Identification of the functional importance of valine-19 residue in streptokinase by N-terminal deletion and site-directed mutagenesis.

Streptokinase (SK) is a bacterial plasminogen activator of multi-domain structure. In deletion analysis of the N-terminal region of SK, the deletion of 20 amino acids (SK delta N20) resulted in the dramatic reduction of plasminogen activator activity compared to deletion of 7 (SK delta N7) and 13 amino acids (SK delta N13). The incubation time to reach maximal active site generation in an equimolar mixture of SK delta N20 and plasminogen was the same as that for wild-type SK. To identify the functional residues important in plasminogen activation, several site-directed mutations were introduced at the region spanning Ser16-Val20 of SK. The results showed that Val19 residue is important for the activity of the SK-plasminogen complex.