Schematic model of fibrinogen and fibrin

Fibrinogen consists of three pairs of polypeptide chains, Aα, Bβ, and γ. The amino-terminal regions of all six chains are joined in the central domain (E domain) by disulfide bonds and form a dimeric structure. The E domain contains the FPA and FPB sequences that are cleaved by thrombin (factor IIa). Carbohydrate groups (O) are located at one site on each of the γ and Bβ chains. Thrombin-catalyzed conversion of fibrinogen to fibrin involves release of FPA and FPB. A nonsubstrate ("secondary") binding site for thrombin is present in the E domain of a,β-fibrin, and depends largely, if not entirely, on the presence of the b15-42 sequence. The binding sites for thrombin, t-PA, factor XIII, and α2-PI are indicated on the fibrinogen or fibrin molecule. The regions in the fibrinogen/fibrin molecule containing the platelet-binding HHLGGAKQAGDV or RGD sequences are indicated by the arrowheads.