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The major clot-forming substrate in the blood plasma of vertebrates. Though fibrinogen represents a small fraction of plasma proteins (normal human plasma has a fibrinogen content of 2–4 mg/ml of a total of 70 mg protein/ml), its conversion to fibrin causes a gelation which blocks the flow of blood. Upon injury, sufficient amounts of the clotting enzyme, thrombin, are generated in about 5 min clotting time to produce a gel. Although clotting in the circulation (thrombosis) can be extremely dangerous, clotting is an essential and normal response for preventing the loss of blood. Individuals born with the hereditary absence of fibrinogen (afibrinogenemia) suffer from severe bleeding, which can be counteracted by transfusing normal plasma or purified fibrinogen.
Fibrinogen is synthesized by the hepatocytes in the liver, and the synthetic rate can be stimulated by hormones. Significant amounts of carbohydrates become attached to the protein before it is secreted into the circulation; alterations in its carbohydrate composition as found in some liver diseases can give rise to abnormal fibrinogens with defective clotting properties.
Clotting is regulated by two enzymes, thrombin and factor XIIIa (fibrinoligase, activated fibrin-stabilizing factor, transglutaminase). Thrombin exerts a dual control by regulating the rate of fibrin formation as well as producing factor XIIIa. In the plasma milieu, the fibrin molecules readily aggregate into a clot. In order to obtain a clot structure of a strength sufficient to stem bleeding, however, it is necessary for the thrombin-modified factor XIII to be activated to XIIIa. Factor XIIIa acts as a transamidating enzyme which strengthens the fibrin clot by creating cross-links between the molecules. Without such cross-links, a clot structure would be like a brick wall without mortar. Individuals with the hereditary absence of factor XIII often suffer from severe bleeding, even though their clotting times are in the normal range. See Blood, Immunoglobulin
a soluble plasma protein belonging to the globulin class; coagulation factor I, which is converted to fibrin by the action of the enzyme thrombin. Molecular weight, approximately 350,000.
The fibrinogen molecule is globular, with a diameter of about 22 nanometers. It consists of two identical subunits, each of which is formed of three dissimilar polypeptide chains designated α, β, and γ, where α and β are peptides split off by the action of thrombin. Fibrinogen is synthesized in the parenchymatous cells of the liver. Its concentration in human plasma is 300–500 mg per 100 milliliters. A hemorrhagic diathesis occurs as a result of fibrinogen insufficiency or the formation of molecules with an anomalous structure.
Fibrinogen obtained by precipitation with ethyl alcohol from plasma is used to arrest bleeding during surgery, in obstetrical and gynecological practice, and in cases of hemophilia and diseases associated with low blood fibrinogen levels. Fibrinogen preparations are also produced for laboratory research. Fibrinogen derived from human blood is used clinically.
REFERENCESAndreenko, G. V. “Sovremennye dannye o khimii i fiziologii fibrinogena.” Uspekhi sovremennoi biologii, 1974, vol. 77, issue 1.
“Models Proposed for the Fibrinogen Molecule and for the Polymerization Process.” Thrombosis Research, 1975, vol. 6.
I. P. BASKOVA