Fibrinogen functions and fibrin assembly

Abstract

Fibrinogen and fibrin play important roles in clot formation, fibrinolysis, cellular and matrix interactions, inflammation, and wound healing. These biological events are regulated to a large extent by clot formation itself and by complementary interactions between specific reactive sites on fibrin(ogen) and extrinsic molecules such as enzymes, proteins including other clotting factors, or cell receptors. Fibrinogen is comprised of two sets of three polypeptide chains termed Aα, Bβ, and γ, all six of which are joined by disulfide bridges to form the amino-terminal E domain. The molecules are elongated trinodular structures consisting of two globular outer D domains that are connected to its central E domain by a coiled-coil segment. These domains contain constitutive binding sites (e.g. Da, Db, γXL, D:D, γ′, thrombin substrate, platelet receptor, leukocyte integrin receptor) as well as interactive sites that become expressed as a result of fibrinogen cleavage by thrombin or that are exposed as a consequence of polymerization (e.g. t-PA binding sites). Other relevant constitutive sites in fibrinogen include two thrombin substrate recognition sites in each E domain plus a high-affinity non-substrate thrombin binding site in each γ′ chain that that also binds factor XIII. Constitutive binding sites on fibrinogen participate in fibrin assembly by self-association (γXL or D:D) or by complementary association with exposed fibrin sites (Da to EA and Db to E B). Other relevant constitutive sites in fibrin include a low-affinity thrombin-binding site in the fibrin E domain that evidently remains as a residual of the substrate binding site.Fibrin polymerization is initiated by thrombin cleavage of fibrinopeptide A (FPA) from fibrinogen Aα chains, exposing two E domain E A sites. Cleavage of fibrinopeptide B (FPB) from Bβ chains exposes another E domain polymerization site, E B, that also interacts with platelets, fibroblasts and endothelial cells. Fibrin generation is followed by end-to-middle intermolecular D-to-E associations forming linear and equilaterally branched double-stranded fibrils, and is accompanied by lateral fibril associations that form multi-stranded fibers. Concomitantly, thrombin-activated factor XIIIa introduces covalent crosslinks into these polymers, mainly between γ chains (at γXL sites forming γ-dimers) and α chains (α-polymers), to complete the mature clot network structure.