What is factor XIII?

Updated: Feb 07, 2020
  • Author: Bishnu Prasad Devkota, MD, MHI, FRCS(Edin), FRCS(Glasg), FACP; Chief Editor: Eric B Staros, MD  more...
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The enzyme factor XIII, which cross-links fibrin, belongs to the blood coagulation system. Also known as fibrin-stabilizing factor, the 320,000-dalton glycoprotein is activated by thrombin in the presence of calcium and has a plasma half-life of approximately 10 days. [3]

Plasma, platelets, monocytes, and monocyte-derived macrophages also contain factor XIII. In plasma, it exists as a heterotetramer made up of paired A and B subunits (A2, B2). In platelets and other cells, factor XIII lacks the B domain, existing instead as an A2 dimer.

Monocytes/macrophages can synthesize factor XIII, [4] while megakaryocytes are probably responsible for synthesizing the factor XIII found in platelets. [5, 6] The primary site for the synthesis of subunit A in plasma factor XIII seems to exist in cells that originate in the marrow. Synthesis of the B subunit occurs in the liver. [6] The factor XIII A chain gene is found on chromosome 6. [6]

Circulation of plasma factor XIII occurs in association with its substrate, fibrinogen. Thrombin cleavage of the Arg37 -Gly38 bond in the A chain, causing the release of an Mr 4500 activation peptide, is the key step in the activation of plasma factor XIII. As a result of this step, the A and B subunits become dissociated and the active site on the free A subunits is exposed.

In platelets, a nonproteolytic process activates cellular factor XIII. In the absence of the B-chain, the elevation of intracytoplasmic Ca2+ during platelet activation causes the zymogen to assume an active configuration. [3, 6]

Stabilizing the fibrin plug through cross-linkage of the alpha and gamma chains of fibrin is plasma XIIIa's main physiologic function. [7] Although a clot will form if factor XIII is absent, it will not be adequate for hemostasis. Components of the clotting and fibrinolytic system, along with multiple adhesive and contractile proteins, are additional factor XIIIa protein substrates. By crosslinking fibrin to alpha 2-antiplasmin, factor XIIIA also protects fibrin from fibrinolysis. [8]

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