Schematic representation of human fibrinogen. The α chain is shown in blue, the β chain in green, and the γ chain in red. The black arrowheads indicate thrombin cleavage sites on the α chain. Glycosylation sites are shown as purple hexagons, while the calcium ions within the γ chains are represented as purple spheres (A). Schematic diagram showing the initial process of fibrin polymerization. The central nodules contain the amino-termini of all six chains(α,β,γ)2 and are referred to as the “E” regions, named after a fragment obtained by limited plasmin digestion of fibrin. They are flanked by two symmetric coiled coils that terminate in the distal “D” nodules. After the cleavage of fibrinopeptide A by thrombin, the newly exposed polymerization site “A” binds to the polymerization pocket “a” that is part of the γ chain of fibrin(ogen). The fibrin monomers thus align in a half-staggered, two-stranded arrangement to form long fibrils. Branch points and junctions occur sporadically (only one type is depicted here), contributing to the formation of a three-dimensional mesh (B).
