Fibrin polymerization and its regulatory role in hemostasis.

Proteolytic conversion of fibrinogen to fibrin results in self-assembly to form a three-dimensional clot matrix that subsequently becomes cross-linked by fXIIIa to form the central structural element of the in vivo thrombus. The process of fibrin formation and assembly leads to new properties that serve to regulate the rate and extent of clotting, cross-linking, and fibrinolysis. These are brought about by the ability of fibrin (1) to bind thrombin at a noncatalytic site, thus limiting its diffusability but at the same time preserving its catalytic potential; (2) to bind fXIII, regulate its activation to fXIIIa, and limit further activation of fXIII once fibrin cross-linking has occurred; and (3) to bind alpha 2-PI, tPA, and plasminogen and regulate the initiation and propagation of fibrinolysis. Additional interactions not covered in this review between fibrin(ogen), and other plasma proteins, cells or matrix components suggest other functions for fibrin that, along with those discussed above, define a critical role in modulating hemostasis, inflammation, and the wound healing process.