Factor xii

Factor XII, also known as Hageman factor, is a protein that plays a crucial role in the blood coagulation system, kinin-kallikrein system, and complement system, all part of the body's immune response. It is classified as a serine protease, a type of enzyme that cuts peptide bonds in proteins.

Structure

Factor XII is a single-chain polypeptide composed of 596 amino acid residues. The protein is synthesized in the liver and circulates in the blood plasma in its inactive form, known as factor XII or prekallikrein. Upon activation, it is converted into its active form, factor XIIa.

Function

Factor XII is part of the intrinsic pathway of blood coagulation, where it acts as the first factor. The activation of Factor XII is triggered by contact with negatively charged surfaces, a process known as contact activation. Once activated, Factor XIIa can then activate Factor XI and prekallikrein, which in turn activates other factors in the coagulation cascade, leading to the formation of a blood clot.

In addition to its role in coagulation, Factor XII also plays a role in the kinin-kallikrein system, where it activates prekallikrein to kallikrein. Kallikrein then cleaves high-molecular-weight kininogen to release bradykinin, a peptide that causes blood vessels to dilate and increases vascular permeability.

Clinical significance

Mutations in the Factor XII gene can lead to a rare disorder known as Factor XII deficiency. This condition is usually asymptomatic and is often discovered during routine blood tests. Despite the crucial role of Factor XII in the coagulation cascade, individuals with Factor XII deficiency do not have a bleeding tendency. This has led to questions about the exact role of Factor XII in coagulation and its potential as a target for anticoagulant drugs.

Factor XII has also been implicated in the pathogenesis of several other conditions, including hereditary angioedema, thrombosis, and inflammation. Research is ongoing to further understand the role of Factor XII in these conditions and to develop potential treatments.

See also

• Blood coagulation
• Factor XI
• Factor XII deficiency
• Hereditary angioedema
• Thrombosis