Intermolecular interactions in staphylokinase-plasmin(ogen) bimolecular complex: function of His43 and Tyr44

Staphylokinase (SAK) forms a 1:1 stoichiometric complex with human plasmin (Pm) and switches its substrate specificity to generate a plasminogen (Pg) activator complex. Site-directed mutagenesis of SAKHis43 and SAKTyr44 demonstrated the crucial requirement of a positively charged and an aromatic residue, respectively, at these positions for optimal functioning of SAK-Pm activator complex. Molecular modeling studies further revealed the role of these residues in making cation-pi and pi-pi interactions with Trp215 of Pm and thus establishing the crucial intermolecular contacts within the active site cleft of the activator complex for the cofactor activity of SAK.