| Von Willebrand Factor |
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Von Willebrand factor is a Blood Glycoprotein involved in Coagulation . It is deficient or defective in Von Willebrand Disease and is involved in a large number of other diseases, including Thrombotic Thrombocytopenic Purpura , Heyde's Syndrome , and possibly Hemolytic-uremic Syndrome . BIOCHEMISTRY Synthesis vWF is a large multimeric Glycoprotein present in Blood Plasma and produced constitutively in Endothelium (in the Weibel-Palade Bodies ), Megakaryocyte s (α-granules of Platelet s), and subendothelial Connective Tissue . Structure The basic vWF monomer is a 2050 Amino Acid protein. Every monomer contains a number of specific domains with a specific function; elements of note are:
Monomers are subsequently N-glycosylated , arranged into dimers in the Endoplasmic Reticulum and into multimers in the Golgi Apparatus by crosslinking of Cysteine residues via Disulfide Bond s. With respect to the glycosylation, vWF is one of the few proteins that carry ABO Blood Group System antigens. Multimers of vWF can be extremely large, >20,000 KDa , and consist of over 80 subunits of 250 kDa each. Only the large multimers are functional. Some cleavage products that result from vWF production are also secreted but probably serve no function. Function Von Willebrand factor is not an Enzyme and therefore has no catalytic activity. Its primary function is binding to other proteins, particularly Factor VIII and it is important in platelet adhesion to wound sites. vWF binds to a number of cells and molecules. The most important ones are:
vWF appears to play a major role blood coagulation, and vWF deficiency or dysfunction (von Willebrand disease) therefore leads to a bleeding tendency, which is most apparent in tissues having high blood flow Shear in narrow vessels. From studies it appears that vWF uncoils under these circumstances, decelerating passing platelets. Catabolism The biological breakdown ( Catabolism ) of vWF is largely mediated by a protein cryptically termed ADAMTS13 (acronym of "''a'' ''d''isintegrin-like ''a''nd ''m''etalloprotease with ''t''hrombo''s''pondin type 1 motif no. ''13''"). It is a Metalloproteinase which Cleaves vWF between Tyrosine at position 842 and Methionine at position 843 (or 1605-1606 of the gene) in the A2 domain. This breaks down the multimers into smaller units, which are degraded by other Peptidase s. ROLE IN DISEASE See Also: Von Willebrand disease
HISTORY vWF is named after Dr. Erik Von Willebrand , a Finnish doctor who in 1924 first described a hereditary bleeding disorder in families from the Åland Islands who had a tendency for cutaneous and mucosal bleeding, including Menorrhagia . Although von Willebrand could not identify the definite cause, he distinguished von Willebrand disease (vWD) from Haemophilia and other forms of Bleeding Diathesis . In the 1950s , vWD was shown to be caused by a plasma factor deficiency (instead of being caused by platelet disorders), and, in the 1970s , the vWF protein was purified. SEE ALSO SOURCES
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