Chemistry and Physiology of the Fibrinogen-Fibrin Transition

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Abstract
This review paper is concerned with clot formation. Thrombin is a proteolytic enzyme of the endopeptidase variety having the very small molecular weight of 8000 g. The monomeric form is fully active and may act differently than the dimeric form. Thrombin hydrolyzes only four peptide bonds in its action on fibrinogen. These are the bonds between arginine and glycine residues. The action mechanism is described. Fibrinogen, the clotting protein of plasma and the primary substrate of thrombin, has a molecular weight of 330,000 g. It is synthesized in the liver, the rate of formation being adjusted to the level in the plasma. Partially clotted fibrinogen also exists in the blood indicating that the fibrinogen-fibrin transition goes on continuously to a small extent. The clotting mechanism of fibrinogen is described.