The Journal of biological chemistry
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Thrombin generation and fibrinogen (Fbg) clotting are the ultimate proteolytic reactions in the blood coagulation pathway. Staphylocoagulase (SC), a protein secreted by the human pathogen Staphylococcus aureus, activates prothrombin (ProT) without proteolysis. The SC.(pro)thrombin complex recognizes Fbg as a specific substrate, converting it directly into fibrin. ⋯ On this basis, a molecular model of the pentameric (SC-(1-325).thrombin)(2). Fbg encounter complex was generated, which explains the coagulant properties and efficient Fbg conversion. The results provide new insight into the mechanism that mediates high affinity Fbg binding and cleavage as a substrate of SC.(pro)thrombin complexes, a process that is central to the molecular pathology of S. aureus endocarditis.
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Staphylocoagulase (SC) is a protein secreted by the human pathogen, Staphylococcus aureus, that activates human prothrombin (ProT) by inducing a conformational change. SC-bound ProT efficiently clots fibrinogen, thus bypassing the physiological blood coagulation pathway. The crystal structure of a fully active SC fragment, SC-(1-325), bound to human prethrombin 2 showed that the SC-(1-325) N terminus inserts into the Ile(16) pocket of prethrombin 2, thereby inducing expression of a functional catalytic site in the cognate zymogen without peptide bond cleavage. ⋯ Comparison of human and bovine fibrinogen as substrates of human and bovine thrombin and the SC-(1-325).(pro)thrombin complexes indicates that the species specificity of SC-(1-325) cofactor activity is determined primarily by differences in conformational activation of bound ProT. These results suggest that the catalytic site in the SC-(1-325).bovine ProT complex is incompletely formed. The current crystal structure of SC-(1-325).bovine thrombin reveals that SC would dock similarly to the bovine proenzyme, whereas the bovine (pro)thrombin-characteristic residues Arg(144) and Arg(145) would likely interfere with insertion of the SC N terminus, thus explaining the greatly reduced activation of bovine ProT.