Species specificity of streptokinase

Streptokinase, a bacterial protein, forms a complex with human plasminogen which results in a conformational change in the plasminogen molecule and the exposure of an active center. The plasminogen-streptokinase complex is an activator of plasminogen and is rapidly converted to a plasmin-streptokinase complex which, in the human, is also an activator of plasminogen. Species differences have been found in the reaction of streptokinase with plasminogen varying from no active complex formation at one extreme to the rapid formation of an active activator complex at the other, with resultant differences in rates of complex formation and the yield of plasmin. Explanation of these species differences at a molecular level are discussed as well as the possible application of complex formation in a variety of biological systems as a mechanism to produce variation in enzyme activities in proportion to the concentration of substrate available.