrombolytic agent

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Staphylokinase (Sak) is a small bacterial-derived protein (15.5 kDa) that can hydrolyze fibrin-rich blood clots by activating the conversion of plasminogen into plasmin. Several studies have shown the effectiveness of Sak as a thrombolytic agent that could be used to treat stroke or cardiac arrest. However, its short half-life in blood circulation and immunogenicity are the main issues that prevent Sak from clinical applications. Herein, we explore two modifications of Sak to improve its therapeutic potential. Covalently attaching PEG (polyethylene glycol) polymers to therapeutic proteins has been widely applied to improve their pharmacokinetic properties, resulting in several FDA-approved protein-based drugs. Although PEGylation of Sak has previously been investigated, the effects of PEGylation on the 3D structure of Sak have not been studied in detail. In the first part of this research project, site-specific PEGylation of the truncated version of Sak was performed in the immunogenic region of the protein, and three covalent PEGylated derivatives of the protein were prepared. NMR studies showed a slight structural perturbation upon PEGylation, mostly close to the PEGylation site, and a direct relationship between the hydrodynamic radius of the PEGylated protein and the PEG size. Together, the results suggest that PEG and Sak∆10 move relatively independently from each other. As a second approach, we introduce cyclic Sak (cyc-Sak), a novel form of staphylokinase with higher stability and improved plasminogen activation activity. Using an Ssp GyrB split intein, the N- and C-terminal ends of the linear Sak were connected by a peptide bond, rendering the protein into a cyclic form (cyc-Sak). This structural modification was generated at the protein expression level in Escherichia coli, and the cyclic protein could be purified by common chromatography techniques. Successful backbone cyclization was confirmed by NMR spectroscopy of the 13C,15N-labeled cyc-Sak and by chemical cleavage assays. Our studies show that mono-PEGylation and intein-mediated backbone cyclization of Sak are powerful strategies to improve its therapeutic potential. These approaches could be combined to facilitate the design of protein-based medications when the stability of therapeutic proteins is an obstacle to their clinical application.
Staphylokinase, protein cyclization, protein PEGylation, intein, NMR Spectroscopy
Baharian, A. (2023). Improving the therapeutic potential of staphylokinase, a potent thrombolytic agent (Master's thesis, University of Calgary, Calgary, Canada). Retrieved from