Factor XIII is the Coagulation Protein That Creates Covalent Bonds Between Fibrin Monomers
Factor XIII is the coagulation protein that exclusively functions to create covalent bonds between fibrin monomers formed during the coagulation process to form a stable fibrin clot. 1
Mechanism of Factor XIII in Fibrin Stabilization
Factor XIII plays a critical role in the final stages of blood coagulation through the following mechanism:
- Factor XIII circulates in blood as a heterotetramer with two catalytic A subunits and two carrier B subunits (FXIII-A₂B₂) 1
- During coagulation, thrombin cleaves an activation peptide from each FXIII-A subunit
- In the presence of calcium, the B subunits dissociate, allowing the A subunits to assume an active configuration (FXIIIa) 1
- FXIIIa functions as a transglutaminase that catalyzes the formation of covalent bonds between fibrin monomers 1, 2
- These covalent bonds are formed specifically between the α and α chains, resulting in γ-γ dimers and α polymers 1
Importance of Factor XIII in Clot Stability
The covalent cross-linking of fibrin by Factor XIII is essential for:
- Converting soluble fibrin monomers into a stable fibrin clot 2
- Preventing premature degradation of fibrin by the fibrinolytic system 3
- Reinforcing the fibrin structure to withstand mechanical stress 1
- Enabling the body to stop potential bleeding episodes effectively 3
Clinical Significance of Factor XIII
The critical role of Factor XIII in clot stability is evidenced by the consequences of its deficiency:
- In Factor XIII deficiency, although initial clot formation occurs, the clot is rapidly degraded by the fibrinolytic system 3
- This results in delayed bleeding, which can be severe and life-threatening 3, 4
- Factor XIII deficiency can be congenital (rare, 1 per million) or acquired (more common) 2
Distinguishing Factor XIII from Other Coagulation Factors
It's important to understand how Factor XIII differs from the other options presented:
- Factor XII (Option A): Initiates the intrinsic pathway of coagulation but does not create covalent bonds between fibrin monomers 5
- Factor II (Option C): Also known as prothrombin, converts to thrombin which cleaves fibrinogen to form fibrin monomers but does not create covalent bonds between them 5
- Factor IX (Option D): Functions in the intrinsic pathway of coagulation, activating Factor X, but is not involved in creating covalent bonds between fibrin monomers 5
The unique role of Factor XIII as the "fibrin-stabilizing factor" that creates covalent bonds between fibrin monomers makes it the correct answer to this question.