Fibrin (also calledFactor Ia) is afibrous, non-globularprotein involved in theclotting ofblood. It is formed by the action of theproteasethrombin onfibrinogen, which causes it topolymerize. The polymerized fibrin, together withplatelets, forms ahemostatic plug or clot over a wound site.
When the lining of a blood vessel is broken, platelets are attracted, forming aplatelet plug. These platelets havethrombin receptors on their surfaces that bind serum thrombin molecules,[1] which in turn convert soluble fibrinogen in the serum into fibrin at the wound site. Fibrin forms long strands of tough insoluble protein that are bound to the platelets.Factor XIII completes the cross-linking of fibrin so that it hardens and contracts. The cross-linked fibrin forms a mesh atop the platelet plug that completes the clot. Fibrin was discovered[2] byMarcello Malpighi in 1666.[3]
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Excessive generation of fibrin due to activation of thecoagulation cascade leads tothrombosis, the blockage of a vessel byagglutination of red blood cells, platelets, polymerized fibrin and other components. Ineffective generation or prematurelysis of fibrin increases the likelihood of ahemorrhage.
Dysfunction or disease of the liver can lead to a decrease in the production of fibrin's inactive precursor,fibrinogen, or to the production of abnormal fibrinogen molecules with reduced activity (dysfibrinogenaemia). Hereditary abnormalities of fibrinogen (the gene is carried on chromosome 4) are both quantitative and qualitative in nature and includeafibrinogenaemia,hypofibrinogenaemia, dysfibrinogenaemia, andhypodysfibrinogenemia.
Reduced, absent, or dysfunctional fibrin is likely to render patients ashemophiliacs.[clarification needed]
Fibrin from various different animal sources is generallyglycosylated with complex type biantennary asparagine-linkedglycans. Variety is found in the degree of corefucosylation and in the type ofsialic acid andgalactose linkage.[4]
Fibrin is formed after thrombin cleavage of fibrinopeptide A (FPA) from fibrinogen Aalpha-chains, thus initiating fibrin polymerization. Double-stranded fibrils form through end-to-middle domain (D:E) associations, and concomitant lateral fibril associations and branching create a clot network.[5][6] Fibrin assembly facilitates intermolecular antiparallel C-terminal alignment of gamma-chain pairs, which are then covalently 'cross-linked' by factor XIII ('plasma protransglutaminase') or XIIIa to form 'gamma-dimers'. The image at the left is a crystal structure of the double-d fragment from human fibrin with two bound ligands. The experimental method used to obtain the image was X-ray diffraction, and it has a resolution of 2.30 Å. The structure is mainly made up of singlealpha helices shown in red andbeta sheets shown in yellow. The two blue structures are the boundligands. The chemical structures of the ligands are Ca2+ ion, alpha-D-mannose (C6H12O6), andD-glucosamine (C6H13NO5).[7]
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