Contact with the N termini in the central E domain enhances the reactivities of the distal D domains of fibrin to factor XIIIa.

The reaction of Factor XIIIa with fibrin is the last enzyme-catalyzed step on the coagulation cascade, leading to the formation of a normal blood clot. The finding that fibrin is preferred by the cross-linking enzyme about 10-fold over the circulating fibrinogen suggests the operation of a unique substrate-level control for the orderly functioning of the physiological process in the forward direction. An important task is to elucidate the molecular mechanism for the transmission of the signal generated by the thrombin-catalyzed cleavage in the central E domain of fibrin to the distant Factor XIIIa-reactive glutamine residues. By focusing on the substrate sites present in gamma chain remnants of D type domains of fibrinogen and by employing the approach of fragment complementation with the regulatory E domain, which represents the thrombin-modified portion of fibrin, we have now succeeded in reconstructing in solution the phenomenon of kinetic enhancement for the reaction with Factor XIIIa. Two D type preparations (truncated fibrinogen, approximately 250 kDa and D', approximately 105 kDa) were obtained by digestion of human fibrinogen with endo Lys-C. Neither product could be cross-linked by Factor XIIIa, but as shown by the incorporation of dansylcadaverine, both were acceptor substrates for the enzyme. The plasmin-derived D (approximately 105-kDa) product, however, could be cross-linked into DD dimers. In all cases, the admixture of E fragments exerted a remarkable boosting effect on the reactions with Factor XIIIa. Even with native fibrinogen as substrate, cross-linking of gamma chains was enhanced in the presence of E. Nondenaturing electrophoresis was used to demonstrate the complex forming potential of E fragments with fibrinogen, truncated fibrinogen, D', or D. The GPRP tetrapeptide mimic of the GPRV N-terminal sequence of the alpha chains in the E fragments, abolished both complex formation and the kinetic boosting effect of E on the reactions of substrates with Factor XIIIa. Thus, the N-terminal alpha chain sequences seem to act as organizing templates for spatially orienting the D domains, probably during the protofibrillar assembly of the fibrin units, for favorable reaction with Factor XIIIa.