Evidence for four different polymerization sites involved in human fibrin formation.

The mechanism of association and the organization of human fibrin were studied by using affinity chromatography. Insolubilized fibrinogen, fibrin monomer, and crosslinked fibrin were used to localize the binding sites on fibrinogen and fibrin derivatives. Four different polymerization sites have been distinguished. A binding site ("a"), available without thrombin action, is present on the fibrinogen fragment D domain. The complementary ("A") is inoperative in fibrinogen and requires thrombin for activation; it is located on the fibrinogen NH2-terminal domain. A third polymerization site ("b") appears to be formed by the alignment of the fragment D domains on two fibrin monomer molecules upon polymerization; this site functions without thrombin mediation and the alignment is stabilized by the Factor XIIIa-catalyzed crosslink bonds. The "b" site is complementary to another thrombin-activated site ("B") on the fibrinogen NH2-terminal domain. The two thrombin activable sites, "A" and "B", are distinguishable, although they are located in the same fibrinogen domain.