Identification of the binding site for plasminogen kringle 5 in the ?-chain of fibrin(ogen) D-fragment

The interaction of the fifth kringle of Glu-plasminogen with fibrin triggers activation and initiation of fibrinolysis, yet the site on fibrin that binds kringle 5 remains unknown. The aim of our work was to determine an amino acid sequence in the D-fragment of fibrin(ogen) molecule, which is complementary to the lysine-binding site (LBS) in kringle 5. We studied the interaction between kringle 5 of plasminogen with polypeptide chains of the D-fragments of fibrin and cyanogen bromide fragments FCB-2 and t-NDSK and showed that kringle 5 bound specifically to αand γ-chains of the D-fragment and the α-chain of FCB-2. Tryptic peptides of D-fragment α-chain were obtained, separated by their ability to bind with the immobilized kringle 5, and then all studied peptides were characterized by MALDI-TOF analysis. The critical amino acid residues of the α-chain of D-fragment, which provide its interaction with kringle 5, turned out to be α171Arg and/or α176Lys. The binding site of Glu-plasminogen complementary to the LBS of kringle 5 is located within Аα168Ala−183Lys, a sequence in a weakly structured loop between two supercoils in the α-chain of the Dfragment of the fibrin(ogen) molecule.