Extended Mechanism of the Plasminogen Activator Staphylokinase Revealed by Global Kinetic Analysis: 1000-fold Higher Catalytic Activity than That of Clinically Used Alteplase

Abstract

The plasminogen activator staphylokinase is a fibrin-specific thrombolytic biomolecule and an attractive target for the development of effective myocardial infarction and stroke therapy. To engineer the protein rationally, a detailed understanding of the biochemical mechanism and limiting steps is essential. Conventional fitting to equations derived on the basis of simplifying approximations may be inaccurate for complex mechanisms such as that of staphylokinase. We employed a modern numerical approach of global kinetic data analysis whereby steady-state kinetics and binding affinity data sets were analyzed in parallel. Our approach provided an extended, revised understanding of the staphylokinase mechanism without simplifying approximations and determined the value of turnover number kcat of 117 s–1 that was 10000-fold higher than that reported in the literature. The model further showed that the rate-limiting step of the catalytic cycle is binding of staphylokinase to plasmin molecules, which occurs via an induced-fit mechanism. The overall staphylokinase effectivity is further influenced by the formation of an inactive staphylokinase·plasminogen complex. Here, we describe a quick and simplified guide for obtaining reliable estimates of key parameters whose determination is critical to fully understand the staphylokinase catalytic functionality and define rational strategies for its engineering. Our study provides an interesting example of how a global numerical analysis of kinetic data can be used to better understand the mechanism and limiting factors of complex biochemical processes. The high catalytic activity of staphylokinase (more than 1000-fold higher than that of the clinically used drug alteplase) determined herein makes this thrombolytic agent a very attractive target for further engineering.