Fibrinogen (Factor I) is a coagulation factor. This protein is essential for blood clot formation and is produced in the liver by the hepatocyte cells. Fibrinogen is released from the liver into the bloodstream as needed, along with over 20 other clotting factors. The Aα, Bβ, and γ chains are transcribed and translated coordinately on the endoplasmic reticulum (ER). The peptide chains are then passed into the ER. Here their signal peptide portions are removed.
When inside the ER, the three chains are assembled initially into Aαγ and Bβγ dimers. They are then assembled into AαBβγ trimers. And finally into (AαBβγ)2 heximers, which are two AαBβγ trimers joined together by numerous disulfide bonds.
The heximer is transferred to the Golgi. From here it is glycosylated, hydroxylated, sulfated, and phosphorylated. The result of this process is the formation of the mature fibrinogen glycoprotein that is secreted into the blood in the event of an injury to body tissue or blood vessel wall.
The fibrinogen circulates as a soluble plasma glycoprotein, which is fibrinogen dissolved in fluid. It is then changed into insoluble fibrin threads. The threads will cross-link to stabilize the injury site by forming a fibrin net. The net adheres to the injury site and aggregated cell fragments called platelets that work together to form a stable clot. The clot reduces blood loss from the injury and remains in place until the site is fully healed. The concentration of fibrinogen in blood plasma is normally 150–400 mg/dl. Levels above or below this normal range are associated with pathological bleeding or Thrombosis.