Coagulation factor XIII A chain

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Coagulation factor XIII A chain (FXIII-A) is a protein made from the F13A1 gene in humans. It is the catalytic part of coagulation factor XIII, the final enzyme activated in blood clotting. In plasma, factor XIII forms a tetramer with two A subunits and two B subunits. The A subunits are the active enzymes, while the B subunits are carriers with no enzymatic activity. Platelet factor XIII has two A subunits identical to the plasma A subunits.

Activation and function
- Activation occurs when thrombin cleaves an activation peptide and calcium is present; the plasma factor XIII loses its B subunits to become active FXIIIa. Platelet FXIII is already in an active form with two A subunits.
- FXIIIa is a transglutaminase that crosslinks fibrin, helping to stabilize the clot. It can also crosslink other proteins, such as alpha-2-plasmin inhibitor or fibronectin, to fibrin.

Deficiency
- Type I deficiency: lack of both A and B subunits.
- Type II deficiency: lack of the A subunit only.
- Symptoms include lifelong excessive bleeding, poor wound healing, and sometimes recurrent pregnancy loss.

Interactions and location
- FXIII A chain interacts with the FXIII B chain.
- The F13A1 gene is located on human chromosome 6.

Summary: FXIII-A is the catalytic part of the clot-stabilizing factor XIII, becoming active after activation and helping form a stronger, longer-lasting blood clot by crosslinking fibrin and other proteins. Defects in the A chain can lead to bleeding problems.