.beta.(Leu121-Lys122) segment of fibrinogen is in a region essential for plasminogen binding by fibrin fragment E

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Abstract
Two sequentially nonidentical regions of human fibrin(ogen), present in fragments D and E, carry specific plasminogen-binding sites. Comparison of the affinity of a variety of fragment E species for immobilized Lys-plasminogen revealed that fragment E3e [(.alpha.20/24-78, .beta.54-122, .gamma.1-53)2] possesses a strong plasminogen-binding site, whereas fragment E3t [(.alpha.20/24-78, .beta.54-120, .gamma.1-53)2] has 30-fold lower affinity for the affinant. Since the 2 fragments differ only in the .beta.(Leu121-Lys122) segment, this suggests that residues .beta.(Leu121-Lys122), present in the triple-helical connector region of fibrin(ogen), are essential for plasminogen binding by fragment E. Reduction and alkylation of fragment E3e lead to the destruction of the plasminogen-binding site, indicating that none of the separated, alkylated polypeptide chains of the fragment are able to bind to plasminogen and probably the coiled-coil superstructure of the connector region is necessary for the maintenance of the plasminogen-binding site of fragment E.