Factor XIII, also known as fibrin stimulating factor is a Zymogen found in he blood of humans along with some other animals. It is activated by Thrombin to Factor XIIIa which enables it to preform it's enzymatic functions of cross-linking fibrin as part of the coagulation cascade. It was first detected in 1944 by Robbins. Factor XIII is a heterotetramer that consists of 2 enzymatic A peptides and 2 non-enzymatic B peptides.
Function
Factor XIII is a transglutaminase that circulates throughout the blood as a heterotetramer. The B subunits bind to the clot structure. When fibrin is present, thrombin cleaves the R37-G38 peptide bond in the A subunit that reveals it's active enzymatic region at the N-terminus. Calcium ions further activate the A subunits through a change in shape. The calcium ions additionally dissociate the non-covalently bound B subunits. The remaining dimer of two active A subunits, FactorXIIIa, crosslinks fibrin by forming isopeptide bonds between glutamines and lysines within the fibrin. The crosslinks make the clot more durable and more resistant to fibrinolysis due to premature enzymatic degradation.
Disease
Factor XIII deficiency is a genetic bleeding disorder. It is an autosomal recessive disease. It is rare with Iran having the leading number of cases in the world. Individuals with this disorder form clots normally, however the clots are unstable and typically degrade which results in long bleeding episodes. Most phenotypical changes in factor XIII deficiency are caused by mutations in the A subunits.
= Signs and Symptoms
The symptoms of Factor XIII deficiency vary but in 80% of cases appear after birth with a bleeding episode stemming from the umbilical stump. Bleeding can occur spontaneously or from various activities
Relevance
Structural highlights
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