Human fibrinogen is a 340 kDa plasma protein produced in the liver. Plasma concentrations are typically 1.7 – 3.5 g/L (5-10 μM). The intact fibrinogen molecule consists of two identical subunits, each consisting of three non-identical polypeptide chains denoted as Aα, Bβ and γ. The letters A and B in the Aα and Bβ chains designate, respectively, fibrinopeptide A (FpA, residues 1-16), and fibrinopeptide B (FpB, residues 1-14), which are cleaved by thrombin upon conversion of fibrinogen to fibrin. The fibrin monomers polymerize in a half-overlap fashion to form insoluble fibrin fibrils. The polymerised fibrin is subsequently stabilized by activated Factor XIII that forms amide linkages between γ chains and, to a lesser extent, α chains of the fibrin molecules.
Proteolysis of fibrinogen by plasmin initially liberates C-terminal residues from the Aα chain to produce fragment X (intact D-E-D, which is still clottable). Fragment X is further degraded to non-clottable fragments Y (D-E) and D. Fragment Y can be digested into its constituent D and E fragments. Proteolysis of crosslinked fibrin by plasmin results in fragment DD (D-Dimer consisting of the D domains of 2 fibrin molecules crosslinked via the γ chains), fragment E (central E domain) as well as DDE in which fragment E is non-covalently associated with DD. The molecular weights of the cleavage fragments produced from human crosslinked fibrin are: 184 kDa for fragment DD, 92 kDa for D, 50 kDa for E, 1.54 kDa for FpA and 1.57 kDa for FpB.
Most of the fibrinogen in the circulation consists of 2 copies of each chain (Aα2, Bβ2, γA2), but in normal plasma approximately 10% of the fibrinogen molecules contain one γA chain and one variant γ chain (termed γ′), in which the c-terminal AGDV residues are replaced with the amino acid sequence VRPEHPAETEYDSLYPEDDL. This variant fibrinogen is commonly referred to as fibrinogen gamma prime (γA/γ′) but has also been called fibrinogen 2 or peak 2 fibrinogen because it elutes separately from fibrinogen 1 (γA2) by ion exchange chromatography. Residues 414-427 of the γ′ chain of fibrin gamma prime (contain a high-affinity binding site for exosite II of thrombin, which allows the active site of bound thrombin to remain available to interact with substrates while demonstrating resistance to heparin mediated inhibition by antithrombin1-4.
References and Reviews
Hantgan RR, Francis CW, Marder VJ; Fibrinogen Structure and Physiology; in Hemostasis and Thrombosis, 3rd Edition, eds. RW Colman, J Hirsh, VJ Marder and EW Salzman, pp 277-300, J.B. Lippincott Co., Philadelphia PA, USA, 1994.
Binnie CG, Lord ST; The Fibrinogen Sequences that Interact with Thrombin; Blood 81, pp 3186-3192, 1993.
Pospisil CH, Stafford AR, Fredenburgh JC, Weitz JI; Evidence that both Exosites on Thrombin Participate in Its High Affinity Interaction with Fibrin; JBC 278, pp 21584-21591, 2003.
Medved L, Weisel JW; Recommendations for Nomenclature on Fibrinogen and Fibrin; JTH 7, pp 355-359, 2009.